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A LevelChemistryCIEExam Questions25. Equilibria (AL)25.1 Acids & Bases

Acids & Bases (CIE A Level Chemistry)

Exam Questions

2 hours8 questions
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1a3 marks

Acidic buffer solutions are made of a weak acid and its salt. pH calculations of buffer solutions can be conducted using the same equation used for weak acids:

Ka = fraction numerator left square bracket straight H to the power of plus right square bracket left square bracket straight A to the power of minus right square bracket over denominator left square bracket HA right square bracket end fraction

State the assumptions made when using this equation for buffer solutions.

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1b1 mark

Suggest a salt that could be used to form a buffer solution with propanoic acid, CH3CH2COOH.

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1c4 marks

A buffer solution contains a 0.15 mol dm-3 concentration of propanoic acid and a propanoate salt with a concentration of [A–] = 0.30 mol dm-3.

At 298 K the value of Ka for propanoic acid is 1.35 × 10–5 mol dm–3.  

i)
Calculate the concentration of [H+] in this buffer solution at 298 K. Give your answer to 2 decimal places. Show your working.
 
[H+] = ...................... mol dm-3
[3]
 
ii)
Calculate the pH of the buffer solution to 2 decimal places.
 
pH = .....................
[1]

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1d2 marks

Buffer solutions can be tailored to a specific pH by 'coarse-tuning' by changing the acid used, and 'fine-tuning' by altering the ratio of [HA] : [A–].

Predict and explain the effect on the pH of the buffer solution by increasing the concentration of the salt only.

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2a2 marks

Two acids used in the same laboratory, which both have the same concentration, 0.1 mol dm-3, can have significantly different pH values. Explain why.

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2b
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Calculate the pH of a solution of 0.1 mol dm-3 HCl.

 Show your working


pH = .........................

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2c
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A sample of 0.1 mol dm-3 methanoic acid has a Ka value of 1.78 x 10-4 mol dm-3 at 298 K.

i)
Write an expression for the acid dissociation constant, Ka, for methanoic acid.
 
         Ka = 
[1]
 
ii)
Calculate the pH of the 0.1 mol dm-3 methanoic acid. Give your answer to two decimal places and show your working.
  
 
pH = ....................
[3]

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1a
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The pH of a 0.150 mol dm–3 solution of piperidine is 11.9.

20.0 cm3 of 0.200 mol dm–3 HCl was slowly added to a 20.0 cm3 sample of 0.150 mol dm–3 piperidine. The pH was measured throughout the addition. One mole of hydrochloric acid reacts one mole of piperidine.

i)
Calculate the number of moles of HCl remaining at the end of the addition.




moles of HCl = .....................
[2]
ii)
Hence calculate the [H+] and the pH at the end of the addition.




pH = .....................
[1]

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1b
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On the following axes shown in Fig. 3.1, sketch how the pH will change during the addition of a total of 20.0 cm3 of 0.200 mol dm–3 HCl.

5-5-3b-m-ph-curve---addition-of-hcl-a
Fig. 3.1

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1c5 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]

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1d2 marks

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

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2a1 mark

This question is about buffers.

Give the meaning of the term buffer solution.

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2b2 marks

Buffers can be acidic but they can also be basic. This is determined by the reactants which are combined to create the buffer

i)
Describe how an acidic buffer is made.

ii)
State the key feature which makes a buffer acidic.

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2c4 marks

Ethanoic acid is a weak acid. Hydrogen carbonate ions act as a weak acid if they are 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]

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2d2 marks

Explain how the solution of ethanoic acid works as a buffer when small amounts of alkali are added.

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2e
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A buffer solution contains 0.10 mol dm-3 concentration of methanoic acid and a 0.20 mol dm-3 concentration of sodium methanoate. The values of Ka for methanoic acid is 1.80 × 10–5 mol dm–3. 


Calculate the pH of this buffer solution at 298 K. Give your answer to 2 decimal places and show your working. 



pH = .......................

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3a1 mark

Calcium phosphate is naturally present in the body and can be taken as a supplement to treat calcium deficiencies.

Write an expression for the Ksp of Ca3(PO4)2.

Ksp = 

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3b
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At 298 K, the solubility of Ca3(PO4)2 is 2.50 x 10–6 mol dm–3

Calculate the solubility product for Ca3(PO4)2 at this temperature. Include the units.

Ksp = .........................

units = .......................

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3c1 mark

Explain why the lattice energy of calcium phosphate is more exothermic than strontium phosphate.

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1a2 marks

3-dodecylbenzenesulfonic acid can be prepared from benzenesulfonic acid.

benzenesulfonic-acid

Suggest the reagents and conditions and name the mechanism for this reaction.

reagents and conditions ..................................................................................................

mechanism ............................................................................................

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1b2 marks

When concentrated sulfuric acid is added to water, dissociation takes place in two stages.

   stage 1 H2SO4 rightwards harpoon over leftwards harpoon H+ + HSO4–

   stage 2 HSO4– rightwards harpoon over leftwards harpoon H+ + SO42–     Ka2 = 1.0 × 10–2 mol dm–3

Ka2 is the acid dissociation constant for stage 2.

i)
Write the expression for the acid dissociation constant Ka2.
 
Ka2 =
 
[1]
 
ii)
H2SO4 is considered a strong acid whereas HSO4– is considered a weak acid.
 
Suggest how the magnitude of the acid dissociation constant for stage 1 compares to Ka2.
 
[1]

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1c
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Benzoic acid, C6H5CO2H, is a weak acid.

The value of the acid dissociation constant, Ka, of benzoic acid is 6.3 x 10-5 mol dm-3 at 298K.

Calculate the pH of the 0.50 g dissolved in 250 cm3 of water.

Give your answer to two decimal places and show your working.





pH = .....................

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1d
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Calculate the pH of the solution formed when 0.060 g of sodium hydroxide is added to 500.00 cm3 of 0.005 mol dm-3 of benzoic acid. Give your answer to two decimal places. Show your working.




pH = ................. 

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2a5 marks

One atom of each of the four elements H, C, N and O can bond together in different ways.

Two examples are molecules of cyanic acid, HOCN, and isocyanic acid, HNCO. The atoms are bonded in the order they are written.

i)
Draw ‘dot-and-cross’ diagrams of these two acids, showing outer shell electrons only.
 







HOCN, cyanic acid







HNCO, isocyanic acid
 
[3]
 
ii)
Suggest the values of the bond angles HNC and NCO in isocyanic acid.
 
HNC ..............................
 
NCO ..............................
 
[1]
 
iii)
Suggest which acid, cyanic or isocyanic, will have the shorter C–N bond length. Explain your answer.
 
[1]

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2b3 marks

Isocyanic acid is a weak acid.

HNCO rightwards harpoon over leftwards harpoon H+ + NCO–    Ka = 1.2 × 10–4 mol dm–3

i)
Calculate the pH of a 0.10 mol dm–3 solution of isocyanic acid and show your working.
 
 
 
pH = ..............................
[2]
 

Sodium cyanate, NaNCO, is used in the production of isocyanic acid. Sodium cyanate is prepared commercially by reacting urea, (NH2)2CO, with sodium carbonate. Other products in this reaction include ammonia and steam.

ii)
Write an equation for the production of NaNCO by this method.
 
[1]

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2c
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Barium hydroxide, Ba(OH)2, is completely ionised in aqueous solutions. During the addition of 30.0 cm3 of 0.100 mol dm–3 Ba(OH)2 to 20.0 cm3 of 0.100 mol dm–3 isocyanic acid, the pH was measured.

i)
Calculate the [OH–] at the end of the addition.
 
[OH–] = .............................. mol dm–3 
[2]
 
ii)
Use your value in (i) to calculate [H+] and the pH of the solution at the end of the addition.
 
final [H+] = .............................. mol dm–3
 
final pH = ..............................
[2]
 
iii)
On the following axes in Fig. 2.1, sketch how the pH changes during the addition of a total of 30.0 cm3 of 0.100 mol dm–3 Ba(OH)2 to 20.0 cm3 of 0.100 mol dm–3 isocyanic acid.
 ph-axes
 
Fig. 2.1
 
[3]

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3a
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This question is about buffer solutions

Calculate the pH of a buffer solution made by dissolving 0.275 g of benzoic acid (Ka  = 6.3 x 10-5 mol dm-3) and 0.525 g of sodium benzoate in 250 cm3 of water. Give your answer to 2 decimal places and show your working.





pH = .................

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3b
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Calculate the pH of a buffer solution made by mixing together 130 cm3 of 0.2 mol dm-3 ethanoic acid (Ka = 1.74 x 10-5 mol dm-3) and 85 cm3 of 0.45 mol dm-3 sodium ethanoate. Give your answer to 2 decimal places and show your working.

 






pH = .....................

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3c
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Calculate the new pH of the buffer solution given in part (b) if 1.00 cm3 of 1.00 mol dm-3 sodium hydroxide is added. Give your answer to two decimal places and show your working. 

Comment on the pH value after the addition of sodium hydroxide.

pH = ...................

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3d2 marks

Write two equations for the separate addition of a small volume of base and acid to the buffer solution given in part (b).

 

equation 1 ....................................................................................

 

equation 2 .....................................................................................

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