Practice · AQA 7405 · Physical chemistry: acids and bases
A-level Chemistry pH calculations with worked answers
Five exam-style pH questions on strong and weak acids, marked with the working.
3 marksCore chemistry3.1.12.2
pH = −log₁₀[H⁺]. Hydrochloric acid is a strong acid.
(a)Calculate the pH of 0.0250
moldm−3 hydrochloric acid. Give your answer to 2 decimal places.
(b)The acid is diluted by a factor of 10 with water. Calculate the new pH.
[1 mark] (c)Calculate
[H+] in a solution of pH 2.70.
3 marksUnfamiliar context3.1.12.2
A student mixes 20.0
cm3 of 0.100
moldm−3 hydrochloric acid with 30.0
cm3 of 0.0500
moldm−3 nitric acid. Both are strong acids.
Calculate the pH of the mixture. Give your answer to 2 decimal places.
[3 marks]4 marksCore chemistry3.1.12.4
Ethanoic acid is a weak acid with
Ka=1.74×10−5 moldm−3 at 298 K.
(a)Write an expression for
Ka for ethanoic acid.
(b)Calculate the pH of 0.100
moldm−3 ethanoic acid. Give your answer to 2 decimal places.
3 marksCore chemistry3.1.12.4
A 0.0500
moldm−3 solution of a weak monoprotic acid HA has pH 3.10.
(a)Calculate
Ka for the acid.
(b)Calculate
pKa for the acid.
5 marksUnfamiliar context3.1.12.4
Propanoic acid has
Ka=1.35×10−5 moldm−3. A student calculates the pH of a 0.0100
moldm−3 solution using the usual weak-acid approximations.
(a)Calculate the pH. Give your answer to 2 decimal places.
[2 marks] (b)State the two approximations used and evaluate whether they are reasonable here.
[3 marks] Questions people ask
How do I find the pH of a strong acid?
A strong monoprotic acid fully dissociates, so [H⁺] equals the acid concentration. Then pH = −log₁₀[H⁺].
How do I find the pH of a weak acid?
Use Ka = [H⁺]² / [HA], assuming the acid barely dissociates so [HA] stays at its starting value. Then [H⁺] = √(Ka × [HA]) and pH = −log₁₀[H⁺].
How do I find the pH of a strong base?
Find [OH⁻], then use Kw = [H⁺][OH⁻] to get [H⁺]. At 298 K, Kw = 1.00 × 10⁻¹⁴ mol² dm⁻⁶.
