Further Acids & Bases Calculations (AQA A Level Chemistry)

Before an acid-base titration can be carried out, to obtain a pH curve, it is important that the pH probe is calibrated.

Describe the steps involved in calibrating a pH probe.

A student titrated 0.10 mol dm^-3 acid into a conical flask containing 25.0 cm³ of 0.1 mol dm^-3 of a base, recording the pH with each addition of acid.

The student repeated the procedure using different combinations of acids and bases.

Figure 1

Identify which curve shown in Figure 1 above, is the acid-base combination for the following:

i) Ammonia and ethanoic acid

ii) Ammonia and nitric acid

iii) Sodium hydroxide and propanoic acid

Identify which indicator given in Table 1 would be most suitable for curve Y.

Explain your decision.

Table 1

A student was provided with the following equipment:

• Calibrated pH probe

• Conical flask containing 25.00 cm³ of 0.100 mol dm^-3 ammonia solution

• 50.00 cm³ burette filled with hydrochloric acid

Describe how the student would carry out a titration to obtain a pH curve.

This question is about buffers.

Give the meaning of the term buffer solution.

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.

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.

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. 

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

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 K_a 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.

Propanoic acid has an acid dissociation constant value, K_a, of 1.34 x 10^-5 mol dm⁻³ at 298K.

A buffer solution containing propanoic acid and sodium propanoate was prepared.

The concentration of the propanoate ions in the buffer solution was 0.177 mol dm⁻³.  

The pH of the buffer solution was 4.01.

i) Give the expression for the acid dissociation constant K_a for propanoic acid.

ii) Calculate the concentration of the propanoic acid in the buffer solution. Give your answer to two decimal places.

A vegetable extract can be used as an indicator in an acid-base titration. At pH 4 the extract is yellow and at pH 9 it is blue.

Suggest a type of an acid-base titration this could be used in. You do not need to name specific acids and bases.

Explain your choice.

A buffer solution was prepared with a pH of 8.0. A couple of drops of each indicator given in Table 1 were added to the buffer solution. The indicators do not react with each other.

State the colour that would be seen in the mixture of indicators and buffer solution.

Table 1

A student carried out a titration using hydrochloric acid and potassium hydroxide by adding small amounts of potassium hydroxide to 25.0 cm³ of 0.2 mol dm^-3 hydrochloric acid at 298K. The student plotted the results of the titration as a graph, see Figure 1.

Use Figure 1 to determine the volume of KOH added at the equivalence point of the reaction.

Figure 1

Suggest a suitable indicator for the reaction in Figure 1.

State the colour change which would occur during the reaction.

The student repeated the experiment, using 20 cm³ of 0.25 mol dm³ potassium hydroxide and 0.25 mol dm³ butanoic acid.

Butanoic acid is a weak acid. State the meaning of the term weak acid.

The student plotted a graph of the results of the reaction between butanoic acid and potassium hydroxide. The results are shown in Figure 2.

Use Figure 2 to determine the value of K_a for butanoic acid at 298K.

State the units for K_a.

Figure 2

This question is about buffers.

i) Explain the term basic buffer solution.

ii) Suggest why buffers are used in the cosmetics industry.

When we exercise, chemical changes occur which can cause our blood pH to drop. If the pH drops below 7.4 a condition called acidosis occurs. This can be very dangerous as many chemical reactions involve proteins which work at pH 7.4. The most important buffer for maintaining acid-base balance in the blood is the carbonic acid, H₂CO₃, hydrogencarbonate, HCO₃^- buffer.

i) Write a balanced equation to show the dissociation of carbonic acid, acting as a buffer solution. 

ii) Explain how the carbonic acid-hydrogencarbonate buffer maintains blood pH at 7.4 when we exercise. Use your answer to part (i).

Much of the atmospheric carbon dioxide we produce is absorbed by dissolving into the oceans, forming carbonic acid. An equilibrium is set up within the oceans using the same buffer solution as in the body, described in part (b).

In this buffer solution there is an almost unlimited amount of hydrogen carbonate ions and the carbonic acid available has a high concentration.

Using your answer to (b) part (i), suggest the changes which would lead to more CO₂ dissolving into the oceans. Explain your answer.

When we exercise, lactic acid CH₃CH(OH)COOH, may be produced by anaerobic exercise, if there is insufficient oxygen reaching the muscles.

The concentration of lactic acid is approximately 0.01 mol dm^-3 in the body directly after exercise.

The dissociation constant, K_a for lactic acid is 1.288 x 10^-4 mol dm^-3.

Give the expression for K_a and calculate the pH of lactic acid.

0.3825 g of an organic monobasic aromatic carboxylic acid containing only carbon, hydrogen and oxygen were dissolved in ethanol. A few drops of indicator were added to the mixture titrated with 0.100 mol dm^-3 sodium hydroxide.

It took 25.50 cm³ of alkali to obtain the first permanent colour change.

Suggest a structure for the acid.

Suggest a suitable indicator for the procedure outlined in part (a) and explain your answer.

The value of the acid dissociation constant, K_a, 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 2 decimal places.

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 2 decimal places.

Explain, using equations, how chemicals in the blood resist changes in pH when hydroxide ions enter the bloodstream.

A buffer solution contains a mixture of ethanoic 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.

Determine the pH of the buffer solution that is 0.050 mol dm^-3 of ethanoic acid and 0.050 dm^-3 of sodium ethanoate. You must show working and give your answer to 2 decimal places.

(The pK_a of ethanoic acid is 4.77)

Calculate the concentration of methanoic acid used in a buffer which has a pH of 3.45 and a methanoate salt concentration of 0.149 mol dm^-3. Give your answer to 3 significant figures.

(K_a of methanoic acid = 1.8 x 10^-4 mol dm^-3)

A 0.25 mol dm^-3 of sodium hydroxide solution, NaOH (aq), was added from a burette during a titration to 25.00 cm³ of a 0.15 mol dm^-3 solution of propanoic acid.

Calculate the pH of the solution after 10.00 cm³ of NaOH (aq) was added. Give your answer to 2 decimal places.

(K_a of propanoic acid = 1.34 x 10^-5 mol dm^-3)

Calculate the pH of the solution after 30.00 cm³ of NaOH (aq) was added.

Sorbic acid is used in food preservatives as a substitute for benzoic acid. It is highly effective in inhibiting the growth of mould which can spoil food and spread fatal diseases. 

The structure of sorbic acid is shown in Figure 1.

Figure 1

(K_a of sorbic acid = 1.7 x 10^-5 mol dm^-3)

25.00 cm³ of 0.020 mol dm^-3 of sorbic acid is titrated with 0.040 mol dm^-3 potassium hydroxide.

i) Give the IUPAC name of sorbic acid.

ii) Calculate the pH of the sorbic acid used in the titration.

During the titration the potassium hydroxide solution is added to sorbic acid. 

i) Calculate the volume in cm³ of potassium hydroxide present at the half equivalence point.

ii) Identity of the major species present at the equivalence point and whether the pH should be greater than, less than, or equal to 7.

What is the pH of a buffer solution made by dissolving 0.250 g of benzoic acid and 0.475 g of sodium benzoate in 250 cm³ of water? Give your answer to 2 decimal places.

(K_a of benzoic acid = 6.3 x 10^-5 mol dm^-3)

Calculate the pH of a buffer solution made by mixing together 120 cm³ of 0.15 mol dm^-3 ethanoic acid and 75 cm³ of 0.55 mol dm^-3 sodium ethanoate. Give your answer to 2 decimal places.

(K_a of ethanoic acid = 1.74 x 10^-5 mol dm^-3)

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

Buffers are solutions that can resist changes in pH if a small volume of acid or base is added.

i) Write an equation to show the action of the buffer given in part (b) if a small volume of acid was added.

ii) Write an equation to show the action of the buffer given in part (b) if a small volume of base was added.

Figure 1 shows a sketch of a graph pH curve when ammonia, NH₃ (aq), is added to a hydrochloric acid, HCl (aq).

38.65 cm³ of NH₃ (aq) was required to neutralise 20.00 cm³ of 0.5 mol dm^-3 HCl (aq)

Figure 1

i) Write two equations to show how hydrochloric acid and ammonia can act as a Brønsted-Lowry acid and base respectively.

ii) Suggest a value for the pH at point 2 on the curve and explain your answer.

iii) Calculate the pH of the solution at point 1. Give your answer to 2 decimal places.

In order to find the K_a of a weak acid, a student carried out a titration. The method was carried out as follows:

Step 1 Transfer 15.00 cm³ of a weak acid to a conical flask.

Step 2 Fill the burette with 0.1 mol dm^-3 sodium hydroxide.

Step 3 Add 1 cm³ of sodium hydroxide from the burette into the beaker and take the pH reading using a pH probe

Step 4 Continue until the pH no longer changes

Step 4 Plot a graph of pH versus volume of base added

The student’s graph is shown below in Figure 2.

The student suggests that the pH value measured is the K_a of the acid. Is the student correct? Explain your answer.

Figure 2

Using Figure 2, calculate the value for K_a of the acid. Give your answer to 3 significant figures and include the units for K_a. 

Figure 3 shows the change in pH as hydrochloric acid is added to sodium carbonate.

Explain using equations the two reactions in this double end point titration.

Figure 3