AQA A-Level Chemistry: Acids, Bases & Buffers

An acidic buffer solution can be made by mixing ethanoic acid (CH₃COOH) with sodium ethanoate (CH₃COONa).

Describe and explain how this buffer solution resists a change in pH when a small amount of acid (H⁺) or alkali (OH⁻) is added to it. In your answer you should:

• state what the buffer is made from and write the equilibrium present;
• explain what happens to the added H⁺ ions;
• explain what happens to the added OH⁻ ions.

(6 marks)

Two acids are each made up to a concentration of 0.0500 mol dm⁻³ at 298 K.

• Acid P is hydrochloric acid (HCl), a strong monoprotic acid.
• Acid Q is propanoic acid (CH₃CH₂COOH), a weak monoprotic acid with $K_a = 1.35 \times 10^{-5}\ \text{mol dm}^{-3}$Ka​=1.35×10−5 mol dm−3.

(a) Calculate the pH of acid P. Give your answer to 2 decimal places. (2 marks)

(b) Calculate the pH of acid Q. State any approximations you make and show that they are justified. Give your answer to 2 decimal places. (3 marks)

(c) Acids P and Q have the same concentration. Explain why acid Q has the higher pH. (1 mark)

A buffer solution is prepared by dissolving 0.0500 mol of ethanoic acid (CH₃COOH) and 0.0250 mol of sodium ethanoate (CH₃COONa) in water and making the total volume up to 500 cm³ at 298 K.

For ethanoic acid, $K_a = 1.74 \times 10^{-5}\ \text{mol dm}^{-3}$Ka​=1.74×10−5 mol dm−3.

Calculate the pH of this buffer solution. Show your working and give your answer to 2 decimal places. (5 marks)

A student placed 25.0 cm³ of an acid HA of unknown strength in a flask and titrated it with 0.100 mol dm⁻³ sodium hydroxide (NaOH), recording the pH after each addition with a pH meter. The results are shown below.

Volume of NaOH added / cm³	pH
0.0	2.93
5.0	4.27
12.5	4.87
20.0	5.47
24.0	6.25
25.0	8.78
26.0	11.29
30.0	11.96
40.0	12.36

The colour-change ranges of three indicators are given below.

Indicator	pH range of colour change
Methyl orange	3.1 – 4.4
Bromothymol blue	6.0 – 7.6
Phenolphthalein	8.2 – 10.0

(a) State, with a reason from the data, whether HA is a strong acid or a weak acid. (2 marks)

(b) Give the volume of NaOH and the pH at the equivalence point. (1 mark)

(c) Choose the most suitable indicator for this titration and justify your choice. (2 marks)

A solution of sodium hydroxide (NaOH) has a concentration of 0.0400 mol dm⁻³ at 298 K. Sodium hydroxide is a strong base and is fully dissociated.

At 298 K, $K_w = [\text{H}^{+}][\text{OH}^{-}] = 1.0 \times 10^{-14}\ \text{mol}^2\,\text{dm}^{-6}$Kw​=[H+][OH−]=1.0×10−14 mol2dm−6.

Calculate the pH of this sodium hydroxide solution. Show your working and give your answer to 2 decimal places. (4 marks)

Ammonia reacts reversibly with water in the following equilibrium:

$\text{NH}_3\text{(aq)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{NH}_4^{+}\text{(aq)} + \text{OH}^{-}\text{(aq)}$NH3​(aq)+H2​O(l)⇌NH4+​(aq)+OH−(aq)

(a) Define, in terms of protons, a Brønsted-Lowry acid and a Brønsted-Lowry base. (2 marks)

(b) Identify a conjugate acid-base pair in the equilibrium above. (1 mark)