A 0.10 mol dm⁻³ aqueous solution of an acid is added slowly to 25 cm³ of a 0.10 mol dm⁻³ aqueous solution of a base - AQA - A-Level Chemistry - Question 14 - 2020 - Paper 3

To determine which acid-base pair has the highest pH at the equivalence point, let's consider the properties of the acid and base involved in each option:

1. Option A: CH₃COOH and NaOH

   • CH₃COOH (acetic acid) is a weak acid, and NaOH (sodium hydroxide) is a strong base. The pH at the equivalence point will be greater than 7 due to the presence of acetate ions.

2. Option B: CH₃COOH and NH₃

   • This involves a weak acid (acetic acid) and a weak base (ammonia). The pH at the equivalence point is likely to be less than 7 because both species do not fully dissociate.

3. Option C: HCl and NaOH

   • HCl is a strong acid, and NaOH is a strong base. At the equivalence point, the pH will be exactly 7.

4. Option D: HCl and NH₃

   • HCl (strong acid) and NH₃ (weak base). The pH at the equivalence point will be acidic, less than 7.

Based on these evaluations, Option A (CH₃COOH and NaOH) will have the highest pH at the equivalence point amongst the listed pairs.