Edexcel A-Level Chemistry: Acids, Bases & Buffers

An acidic buffer solution is prepared by mixing methanoic acid (HCOOH) with sodium methanoate (HCOONa).

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

• state the two components of the buffer and write the equilibrium that is set up;
• explain, with an equation, what happens to added H⁺ ions;
• explain, with an equation, what happens to added OH⁻ ions.

(6 marks)

A buffer solution is prepared by adding 30.0 cm³ of 0.150 mol dm⁻³ sodium hydroxide (NaOH) to 50.0 cm³ of 0.200 mol dm⁻³ butanoic acid (CH₃CH₂CH₂COOH) at 298 K.

For butanoic acid, $K_a = 1.51 \times 10^{-5}\ \text{mol dm}^{-3}$Ka​=1.51×10−5 mol dm−3.

Calculate the pH of the buffer solution that is formed. Show your working and give your answer to 2 decimal places. (6 marks)

A solution of nitric acid (HNO₃), a strong monoprotic acid, has a concentration of 0.250 mol dm⁻³ at 298 K.

A student measures out 25.0 cm³ of this solution and dilutes it with distilled water to a total volume of 250.0 cm³.

(a) Calculate the pH of the original 0.250 mol dm⁻³ nitric acid. Give your answer to 2 decimal places. (2 marks)

(b) Calculate the pH of the diluted solution, and hence state the change in pH caused by the dilution. Give pH values to 2 decimal places. (3 marks)

A technician titrates 25.0 cm³ of 0.100 mol dm⁻³ aqueous propylamine (CH₃CH₂CH₂NH₂), a weak base, against 0.100 mol dm⁻³ hydrochloric acid. The pH is followed throughout. The equivalence point occurs after 25.0 cm³ of acid has been added, where the pH is 5.40; the steep, near-vertical section of the curve runs from about pH 7.2 down to pH 3.5.

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

Indicator	pH range of colour change
Thymol blue (acid range)	1.2 – 2.8
Methyl red	4.2 – 6.3
Phenol red	6.6 – 8.0
Thymolphthalein	9.3 – 10.5

(a) Explain why the equivalence-point pH is below 7 for this titration. (2 marks)

(b) Choose the most suitable indicator from the table and justify your choice, explaining why one of the rejected indicators is unsuitable. (3 marks)

Blood plasma is kept within the narrow range pH 7.35 – 7.45 by the carbonic-acid / hydrogencarbonate buffer system:

$\text{H}_2\text{CO}_3\text{(aq)} \rightleftharpoons \text{H}^{+}\text{(aq)} + \text{HCO}_3^{-}\text{(aq)}$H2​CO3​(aq)⇌H+(aq)+HCO3−​(aq)

During vigorous exercise, muscle cells release lactic acid (a source of H⁺ ions) into the blood.

Explain how this buffer system limits the fall in blood pH when this extra acid enters the blood. Refer to the equilibrium above in your answer. (4 marks)

Chloroethanoic acid (ClCH₂COOH) is a weak monoprotic acid.

(a) State what is meant by a weak acid. (1 mark)

(b) In a particular solution of chloroethanoic acid at 298 K, the hydrogen-ion concentration is $[\text{H}^{+}] = 2.7 \times 10^{-3}\ \text{mol dm}^{-3}$[H+]=2.7×10−3 mol dm−3. Calculate the pH of this solution. Give your answer to 2 decimal places. (2 marks)