Acids & Bases | CIE A Level Chemistry Exam Questions 2025

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:

K_a = $\frac{[H^{+}] [A^{-}]}{[HA]}$

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 K_a 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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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 $⇌$ H⁺ + NCO^– K_a = 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

7 marks

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.

Fig. 2.1

[3]

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Acids & Bases (CIE A Level Chemistry)

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