Acid-base Equilibria (Edexcel A Level Chemistry)

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?

			Acid	Conjugate base
	☐	A	NH3	OH–
	☐	B	NH3	N
	☐	C	H2O	OH–
	☐	D	H2O	N

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.

(3)

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]

(4)

(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.

(2)

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

(3)

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.

(4)

(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.

(4)

(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]