DP Chemistry (last assessment 2024)

Question 17N.3.sl.TZ0.20b

Date	November 2017	Marks available	[Maximum mark: 2]	Reference code	17N.3.sl.TZ0.20b
Level	sl	Paper	3	Time zone	TZ0
Command term	Calculate	Question number	b	Adapted from	N/A

[Maximum mark: 2]

17N.3.sl.TZ0.20b

Excess acid in the stomach can cause discomfort and more serious health issues.

(b)

The pH is maintained in different fluids in the body by the use of buffers.

Calculate the pH of a buffer solution of 0.0200 mol dm^–3 carbonic acid, H₂CO₃, and 0.400 mol dm^–3 sodium hydrogen carbonate, NaHCO₃. The pK_a of carbonic acid is 6.35.

[2]

Markscheme

ALTERNATIVE 1

pH = «pK_a + log $\frac{{\left[ {{{\text{A}}^ - }} \right]}}{{\left[ {{\text{HA}}} \right]}}$ =» 6.35 + log ( $\frac{{0.400}}{{0.0200}}$ )