Acids & Bases (Cambridge (CIE) A Level Chemistry): Exam Questions

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 with one mole of piperidine.

i) Calculate the number of moles of HCl remaining at the end of the addition. Show your working.

[2]

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

[2]

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

25.0 cm³ of 0.100 mol dm^-3 nitric acid, HNO₃ (aq), is added to 0.100 mol dm^-3 ammonia, NH₃ (aq).

i) Construct an equation for this reaction.

[1]

ii) Explain why the salt produced in this reaction is acidic.

[2]

i) Construct an equation for the reaction between nitric acid and water.

[1]

ii) Identify the conjugate acid formed in this reaction.

[1]

This question is about buffers.

Define the term buffer solution.

Buffers can be acidic or basic, depending on the substances mixed to create them.

i) State the two types of substance that must be mixed to prepare an acidic buffer solution.

[1]

ii) State the key feature which makes a buffer acidic.

[1]

Ethanoic acid is a weak acid. Hydrogen carbonate ions act as a weak acid in aqueous solution.

i) Construct equilibrium equations for each of these weak acids.

[2]

ii) A solution was made up containing sodium hydrogen carbonate and sodium carbonate. Explain how this solution would act as a buffer if a small amount of acid was added to it.

[2]

Explain how a buffer solution made from ethanoic acid and sodium ethanoate works when small amounts of alkali are added.

A buffer solution contains 0.10 mol dm^-3 methanoic acid and 0.20 mol dm^-3 sodium methanoate. The value of K_a for methanoic acid is 1.80 × 10^-5 mol dm^-3.

Calculate the pH of this buffer solution at 298 K. Give your answer to 2 decimal places and show your working.

Write an expression for the solubility product, K_sp, of Ca₃(PO₄)₂.

At 298 K, the solubility of Ca₃(PO₄)₂ is 2.50 x 10^-6 mol dm^-3.

Calculate the solubility product, K_sp, for Ca₃(PO₄)₂ at this temperature. State the units. Show your working.

Explain why the lattice energy of calcium phosphate is more exothermic than that of strontium phosphate.

i) Define the term pH.

[1]

ii) Calculate the pH of 0.025 mol dm^-3 Ba(OH)₂ (aq) at 298 K. Show your working.

[3]

A buffer solution has 0.40 mol dm^-3 CH₃COOH and 0.60 mol dm^-3 CH₃COONa.

i) Calculate the pH of the buffer (K_a = 1.75 x 10^-5). Show your working.

[3]

ii) Construct an equation and explain how the buffer resists pH change when NaOH is added.

[2]

Silver sulfate, Ag₂SO₄, is a sparingly soluble salt.

i) Define the term solubility product, K_sp.

[2]

ii) The K_sp for Ag₂SO₄ is 1.20 x 10^-5 mol³ dm^-9.

Calculate the solubility of Ag₂SO₄ in water in mol dm^-3. Show your working.

[3]

i) Predict whether the solubility of Ag₂SO₄ in 0.10 mol dm^-3 Na₂SO₄ (aq) is greater than, less than, or equal to its solubility in pure water.

[1]

ii) Explain your answer to (d)(i).

[2]

3-dodecylbenzenesulfonic acid can be prepared from benzenesulfonic acid.

Identify the reagents and conditions, and state the name of the mechanism, for this reaction.

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 an expression for the acid dissociation constant 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]

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 solution formed when 0.50 g of benzoic acid is dissolved in 250 cm³ of water.

Give your answer to two decimal places and show your working.

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 benzoic acid.

Give your answer to two decimal places. Show your working.

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 HOCN (cyanic acid) and HNCO (isocyanic acid), showing outer shell electrons only.

[3]

ii) Suggest the values of the bond angles HNC and NCO in isocyanic acid.

[1]

iii) Suggest which acid, cyanic or isocyanic, will have the shorter C–N bond length. Explain your answer.

[1]

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. Show your working.

[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) Construct an equation for the production of NaNCO by this method.

[1]

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.

Calculate [OH^-] and the pH of the solution at the end of the addition. Show your working.

On 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

This question is about buffer solutions.

Calculate the pH of a buffer solution made by dissolving 0.275 g of benzoic acid (K_a = 6.3 x 10^-5 mol dm^-3) and 0.525 g of sodium benzoate in 250 cm³ of water. Show your working.

Calculate the pH of a buffer solution made by mixing together 130 cm³ of 0.2 mol dm^-3 ethanoic acid (K_a = 1.74 x 10^-5 mol dm^-3) and 85 cm³ of 0.45 mol dm^-3 sodium ethanoate. Show your working.

1.00 cm³ of 1.00 mol dm^-3 sodium hydroxide is added to the buffer solution in part (b).

i) State what the comparison of the pH values before and after the addition of NaOH demonstrates about the buffer solution.

[1]

ii) Calculate the new pH of the buffer solution after the addition. Show your working.

[4]

Construct two equations for the separate addition of a small volume of acid and a small volume of base to the buffer solution in part (b).