DP Chemistry (last assessment 2024)

Question 19M.3.hl.TZ1.b

Date	May 2019	Marks available	[Maximum mark: 2]	Reference code	19M.3.hl.TZ1.b
Level	hl	Paper	3	Time zone	TZ1
Command term	Calculate	Question number	b	Adapted from	N/A

[Maximum mark: 2]

19M.3.hl.TZ1.b

Alcohol dehydrogenase (ADH) catalyses the oxidation of methanol. The products of oxidation, methanal and methanoic acid, are toxic.

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.

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Options » B: Biochemistry » B.7 Proteins and enzymes (HL only)

Options » B: Biochemistry

Question 19M.3.hl.TZ1.b

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