Ka, pKa and weak acid pHEdexcel A-Level Chemistry: Mind map

Exam questions on Ka, pKa and weak acid pH

1. A food technologist analyses a 0.100 mol dm⁻³ solution of ethanoic acid, CH₃COOH, a weak monobasic acid, at 298 K. For ethanoic acid, $K_a$ = 1.74 × 10⁻⁵ mol dm⁻³.
   Calculate the pH of the 0.100 mol dm⁻³ ethanoic acid solution. State the assumptions you make.2 marks
2. A chemist prepares a 0.0500 mol dm⁻³ solution of methanoic acid, HCOOH, for use as a descaling agent. For methanoic acid, $K_a$ = 1.78 × 10⁻⁴ mol dm⁻³ at 298 K. She compares it with propanoic acid, which has pKₐ = 4.87.
   The chemist states that the pH of the methanoic acid solution is greater than that of a 0.0500 mol dm⁻³ solution of a strong monobasic acid. Explain why.2 marks
3. A student dissolves 0.440 g of a pure solid weak monobasic acid, HA (molar mass 88.0 g mol⁻¹), in water and makes the solution up to 100 cm³. A calibrated pH meter reads 3.06 at 298 K.
   Calculate $K_a$ for the acid HA. State the assumptions you make.3 marks