Acid-base Equilibria (Edexcel A Level Chemistry): Exam Questions

This question is about acids and buffer solutions.

Ethanoic acid, CH₃COOH, is a monobasic acid.

CH₃COOH + H₂O ⇌ CH₃COO^–+ H₃O⁺

Give a reason why only the proton from the carboxylic acid group, and not from the methyl group, is donated to a water molecule.

The reaction of ammonia with water can be represented by

NH₃ + H₂O ⇌ N+ OH^–

Which is the acid-conjugate base pair?

A commercial nitric acid solution, HNO₃ (aq), has a concentration of 15.9 mol dm^–3.

A 15.0 cm³ sample was made up to 100 cm³ by adding deionised water.

Calculate the pH of this diluted solution.

Propanoic acid is a weak acid.

i) Calculate the pH of a 0.100 mol dm^–3 solution of propanoic acid at 298 K. Give your answer to an appropriate number of significant figures.

[K_a = 1.35×10^–5 mol dm^–3 at 298 K]

(3)

ii) State two assumptions that you made in the calculation in d(i).

(2)

A buffer solution was made using 20.0 cm³ of a butanoic acid solution, of concentration 0.100 mol dm^–3 and 30.0 cm³of sodium butanoate solution, of concentration 0.305 mol dm^–3.

[K_a = 1.52×10^–5 mol dm^–3 at 298 K]

i) Calculate the pH of this buffer solution at 298 K.

(4)

ii) Explain why the pH of the buffer solution hardly changes when a few drops of sodium hydroxide solution are added to it. Include an equation or equations in your answer. Use C₃H₇COOH as the formula for butanoic acid.

(2)

Boric acid, H₃BO₃, is a weak acid with antiseptic properties.

Boric acid can be prepared by reacting borax, Na₂B₄O₇.10H₂O, with hydrochloric acid.

Write the equation for this reaction. State symbols are not required.

The formula of boric acid can also be written as B(OH)₃.

i) Complete the dot-and-cross diagram of a molecule of boric acid. Show the outer shell electrons only.

Use dots (•) for the hydrogen electrons, crosses (×) for the oxygen electrons and triangles ( Δ ) for the boron electrons.

(2)

ii) What are the O—B—O and B—O—H bond angles in a molecule of boric acid?

(1)

Boric acid is a solid with melting temperature 171°C.

What are the strongest interactions between the molecules in solid boric acid?

In aqueous solution, boric acid dissociates into ions in three stages.

The equation for the first dissociation is

H₃BO₃(aq) ⇌ H⁺(aq) + H₂BO₃^-(aq)

pK_a for this dissociation is 9.24

i) Calculate the pH of a 0.0500 mol dm⁻³ solution of boric acid from the pK_a value for the first dissociation.

(3)

ii) State any assumptions you made in your calculation in (d)(i).

(2)

Boric acid can undergo further dissociation. Which is the conjugate acid of the HBO₃^2– ion?

This question is about weak acids.

A weak acid, HX, has a K_a value of 5.25 × 10⁻⁵ mol dm⁻³. A solution was formed by mixing 10.5 cm³ of 0.800 mol dm⁻³ dilute sodium hydroxide with 25.0 cm³ of 0.920 mol dm⁻³ HX(aq).

Calculate the pH of the solution formed, showing all your working.

i) Propanoic acid, CH₃CH₂COOH, is a weak acid. On the grid below, sketch the change in pH during the addition of 50.0 cm³ of 0.100 mol dm⁻³ sodium hydroxide solution to 25.0 cm³ of 0.100 mol dm⁻³ propanoic acid solution.

(4)

ii) Explain how you would use the graph in (b)(i) to obtain the value of the acid dissociation constant, K_a , for propanoic acid. You are not expected to calculate this value.

(2)

This is a question about buffer solutions.

A buffer solution always

A buffer solution with a pH of 3.90 is required. Calculate the mass, in grams, of sodium ethanoate that should be added to 50.0 cm³ of an ethanoic acid solution of concentration 0.800 mol dm^–3 to form this buffer solution.

Give your answer to an appropriate number of significant figures.

[K_a for ethanoic acid = 1.74 × 10^–5 mol dm^–3]

One of the systems controlling the pH of blood is the carbonic acid-hydrogencarbonate buffer system.

H₂CO₃ H⁺ + HCO₃^–

Explain how this buffer system helps to control the pH of blood when extra carbon dioxide is present due to strenuous exercise.

A weak acid-strong base titration curve can be used to demonstrate buffer action.

i) Draw a titration curve for the addition of 100 cm³ of sodium hydroxide solution of concentration 0.100 mol dm^–3 to 40.0 cm³ of propanoic acid solution of concentration 0.100 mol dm^–3 which has a pH of 3.0.

Show the part of the curve that demonstrates buffer action.

(4)

ii) Describe, without calculation, how you would use your curve to determine the value of K_a for propanoic acid.

(2)

This question is about acids and bases.

State what is meant by a Brønsted-Lowry acid.

Identify the acid-base conjugate pairs in this reaction.

CH₃COOH + HCOOH → CH₃COOH₂⁺ + HCOO^–

................... + ................... → .................... + ....................

Write the expression that defines the pH of a solution.

Calculate the concentration of hydrogen ions, in mol dm^-3, in a solution with a pH of 2.76

Explain why the pH of a 1 × 10^-8 mol dm^-3 solution of nitric acid, HNO₃, is not 8.

[Ionic product of water, K_w = 1.00 × 10^-14 mol² dm^-6]