Syllabus Edition

First teaching 2023

First exams 2025

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Proton Transfer Reactions (HL IB Chemistry)

Exam Questions

9 hours138 questions
1a1 mark

Define a Brønsted–Lowry acid.

1b1 mark

Which species in the following reaction acts as a Brønsted-Lowry base.

HSO4- (aq) + H3O+ (aq) rightwards harpoon over leftwards harpoon H2SO4 (aq) + H2O (l)]

1c1 mark

Which species in the following equation is acting as a Brønsted-Lowry acid.

CO32- (aq) + H+ (aq) rightwards harpoon over leftwards harpoonHCO3- (aq)

1d2 marks

Explain, using the Brønsted-Lowry theory, how water can act either as an acid or a base.

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2a1 mark

Describe the difference between an amphiprotic and amphoteric species.

2b2 marks

Write an equation to show ammonia, NH3, acting as both a Brønsted–Lowry base and a Brønsted–Lowry acid.

   Brønsted–Lowry base: ....................

   Brønsted–Lowry acid:  ....................

2c1 mark

The equations for two acid-base reactions are given below.

HCO3 (aq) + H2O (l)  H2CO3 (aq) + OH (aq)
HCO3 (aq) + H2O (l) not stretchy rightwards harpoon over leftwards harpoon CO32– (aq) + H3O+ (aq)

Identify two different amphiprotic species in the above reactions.

2d2 marks

i)        State what is meant by the term conjugate base.

[1]

ii)       State the conjugate base of the hydroxide ion, OH– 

[1]

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3a1 mark

State an equation for the reaction of magnesium carbonate with dilute hydrochloric acid.

3b1 mark

State an equation for the reaction of lithium oxide with dilute nitric acid.

3c2 marks

Which acid and base would be required to produce ammonium sulfate, (NH4)2SO4

3d3 marks

Nitric acid and calcium hydroxide react together.

i)        State the type of reaction that takes place.

[1]

ii)       State the formula of the products of the reaction.

[1]

iii)      State the sign of the enthalpy change for this reaction.

[1]

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4a1 mark

Identify one conjugate acid-base pair in the reaction.

OCl(aq) + H2O (l) rightwards harpoon over leftwards harpoon OH(aq) + HOCl (aq)
4b1 mark

State an equation for the reaction of aluminium hydroxide with dilute sulfuric acid.

4c1 mark

State an equation for the reaction of calcium hydrogencarbonate with dilute phosphoric acid, H3PO4.

4d3 marks

Write the formulae for the following:

i)   Carbonic acid. 

[1]

ii)   Ammonium sulfate. 

[1]

iii)   Magnesium ethanoate. 

[1]

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5a1 mark

Using section 18 of the data booklet, identify an indicator that would show a yellow colour in ammonia solution?

5b2 marks

Suggest two characteristics that make a good indicator for a titration.

5c
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1 mark

A typical set of acid-base titration results in shown in the table.

  Rough Run 1 Run 2
Initial burette reading /+0.05 mL 0.00 0.30 0.60
Final burette reading/ +0.05 mL 24.15 22.55 22.95

Determine the mean volume from these results.

5d1 mark

What is the recorded uncertainty on the mean volume calculated in part c)?

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6a1 mark

State the relationship between pH and hydrogen ion concentration.

6b
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1 mark

Determine the pH of 0.200 mol dm-3 hydrochloric acid.

6c
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1 mark

Determine the hydrogen ion concentration in a sample of lake water of pH 5.60.

6d1 mark

The table below shows the hydrogen ion concentration in three solutions:

  P Q R
[H+] 0.001 1 x 10-5 1.00

List the three solutions in order from low pH to high pH

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7a1 mark

State what is meant by the ionic product of water.

7b
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1 mark

Calculate the concentration of [H+] in a solution of sodium hydroxide, NaOH, whose concentration is 0.001 mol dm-3.

7c
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1 mark

Calculate the pH of 0.001 mol dm-3 NaOH solution.

7d
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1 mark

The ionic product of water is 2.916 x 10-14 mol2 dm-6 at 313 K. What is the pH of water at this temperature?

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8a
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2 marks

State one advantage and one disadvantage of using a pH meter instead of universal indicator to measure pH.

8b2 marks

State the name and formula of a strong alkali and a weak alkali.

8c1 mark

State the meaning of the term dissociation as applied to acids and bases

8d2 marks
Write equations for the dissociation of:

   Nitric acid, HNO3: ...................................      

         Methanoic acid, HCOOH: ............................

8e1 mark

Identify the formula of the weakest conjugate base produced in the two acids in part d).

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9a2 marks

Explain the difference between the terms strong acid and weak acid. 

9b2 marks

Other than measuring the pH, describe how you could distinguish between dilute solutions of the same concentration of hydrochloric acid and ethanoic acid.

9c1 mark

A solution of 2.00 mol dm-3 hydrochloric acid was added to marble chips and the volume of carbon dioxide recorded. A graph of the result is shown below:


8-2-ib-sl-sq-easy-q5c-question

On the same graph, sketch the result of repeating the experiment with 2.00 mol dm-3 ethanoic acid.

9d1 mark

The same experiment in part c) can be carried out by measuring how the mass of the reaction flask changes with time.
Sketch a graph of the expected result.

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10a2 marks

On the axes below, draw a sketch graph to show the neutralisation of ethanoic acid by sodium hydroxide:

U7Pfi2Bz_q2-18-1-sq-hl

10b2 marks

Write an equation for the reaction between ethanoic acid and sodium hydroxide and identify the species acting as a Lewis base in the reaction.

10c2 marks

Identify the type of titration taking place from the curve and indicate where the buffer region is found on this curve.

q3-18-1-sq-hl-answer
10d2 marks

Identity on the graph the point at which pKa= pH and find the pKa of the acid.

2-2

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11a3 marks

Explain how an acid-base indicator works.

11b2 marks

Phenolphthalein, C20H14O4, is an acid-base indicator. State the formula and colour of the conjugate base of phenolphthalein.

11c3 marks

Explain how suitable indicators are chosen for titrations.

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12a1 mark

Outline what is meant by a buffer solution.

12b2 marks

Outline how a buffer solution can be made starting from 1.0 mol dm-3 ethanoic acid and 1.0 mol dm-3 sodium hydroxide.

12c4 marks

Use suitable equations to explain how the buffer in b) functions when a small quantity of acid is added.

12d1 mark

State the composition of a basic buffer

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13a1 mark

Explain what is meant by the term hydrolysis in acids and bases.

13b3 marks

Salts can be acidic, basic or neutral. Explain how you can predict whether a salt is likely to be acidic. Include an equation in your answer.

13c3 marks

Deduce which of the following salts are acidic, basic or neutral:

CH3COONa      NH4Cl      KCl

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14a
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1 mark

Hydrocyanic acid, HCN, is used in the synthesis of polymers and pharmaceuticals. It is a weak acid.

Write an equation to show the dissociation of hydrocyanic acid.

14b1 mark

Pyridine is an organic compound with the chemical formula C5H5N. It is a weak base.

Write an equation to show how pyridine acts as a base.

14c2 marks

Write an equation to show the reaction between hydrocyanic acid and pyridine and identify two conjugate acid-base pairs.

14d2 marks

Using Table 1, deduce which of the two acids, ethanoic, CH3COOH, or hydrocyanic is the stronger acid.

Acid pKa
ethanoic acid 4.76
hydrocyanic acid 9.20

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15a4 marks

Using Table 1, determine which of chloroethanoic acid, dichloroethanoic acid and trichloroethanoic acid is the stronger acid.

Table 1

Name of Acid Formula pKa
chloroethanoic acid CH2ClCOOH 2.87
dichloroethanoic acid CHCl2COOH 1.35
trichloroethanoic acid CHCl3COOH 0.66

15b1 mark

Write the Ka expression for dichloroethanoic acid, CHCl2COOH.

15c1 mark

Methylamine, CH3NH2, is a substance used to synthesise many commercially available compounds. State the Kb expression for methylamine.

15d1 mark

State the relationship between Ka and Kb for an acid and its conjugate base.

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16a
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4 marks

A solution of 0.01 mol dm-3 ethanoic acid has a pH of 3.37 at 298 K. Determine the Ka of ethanoic acid.

16b
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5 marks

A solution of 0.10 mol dm-3 methylamine, CH3NH2, has a pH of 11.80 at 298 K. Determine the Kb at this temperature.

16c
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2 marks

Determine the [H+] in a 0.10 mol dm-3 solution whose Ka = 1.00 x 10-8 at 298 K.

16d
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2 marks

Determine the pOH of the solution in part c).

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17a
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3 marks

The pKa of methanoic acid is 3.75 at 298 K.

i)
Write the formula of the conjugate base of methanoic acid.

[1]

ii)
Determine the pKb of the conjugate base

[2]

17b1 mark

The pKa of ethanoic acid is 4.76. Determine whether the conjugate base of methanoic acid is weaker or stronger than the conjugate base of ethanoic acid.

17c
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1 mark

At 283 K the pKw of pure water is 14.54. Determine the pH at this temperature.

17d2 marks

Comment on the acid-base nature of water at 283 K in part c).

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1a1 mark

Malonic acid is a naturally occurring acid found in fruits and vegetables and is shown in Figure 1.

Figure 1 

q1a-figure-1

The first dissociation of malonic acid is:

C3H4O4 (aq) + H2O (l) C3H3O4- (aq) + H3O+ (aq)

Identify one conjugate acid-base pair from the equation.

1b3 marks

The equilibrium constant for the first dissociation of malonic acid is 1.48 x 10-3.

State, with a reason, the strength of malonic acid.

1c1 mark

The anion C3H3O4- may be classified as amphiprotic. Explain the meaning of amphiprotic and write equations, using C3H3O4-, to illustrate your answer.

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2a1 mark

Salicylic acid has the structure shown below in Figure 1.                             

Figure 1

q2a-figure-1

Draw the structure of the conjugate base of salicylic acid, showing all the atoms and all the bonds.

2b2 marks

Predict what would be seen if a small amount of copper (II) oxide was added to an aqueous solution of salicylic acid, HOC6H4COOH, and warmed.

Write a balanced equation for the reaction.

2c1 mark

Suggest, with a reason, whether salicylic acid is likely to be soluble in water.

2d1 mark

Determine the relative molecular mass, Mr, of salicylic acid using Table 6 from the Data book.

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3a3 marks

Write balanced equations to show the separate reactions between ethanoic acid and calcium carbonate, CaCO3, magnesium oxide, MgO, and aluminium hydroxide, Al(OH)3.

3b2 marks

In Table 1 below, suggest the names and formulae of the acids and bases needed to make the specified salts.

Table 1

Acid

Base

Salt

 

 

Copper nitrate, Cu(NO3)2

 

 

Calcium phosphate, Ca3(PO4)2

3c3 marks

The ethanoate ion, CH3COO-, carbon dioxide, CO2, and the ethoxide ion, CH3CH2O-, all contain carbon oxygen bonds. 

Deduce the order in carbon to oxygen bond length from shortest to longest and explain your answer.

3d2 marks

Ethanoic acid, CH3COOH, shows two absorptions in an infrared spectrum that are not present in the spectrum of ethanol.

Using Section 20 of the Data book, state the wavenumber range of these absorptions and the bonds that cause them.

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4a2 marks

Glycolic acid, C2H4O3, is a colourless, odourless crystalline solid that is highly soluble in water and behaves as a Brønsted–Lowry acid.

i)

Define the term Brønsted–Lowry acid.

[1]

ii)

State one difference between Brønsted–Lowry acids and the traditional theory of acids as substances that dissociate in water to form hydrogen ions.

[1]

4b1 mark

The systematic IUPAC name for glycolic acid is 2-hydoxyethanoic acid.

Draw the structural formula for its conjugate base, showing all the atoms and bonds.

4c2 marks

Write an equation for the reaction between glycolic acid, C2H4O3, and limescale, CaCO3. State and explain one observation you would make.

4d1 mark

State one reason why you would use glycolic acid to remove the limescale in a kettle at home, but not hydrochloric acid.

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5a2 marks

An alkaline solution is formed when sodium hydrogencarbonate is dissolved in water.

Write an equation for the reaction and explain why the solution is alkaline.

5b2 marks

State whether the HCO3- ion is behaving as a Brønsted–Lowry acid or as a base and give a reason for your answer.

5c1 mark

Carbon dioxide gas dissolves in rainwater to form carbonic acid. State the formula of the conjugate base of carbonic acid.

5d1 mark

Carbonic acid and sulfuric acid can be described as diprotic acids. Explain the meaning of diprotic.

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6a2 marks

The equilibrium constant for the first dissociation of formic acid is 1.8 x 10-4 mol dm-3.

State, with a reason, the strength of formic acid.

6b1 mark

Outline one laboratory method used to distinguish between equimolar solutions of formic acid and hydrochloric acid, giving the expected observations.

6c2 marks

Formic acid has the chemical formula HCOOH. Identify the conjugate base of formic acid and state whether it is a weak or strong conjugate base.

6d2 marks

Draw the structure of formic acid and give its systematic IUPAC name.

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7a2 marks

The pH of an aqueous solution of salicylic acid at 298 K is 3.85. Determine the concentration of hydroxide ions in the solution, using Section 2 of the Data booklet.

7b3 marks

A and B are two solutions of the same concentrations that have pH values of 3 and 6 respectively. 

i)
Identify which is the stronger acid and calculate the concentration of hydrogen ions in each solution.
[2]
ii)
Calculate the ratio of the hydrogen ion concentrations in both A and B.
[1]
7c3 marks

The variation of conductivity and concentration of a strong and weak monoprotic acid are shown in Figure 1.

Identify the strong and weak acid from the information given and justify your choices.

VcCyqkCr_1

7d1 mark

For acid 1 and acid 2 in part (c) compare the volume of 0.2 mol dm-3 NaOH required to neutralise 20 cm3 of 0.1 mol dm-3 solutions of the acids.

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8a3 marks

The concentrations of solutions of weak acids can be determined by titration against standard solutions of alkalis, such as sodium hydroxide.

i)
Explain what is meant by the term standard solution.
[1]
ii)
State the name of the indicator which should be used for this titration and what would be observed at the equivalence point of the reaction if the sodium hydroxide is placed in the burette.
[2]
8b3 marks

A solution of 25.0 cm3 ethanoic acid was titrated against 0.150 mol dm-3 NaOH (aq) and it was found that 22.35 cm3 of the NaOH was needed for complete neutralisation.

Write an equation for the reaction and determine the concentration of the ethanoic acid.

8c2 marks

A solution of 0.1 mol dm-3 ammonia has a pH of approximately 11. Predict how the pH value of 0.1 mol dm-3 sodium hydroxide solution would compare and calculate its value.

8d2 marks

Write an equation for the reaction between ammonia and water and use it to classify each product as a Brønsted–Lowry acid or base.

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9a2 marks

Glycolic acid, C2H4O3, is an organic acid sometimes used to remove limescale, CaCO3, from electric kettles and coffee machines.

Predict, with a reason, a difference in the reaction between the same concentration of sulfuric acid and glycolic acid with samples of calcium carbonate.

9b3 marks

Another acid that is sometimes used to descale kettles is sulfamic acid, NH2SO3H. Sulfamic is classed as a strong monoprotic acid.

i)
Explain the meaning of the term strong monoprotic acid.
[1]
ii)

Calculate the pH of a 0.136 mol dm-3 solution of sulfamic acid and determine the concentration of hydroxide ions in the solution at 298 K.

[2]

9c2 marks

A solution of hydrochloric acid has a pH of 1 and a solution of carbonic acid has a pH of 5. Determine the ratio of hydrogen ion concentrations of hydrochloric acid to carbonic acid.

9d4 marks

Outline two ways, apart from using pH, which could allow you to distinguish between two solutions of carbonic acid and hydrochloric acid that have the same concentration.

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10a1 mark

Four solutions of acids with identical concentrations are prepared. The equilibrium constants of these acids are given in Table 1.

Table 1

Acid

Kc mol dm-3 at 298 K

HCN

4.9 x 10-10

HF

6.8 x 10-4

CH3COOH

1.7 x 10-5

HCl

1.3 x 106

Write down the acid dissociation equation for HCN.

10b2 marks

Use the information in part (a) to complete this question.

i)
Write down the list of acids in part (a) in order of decreasing pH.
[1]
ii)
Write down the list of acids in order of increasing concentration of molecules of the acid present in the solution.
[1]
10c2 marks

State the name and formula of all the chemical species present in the solution of CH3COOH.

10d1 mark

Write the name and formula of the conjugate base of HF.

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11a3 marks

Ethanoic acid, CH3COOH (aq) , is titrated with 0.16 mol dm-3 potassium hydroxide and the following graph is obtained.


q2a_18-1_ib_hl_medium_sq

Explain why the equivalence point is greater than pH 7 for this titration.

11b2 marks

Explain what is meant by a buffer solution and describe where the ‘buffer region’ on the graph would occur. 

11c4 marks

Explain the shape of the pH curve up to the equivalence point. 

11d3 marks

Explain why potassium hydroxide can act as a Brønsted-Lowry base and Lewis base.

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12a3 marks

This question is about Brønsted-Lowry acids and bases.

i)
Give the meaning of the term Brønsted-Lowry acid. 
[1]
ii)
Explain the term weak acid.
[2]
12b2 marks

When an acid and a base react they produce a conjugate base and a conjugate acid. 

acid + base  ⇋  conjugate base + conjugate acid 

Write an equation to show how hydrochloric acid behaves as a strong acid when it reacts with water, and state the role of water in this reaction.

12c4 marks

Ethanoic acid is a weak acid. Hydrogen carbonate ions can also act as a weak acid if in an aqueous solution.

i)
Write equations for each of these weak acids at equilibrium.
[2]
ii)
A solution was made up containing sodium hydrogen carbonate and sodium carbonate. Explain how this solution would act as a buffer if a small amount of acid was added to it. 
[2]
12d4 marks

Explain how a solution containing ethanoic acid and ethanoate ions can act as a buffer. 

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13a5 marks

A student performed a titration of 25.0 cm3 of 0.100 mol dm-3 hydrochloric acid, HCl (aq) , with 0.100 mol of sodium hydroxide, NaOH (aq).

i)
Draw the expected pH curve on the graph and indicate the equivalence point for this.
[2]

q4a_18-1_ib_hl_medium_sq 
ii)

Explain why the salt produced in this reaction is neutral.

[3]

13b5 marks

The student repeated the titration using two different chemicals, 25.0 cm3 of 0.100 mol dm-3 nitric acid, HNO3 (aq) , and 0.100 mol dm-3 ammonia, NH3 (aq).

i)
State the equation for this reaction. 
[1]
ii)
Explain why the salt produced in this reaction is acidic.
[4]
13c2 marks

State the equation for reaction between nitric acid and water, this reaction and identify the conjugate acid formed in the reaction. 

13d3 marks

The image below shows the hexaaquaaluminium ion, [Al(H2O)6]3+. Explain why this can behave as an acid.

q4d_18-1_ib_hl_medium_sq

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14a3 marks

25.0 cm3 of 0.100 mol dm-3 propanoic acid, CH3CH2COOH (aq) , is titrated with of 0.100 mol dm-3 sodium hydroxide. The pH curve for this titration is shown below.

q5a_18-1_ib_hl_medium_sq

 

i)
Label the equivalence point and half equivalence point on the curve.
[2]

ii)
Explain what is meant by the half equivalence point.
[1]
14b3 marks

The student used the pH range of the indicators to determine which was the best one to use for this titration. 

i)
Using Section 18 of the Data Booklet highlight on the graph the pH range of bromocresol green for this titration.
[1] 
ii)
Using Section 18 of the Data Booklet suggest a suitable choice of indicator for this titration and state the colour change you would expect to see. 
[2]
14c5 marks

The end point of an indicator depends on its pKa 

i)
Explain the connection between the pH range of an indicator that is a weak acid and the value of pKa for the indicator. 
[3]
ii)
Explain how the student can calculate the Ka of propanoic acid by using the pH curve.
[2]
14d1 mark

A buffer solution contains a mixture of propanoic acid and its salt. A small amount of nitric acid is added to the buffer.

Write an equation, including state symbols, showing how this buffer can resist the change in pH.

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15a2 marks

At 298K, water molecules dissociate into equal quantities of ions, and the pH is 7.

i)
Write an equation to show the dissociation of water.
[1]
ii)
At 313 K, the pH of water is 6.77. Explain why water is still neutral with a pH of 6.77.
[1]
15b5 marks

The ionic product of water, Kw, can be used to find the pH of a strong base. Changing the temperature will affect the value for Kw.

i)
Give the expression and units for the ionic product of water, Kw.
[2]
ii)
As temperature increases, the value for Kw also increases. Explain why.
[3]
15c3 marks

Determine the pH of pure water at 40 °.

Kw of pure water at 40 ° is 2.92 x 10-14 mol2 dm-3

15d3 marks

Strong bases fully ionise in water, as shown by the equation of dissociation of sodium hydroxide:

NaOH (aq) →  Na+ (aq) + OH- (aq)

At 298K, Kw is 1 x 10-14 mol2 dm-6.

Calculate the pH of a 0.05 mol dm-3  solution of NaOH at 298 K.      

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16a1 mark

Weak acids do not fully ionise in solution. The acid dissociation constant, Ka is used to determine the hydrogen ion concentration.

Write an expression for the acid dissociation constant, Ka for the acid HA.

16b3 marks

The pH of a 0.15 mol dm-3 solution of HCN is 5.08 at 298 K. Calculate the value of Ka for HCN at 298 K.

Give your answer to two decimal places.

16c4 marks

A sample of 0.01 mol dm-3 butanoic acid has a Ka value of 1.51 x 10-5 mol dm-3.

i)
Write an expression for the acid dissociation constant, Ka, for butanoic acid.
[1]
ii)
Calculate the pH of the 0.01 mol dm-3 butanoic acid. Give your answer to two decimal places.
[3]
16d4 marks

0.50 moles of ammonia was dissolved in water to make a 1.00 dm3 solution. This solution has a hydroxide ion concentration of 6.40 x 10-3 mol dm-3

i)
Write an expression for the base dissociation constant, Kb, of ammonia.
[1]
ii)
Calculate a value for pKb for ammonia.
[3]

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17a3 marks

The pH curve shown below was obtained when a 0.150 mol dm–3 solution of sodium hydroxide was added to 25.0 cm3 of an aqueous solution of ethanoic acid. The half equivalence point is where half of the volume of sodium hydroxide required for neutralisation has been added to the ethanoic acid.

i)
Label the graph with an X to show the position of the half equivalence point.
[1]

q3a_medium_calculations-involving-acids--bases_ib-dp-chem_hl

ii)
When half of the ethanoic acid solution has been neutralised, the remaining ethanoic acid concentration is equal to that of the sodium ethanoate that had formed. Calculate the pH at this point.

Ka of ethanoic acid = 1.75 x 10-5 mol dm-3.
[2]
17b6 marks

A different titration was performed using 0.100 mol dm-3 ammonia solution, NH3 (aq) (Kb = 4.75 at 298K) and 25.00 cm3 of 0.100 nitric acid, HNO3 (aq). 

Calculate the pH of the ammonia solution before it was added to the nitric acid.

17c2 marks

The titration is repeated using 0.200 mol dm-3 sodium hydroxide, NaOH (aq), instead of ammonia.

Determine whether the salt formed in this titration will be acidic, basic or neutral.

17d5 marks

Determine the pH of the solution if 150 cm3 of 0.30 mol dm-3 sodium hydroxide, NaOH (aq), is mixed with 200 cm3 0.10 mol dm-3 of nitric acid, HNO3 (aq).

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18a1 mark

Monochloroacetic acid, ClH2COOH, is a skin irritant that is used in “chemical peels” intended to remove the top layer of dead skin from the face and ultimately improve the complexion.

Write an expression for the acid dissociation constant, Ka, of monochloroacetic acid.

18b4 marks

Calculate the pH of a 0.05 M solution of monochloric acid. 

The value of Ka for monochloroacetic acid is 1.35 x 10-3 mol dm-3

18c2 marks

Using Section 2 of the Data Booklet, calculate the value of [OH-] for the solution of monochloric acid.

18d2 marks

Calculate the percentage dissociation for the solution of monochloric acid.

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19a2 marks

State the relationship between the following expressions for conjugate acid-base pair

i)
Ka and Kb
[1]
ii)
pKa and pKb
[1]
19b4 marks

Use Table 1 to calculate the following for the conjugate bases at 298 K.

Table 1

CH3CH2COOH pKa = 4.87
CH(Cl2)COOH pKa = 1.35
CH(CH3)2COOH pKa = 4.84

i)
pKb of CH3CH2COO-
[1]
ii)
Kb of CH(Cl2)COO-
[2]
iii)
Ka of (CH3)2CHCOOH
[1]
19c4 marks

A student performs a titration using a 0.10 mol dm-3 ammonia, NH3 (aq), and a hydrochloric acid and 0.10 hydrochloric acid, HCl (aq). q5c_medium_calculations-involving-acids--bases_ib-dp-chem_hl

Volume of HCl (cm3)

i)
State the equation for the overall reaction that is occurring.
[1]
ii)
Mark on the curve the point at which the pOH is equal to pKb of the weak base and deduce the pKb of the acid.
[3]
19d1 mark

The student repeats the titration with 0.10 mol dm-3 ethanoic acid, CH3COOH (aq) which has a pKa value of 4.76. A sketch of the pH curve obtained is shown below.

q5d_medium_calculations-involving-acids--bases_ib-dp-chem_hl

Explain why it is difficult to determine the equivalence point for this reaction accurately.

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1a2 marks

Explain why an ammonium ion can not behave as a Brønsted-Lowry base.

1b2 marks

State and explain the acid-base character of aqueous ammonia at 298 K.

1c2 marks

Acids can be classed as monoprotic, diprotic and triprotic. Sulfuric acid is a diprotic acid. 

i)
State the equation for the first ionisation step of sulfuric acid, including state symbols.
[1]
ii)
Label the conjugate acid and base pairs in your answer to part i).
[1]
1d1 mark

The second ionisation step is for the ionisation of sulfuric acid is as follows.

HSO4- (aq) + H2O (aq) ⇌ SO42- (aq) + H3O+ (aq)

Suggest why the second ionisation step reaches equilibrium.

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2a2 marks

Sodium hydrogen carbonate solution, NaHCO3 (aq) , can act as an amphiprotic species. State the equation for the reaction fo NaHCO3 (aq) with the following compounds:

i)
Sodium hydroxide solution. 
[1]
ii)
Hydrochloric acid. 
[1]

2b3 marks

Using your answer to part a) i) and ii), explain why NaHCOis amphiprotic. 

2c3 marks

Phosphine is usually prepared by heating white phosphorus, one of the allotropes of phosphorus, with concentrated aqueous sodium hydroxide.

The equation for the reaction is.

P4 (s) + 3OH (aq) + 3H2O (l) → PH3 (g) + 3H2PO2 (aq)

Identify the amphiprotic species in this reaction giving the formulas of both species it is converted to when it behaves in this manner.

2d4 marks

1.68 g of white phosphorus was used to make phosphine

i)
Calculate the amount, in mol, of white phosphorus used.
[1]
ii)
This phosphorus was reacted with 50.0 cm3 of 3.00 mol dm−3 aqueous sodium hydroxide. Deduce, showing your working, which was the limiting reagent.
[1]
iii)
Determine the excess amount, in mol, of the other reagent.
[1]
iv)
Using section 2 of the data booklet. Determine the volume of phosphine, measured in cm3 at standard temperature and pressure, that was produced.
[1]

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3a2 marks

Oxalic acid, H2C2O, is a weak diprotic acid and can be used in titrations. State the equation for the reaction of oxalic acid with sodium hydroxide. 

3b2 marks

The ionisation of oxalic acid occurs in two steps. State equations for both of these steps. 

3c3 marks

Tartaric acid shown below behaves as a Brønsted-Lowry acid when it reacts with calcium hydroxide, Ca(OH)2. Sketch the structure of the salt formed from this reaction. 

8-1-ib-sl-sq-hard-q3c-tartaric-acid

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4a4 marks

Using ionic equations state how HPO42- can behave as an amphiprotic and amphoteric species.

4b2 marks

Gallium oxide behaves as an amphoteric oxide. State two equations to show how gallium oxide reacts with a strong monoprotic acid and strong base.

Reaction with strong monoprotic acid .........................................................................................................................................

Reaction with strong base .........................................................................................................................................

4c1 mark

Identify the Br∅nsted-Lowry acids in the following reaction.

CH3CH2O- (aq) + H2O (l) rightwards harpoon over leftwards harpoonCH3CH2OH (aq) + OH- (aq)

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5a
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4 marks

A solution of hydrochloric acid of concentration 0.001 mol dm-3 has a pH value of 3. Suggest, giving a reason, the pH of the following solutions of acids:

i)
0.01 mol dm-3 hydrochloric acid
[2]

ii)
0.01 mol dm-3 ethanoic acid
[2]
5b
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1 mark

A solution of 0.01 mol dm-3 ethanoic acid has a concentration of hydrogen ion of 1 x 10-4 mol dm-3.  Determine the percentage of ethanoic acid molecules that have dissociated.

5c1 mark

Two separate titrations are carried out using 25.00 cm3 of 0.01 mol dm-3 solutions of hydrochloric acid followed by ethanoic acid, against 0.01 mol dm-3 sodium hydroxide. 

State what difference(s) would be observed in the two titrations.  

5d
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2 marks

Suggest a suitable indicator for the titration of hydrochloric acid and sodium hydroxide in part c), and state the colour changes observed.

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6a3 marks

Show how the ionic product for water is derived from the dissociation of water and give it units.

6b
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1 mark

Determine the pH of 0.001 mol dm-3 sodium hydroxide.

6c4 marks

Suggest, with a reason, how the magnitude of Kw changes with increasing temperature.

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7a1 mark

Malonic acid is a weak dibasic carboxylic acid with the formula C3H4O4. Draw the displayed structure of malonic acid.

7b2 marks

Suggest, with a reason, which of the two acids, ethanoic or malonic, has a higher pH?

7c1 mark

Apart from testing the pH, suggest how equimolar solutions of malonic acid and ethanoic acid may be distinguished.

7d2 marks

Write the formulas of two conjugate bases that can be formed from malonic acid.

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8a2 marks

Marble chips are added separately to solutions of the same concentration of ethanoic acid and hydrochloric acid. State one similarity and one difference you would expect to observe in the reactions.

8b1 mark

Write an equation for the reaction between marble chips and ethanoic acid.

8c
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3 marks

Determine the volume, in cm3, of 2.25 mol dm-3 ethanoic acid needed to completely react with 1.50 g of marble chips. 

8d
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3 marks

Determine the volume of CO2, in cm3, produced at 273 K and 101 kPa in part c).

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9a4 marks

Sketch the titration curve when 50 cm3 of 0.1 mol dm-3 HNO3 (aq) is titrated against 25 cm3 of 0.1 mol dm-3 NH3 (aq).

18-1-hl-sq-q2-q
9b1 mark

Select a suitable indicator for the titration from table 22 of the Data booklet.

9c
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5 marks

Calculate the pH of 0.1 mol dm-3 ammonia. 

pKb of ammonia = 4.75

9d1 mark

Deduce whether the pH of 0.1 mol dm-3 ethylamine would be higher or lower than 0.1 mol dm-3 ammonia solution.

(pKb of ethylamine is 3.35)

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10a1 mark

Indicators are solutions of weak acids or bases. Methyl red has the molecular formula C15H15N2O2.

q3

Draw the structure of the conjugate base of methyl red.

10b1 mark

What will be seen if a few drops of methyl red are added during a titration of 50 cm3 of 0.1 mol dm-3 HCl (aq) against 25 cm3 of 0.1 mol dm-3 NaOH ( aq). 

10c3 marks

The pKa of methyl red is 5.1. Explain how this relates to the acid-base character of methyl red when added to water.

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11a3 marks

Using Table 1, discuss the relationship between the chemical structures and acidity of chloroethanoic acid, dichloroethanoic acid and trichloroethanoic acid.

Table 1

Name of Acid Formula pKa
chloroethanoic acid CH2ClCOOH 2.87
dichloroethanoic acid CHCl2COOH 1.35
trichloroethanoic acid CHCl3COOH 0.66
11b4 marks

This question is about acid buffers.

i)
Explain how you could make a buffer given a supply of the following:
20 cm3 of 0.10 mol dm-3 chloroethanoic acid
20 cm3 of 0.10 mol dm-3 potassium hydroxide
[3]
ii)
Determine the new concentration of each reactant in the buffer.

[1]

11c
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3 marks

20 cm3 of 0.05 mol dm-3 dichloroethanoic acid was reacted with 10 cm3 of 0.10 mol dm-3 sodium hydroxide. Suggest, with a reason, a pH value for the resulting solution.

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12a
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1 mark

Determine the Ka of benzoic acid

pKa at 298 K = 4.2

12b1 mark

Using the Ka value for benzoic acid, state and explain its acidic character.

12c
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5 marks

Benzoic acid has a solubility of 0.344 g / 100 g water at 293 K. Determine the hydrogen ion concentration and pH of saturated benzoic acid solution at this temperature.

12d1 mark

What assumption is made in the calculation in part c)?

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13a3 marks

Nitric acid, HNO3, and hydrocyanic acid, HCN, can be made from ammonia. Hydrocyanic acid has a pKa of 9.21.

Formulate equations for the dissociation of each acid and distinguish between the terms strong and weak in this context.

13b
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2 marks

Write an expression for the acid dissociation constant, Ka, of hydrocyanic acid and calculate the Ka at 298 K.

13c
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2 marks

Determine the hydrogen ion concentration and pH of 0.15 mol dm-3 hydrocyanic acid.

13d
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2 marks

Write an expression to show the ionisation of the conjugate base of hydrocyanic acid and calculate it Kb value.

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14a3 marks

Calculate the pH of a solution made by mixing 50.0 cm3 of 0.200 mol dm-3 HCl (aq) with 50.0 cm3 of 0.100 mol dm-3 NH3 (aq)

14b
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5 marks

A 0.100 mol dm-3 solution of NH3 (aq) contains 1.28 x 10-3 mol dm-3 in hydroxide ion.

i)
Determine the pH of the solution.

[3]

ii)
Comment on the relative base strength of 0.100 mol dm-3 NaOH (aq) compared to 0.100 mol dm-3 NH3 (aq)

[2]

14c3 marks

Determine the base dissociation constant, Kb for ammonia using the information in part b).

14d
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4 marks

The pH of pure water is 6.92 at 328 K and Kb for NH3 (aq) at this temperature is 1.80 x10-5.

Determine the pKa of [NH4+] at this temperature.

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