DP Chemistry (first assessment 2025)

Question 19M.2.HL.TZ2.23b

Date	May 2019	Marks available	[Maximum mark: 1]	Reference code	19M.2.HL.TZ2.23b
Level	HL	Paper	2	Time zone	TZ2
Command term	Determine	Question number	b	Adapted from	N/A

[Maximum mark: 1]

19M.2.HL.TZ2.23b

(b)

Some antacids contain carbonates.

Determine the pH of a buffer solution which contains 0.160 mol dm⁻³ CO₃²⁻ and 0.200 mol dm⁻³ HCO₃⁻, using section 1 of the data booklet.

pK_a (HCO₃⁻) = 10.32

[1]

Markscheme

«pH = pK_a $+ \log \frac{{[{{\text{A}}^ - }]}}{{[{\text{HA]}}}} = 10.32 + \log \frac{{0.160}}{{0.200}} = 10.32 - 0.097$ »
«pH =»10.22 [✔]

Examiners report

This was generally a well-answered question. Most candidates who did not receive the mark inverted the concentration of the conjugate base/concentration of the acid in the calculation.

Syllabus sections

Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.1—Proton transfer reactions » Reactivity 3.1.17—The pH of a buffer solution depends on both: • the pKa or pKb of its acid or base • the ratio of the concentration of acid or base to the concentration of the conjugate base or acid. Solve problems involving the composition and pH of a buffer solution, using the equilibrium constant.

Question 19M.2.HL.TZ2.23b

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