Question 19M.3.sl.TZ1.16
| Date | May 2019 | Marks available | [Maximum mark: 5] | Reference code | 19M.3.sl.TZ1.16 |
| Level | sl | Paper | 3 | Time zone | TZ1 |
| Command term | Determine, Formulate, Outline | Question number | 16 | Adapted from | N/A |
Mild heartburn is treated with antacids such as calcium carbonate.
Formulate an equation for the neutralization of stomach acid with calcium carbonate, CaCO3 (s).
[1]
CaCO3 (s) + 2HCl (aq) → CO2 (g) + CaCl2 (aq) + H2O (l) [✔]
Note: Accept balanced ionic equations involving “H+” or “H3O+”.
Do not accept “H2CO3”.
About two-thirds of the candidates could write an equation for the reaction of calcium carbonate with an acid. It was refreshing to find that “H2CO3” was only rarely encountered as a product.
Determine the volume of CO2 (g), in dm3, produced at STP, when 1.00 g of CaCO3 (s) reacts completely with stomach acid.
Mr CaCO3 = 100.09
[2]
n CaCO3 = « =» 0.00999 «mol» [✔]
volume CO2 = «0.00999 mol × 22.7 dm3 mol–1 = » 0.227 «dm3» [✔]
Note: Accept 0.224 «dm3» if 22.4 dm3 mol–1 is used as molar volume.
Award [2] for correct answer.
It was encouraging that over half the students were able to calculate the volume of gas produced from a given mass of calcium carbonate.
Acid secretion can be regulated by other types of drugs such as omeprazole and ranitidine. Outline how each of these drugs acts to reduce excess stomach acid.
Omeprazole:
Ranitidine:
[2]
Omeprazole:
inhibits enzyme/«gastric» proton pump «which secretes H+ ions into gastric juice»
OR
inhibits the H+/K+-ATPase system [✔]
Ranitidine:
inhibits/blocks H2/histamine receptors «in cells of stomach lining»
OR
prevents histamine binding to H2/histamine receptors «and triggering acid secretion» [✔]
Note: Accept “H2-receptor antagonist” for M2.
Many students correctly remembered the modes of action of Omeprazole and Ranitidine, though needless to say a few confused these and weaker students thought they acted like conventional antacids.