DP Chemistry (first assessment 2025)

Question 19M.2.HL.TZ1.b

Date	May 2019	Marks available	[Maximum mark: 2]	Reference code	19M.2.HL.TZ1.b
Level	HL	Paper	2	Time zone	TZ1
Command term	Calculate	Question number	b	Adapted from	N/A

[Maximum mark: 2]

19M.2.HL.TZ1.b

Enzymatic activity is studied in buffered aqueous solutions.

Calculate the ratio in which 0.1 mol dm⁻³ NaH₂PO₄ (aq) and 0.1 mol dm⁻³ Na₂HPO₄ (aq) should be mixed to obtain a buffer with pH = 6.10. Use section 1 of the data booklet.

pK_a (NaH₂PO₄) = 7.20

[2]

Markscheme

«pH = pK_a + log $\frac{{[{\text{HPO}}_4^{2 - }]}}{{{\text{[}}{{\text{H}}_2}{\text{PO}}_4^ - ]}}$ / 6.10 = 7.20 + log $\frac{{[{\text{HPO}}_4^{2 - }]}}{{{\text{[}}{{\text{H}}_2}{\text{PO}}_4^ - ]}}$ »

log $\frac{{[{\text{HPO}}_4^{2 - }]}}{{{\text{[}}{{\text{H}}_2}{\text{PO}}_4^ - ]}}$ = «6.10 – 7.20 =» –1.10
OR
$\frac{{[{\text{HPO}}_4^{2 - }]}}{{{\text{[}}{{\text{H}}_2}{\text{PO}}_4^ - ]}}$ = «10^–1.10 =» 0.079 [✔]

NaH₂PO₄ : Na₂HPO₄ = 12.6 : 1 [✔]

Note: Award [2] for correct final answer.

Examiners report

The calculation of the ratio of a conjugate acid-base pair to create a buffer with a specific pH was poorly done. Some candidates wrote a ratio without indicating which compound each value referred to and thus could not score. Many candidates used the concentrations of 0.1 mol dm⁻³ for both compounds and could not proceed.

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.TZ1.b

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