Weak acids and KaAQA A-Level Chemistry: Mind map

Exam questions on Weak acids and Ka

1. Methanoic acid, HCOOH, is a weak acid found in ant stings. In aqueous solution it dissociates slightly: HCOOH ⇌ H⁺ + HCOO⁻. At 298 K its acid dissociation constant, Ka, is 1.78 × 10⁻⁴ mol dm⁻³. A solution of methanoic acid has a concentration of 0.0500 mol dm⁻³.
   Calculate the pH of the 0.0500 mol dm⁻³ solution of methanoic acid.2 marks
2. A chemist measures the pH of a 0.150 mol dm⁻³ solution of a weak monoprotic acid, HA, at 298 K and finds it to be 2.85.
   State two assumptions made when calculating Ka for HA from the pH of its solution.2 marks
3. Ethanoic acid, CH₃COOH, is the weak acid in vinegar. At 298 K its acid dissociation constant, Ka, is 1.74 × 10⁻⁵ mol dm⁻³.
   Calculate the pH of a 0.250 mol dm⁻³ solution of ethanoic acid. State the assumptions that you make.3 marks