A student is asked to determine the acid dissociation constant, Kₐ, for butanoic acid, CH3CH2CH2COOH.
Kₐ = [CH3CH2CH2COO⁻][H⁺] / [CH3CH2CH2COOH]
The student is told to measure the pH of eight buffer solutions containing CH3CH2CH2COOH and the salt sodium butanoate, CH3CH2CH2COO⁻Na⁺. The salt provides butanoate ions, CH3CH2CH2COO⁻, as the base.
Each buffer solution contains a different ratio of [CH3CH2CH2COOH] to [CH3CH2CH2COO⁻].
Each buffer solution is prepared by mixing different volumes of distilled water, 2.00 mol dm⁻³ CH3CH2CH2COOH and 2.00 mol dm⁻³ CH3CH2CH2COO⁻Na⁺.

Q: What topic does this question test?

This question tests Equilibria in A-Level Chemistry (syllabus code 9701). It is worth 16 marks.

Q: Which exam paper is this question from?

This question appeared in the Cambridge A-Level Chemistry May/June 2021 examination, Paper 5 Variant 2 (9701_s21_qp_52.pdf).

Question

A student is asked to determine the acid dissociation constant, Kₐ, for butanoic acid, CH3CH2CH2COOH.
Kₐ = [CH3CH2CH2COO⁻][H⁺] / [CH3CH2CH2COOH]
The student is told to measure the pH of eight buffer solutions containing CH3CH2CH2COOH and the salt sodium butanoate, CH3CH2CH2COO⁻Na⁺. The salt provides butanoate ions, CH3CH2CH2COO⁻, as the base.
Each buffer solution contains a different ratio of [CH3CH2CH2COOH] to [CH3CH2CH2COO⁻].
Each buffer solution is prepared by mixing different volumes of distilled water, 2.00 mol dm⁻³ CH3CH2CH2COOH and 2.00 mol dm⁻³ CH3CH2CH2COO⁻Na⁺.

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