Equal volumes of two solutions, each with the same concentration, are mixed together at 298 K - AQA - A-Level Chemistry - Question 15 - 2022 - Paper 3

To determine which two solutions will result in a pH greater than 7 upon mixing, we need to analyze the nature of the solutions:

1. HCOOH and HCOOK: These are a weak acid and its conjugate base, thus they form a buffer solution with a pH around 4 to 5, which is < 7.

2. KOH and CH3COOH: KOH is a strong base and CH3COOH is a weak acid. Mixing these would result in the neutralization of the weak acid by the strong base, leading to a resulting solution that can have a pH greater than 7 due to the excess of hydroxide ions.

3. NH3 and HCl: NH3 is a weak base and HCl is a strong acid. Mixing these would result in a solution with a pH lower than 7 due to the strong acid dominating.

4. NH4Cl and KCl: NH4Cl is a salt that could produce a weak acid solution (NH4+) and KCl does not affect the pH significantly. Thus, the mixture would not yield a pH > 7.

Based on this analysis, the correct choice is B. KOH and CH3COOH, as it results in a solution with a pH > 7.