Weak acids and KaAQA A-Level Chemistry: Flashcards
What these 13 flashcards ask
- What is a weak acid?
- Write the expression for Ka for HA.
- What are the units of Ka for a monoprotic acid?
- Define pKa.
- How do you convert pKa to Ka?
- Does a stronger weak acid have a larger or smaller pKa?
- State the two assumptions used to calculate the pH of a weak acid.
- Which formula gives [H⁺] for a weak acid?
- How do you find Ka from the pH of a weak acid solution of known concentration?
- What is the pH of 0.100 mol dm⁻³ ethanoic acid (Ka = 1.74 × 10⁻⁵ mol dm⁻³)?
- Why does a weak acid have a higher pH than a strong acid of the same concentration?
- Why is water not included in the Ka expression?
- What is the difference between strong and concentrated?
Exam questions on Weak acids and Ka
- 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
- 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
- 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
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).