3.1.12.4 Weak acids and bases Ka for weak acids (A-level only)

2,4,6-Trichlorophenol is a weak monoprotic acid, with K_a = 2.51 × 10⁻⁸ mol dm⁻³ at 298 K.

What is the concentration, in mol dm⁻³, of hydrogen ions in a 2.00 × 10⁻³ mol dm⁻³ solution of 2,4,6-trichlorophenol at 298 K?

• 5.02 × 10⁻¹¹

• 7.09 × 10⁻⁶

• 1.26 × 10⁻⁵

• 3.54 × 10⁻³

The table shows the pK_a values for two acids.

Which statement is correct?

• Propanoic acid is a stronger acid than butanoic acid.

• The value of K_a for propanoic acid is greater than that for butanoic acid.

• The value of K_a for propanoic acid is 1.35 × 10⁻⁵ mol dm⁻³

• The value of K_a for butanoic acid is 6.61 × 104 mol dm⁻³

The acid dissociation constant, K_a, of a weak acid HA has the value 2.56 × 10^–4 mol dm^–3.

What is the pH of a 4.25 × 10^–3 mol dm⁻³ solution of HA?

• 5.96

• 3.59

• 2.98

• 2.37

This question is based on the reactions and compounds shown in the scheme below.

A 0.100 mol dm⁻³ solution of X is found to have a pH of 2.50. The value of K_a in mol dm⁻³ is

• 3.16 × 10⁻²

• 3.16 × 10⁻³

• 1.00 × 10⁻⁴

• 1.00 × 10⁻⁵

This question is about the following reaction scheme which shows the preparation of polymer P.

K is a weak acid with a pK_a of 9.95. The pH of a 0.10 mol dm⁻³ solution of K is

• 4.48

• 4.98

• 5.48

• 5.98

This question refers to the reaction sequence in the image.

HCN is a weak acid with a pK_a value of 9.40. If a 0.010 mol dm⁻³ solution of HCN was used in the first step, the concentration of cyanide ions, in mol dm⁻³, would be

• 2.0 × 10⁻⁶

• 6.4 × 10⁻⁵

• 2.0 × 10⁻⁵

• 3.1 × 10⁻¹