Acids & Bases | CIE A Level Chemistry Structured 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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1a2 marks

3-dodecylbenzenesulfonic acid can be prepared from benzenesulfonic acid.

benzenesulfonic-acid

Suggest the reagents and conditions and name the mechanism for this reaction.

reagents and conditions ..................................................................................................

mechanism ............................................................................................

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1b2 marks

When concentrated sulfuric acid is added to water, dissociation takes place in two stages.

stage 1 H₂SO₄ $⇌$ H⁺ + HSO₄^–

stage 2 HSO₄^– $⇌$ H⁺ + SO₄^2– K_a2 = 1.0 × 10^–2 mol dm^–3

K_a2 is the acid dissociation constant for stage 2.

i)

Write the expression for the acid dissociation constant K_a2.

K_a2 =

[1]

ii)

H₂SO₄ is considered a strong acid whereas HSO₄^– is considered a weak acid.

Suggest how the magnitude of the acid dissociation constant for stage 1 compares to K_a2.

[1]

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1c

4 marks

Benzoic acid, C₆H₅CO₂H, is a weak acid.

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 two decimal places and show your working.

pH = .....................

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1d

5 marks

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 two decimal places. Show your working.

pH = .................

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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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1a

3 marks

The pH of a 0.150 mol dm^–3 solution of piperidine is 11.9.

20.0 cm³ of 0.200 mol dm^–3 HCl was slowly added to a 20.0 cm³ sample of 0.150 mol dm^–3 piperidine. The pH was measured throughout the addition. One mole of hydrochloric acid reacts one mole of piperidine.

i)

Calculate the number of moles of HCl remaining at the end of the addition.

moles of HCl = .....................

[2]

ii)

Hence calculate the [H⁺] and the pH at the end of the addition.

pH = .....................

[1]

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1b

3 marks

On the following axes shown in Fig. 3.1, sketch how the pH will change during the addition of a total of 20.0 cm³ of 0.200 mol dm^–3 HCl.

Fig. 3.1

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1c5 marks

The student repeated the titration using two different chemicals, 25.0 cm³ of 0.100 mol dm^-3nitric acid, HNO₃ (aq), and 0.100 mol dm^-3ammonia, NH₃ (aq).

i)

State the equation for this reaction.

[1]

ii)

Explain why the salt produced in this reaction is acidic.

[4]

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1d2 marks

State the equation for the reaction between nitric acid and water and identify the conjugate acid formed in the reaction.

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Acids & Bases (CIE A Level Chemistry)

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