| Date | November 2017 | Marks available | 2 | Reference code | 17N.3.sl.TZ0.20 |
| Level | SL | Paper | 3 | Time zone | TZ0 |
| Command term | Calculate | Question number | 20 | Adapted from | N/A |
Question
Excess acid in the stomach can cause discomfort and more serious health issues.
Explain how ranitidine (Zantac) reduces stomach acid production.
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 pKa of carbonic acid is 6.35.
Markscheme
blocks/binds to H2/histamine receptors «in cells of stomach lining»
OR
prevents histamine binding to H2/histamine receptors «and triggering acid secretion»
prevents parietal cells from releasing/producing acid
Accept "H2-receptor antagonist/H2RA" OR "blocks/inhibits action of histamine" for M1.
ALTERNATIVE 1
pH = «pKa + log \(\frac{{\left[ {{{\text{A}}^ - }} \right]}}{{\left[ {{\text{HA}}} \right]}}\) =» 6.35 + log (\(\frac{{0.400}}{{0.0200}}\))
«pH =» 7.65
ALTERNATIVE 2
Ka = 4.5 x 10–7
«Ka = 0.400 x \(\frac{{\left[ {{{\text{H}}^ + }} \right]}}{{0.0200}}\), [H+] =» 2.3 x 10–8 «mol dm–3»
«pH =» 7.64
Award [2] for correct final answer.
Do not accept “pH = 8”.
