| Date | May 2009 | Marks available | 2 | Reference code | 09M.3.sl.TZ2.D2 |
| Level | SL | Paper | 3 | Time zone | TZ2 |
| Command term | Deduce | Question number | D2 | Adapted from | N/A |
Question
The walls of the human stomach contain cells that produce gastric juices. Sodium hydrogencarbonate is an antacid often used to neutralize excess acid.
State an equation for the reaction of stomach acid with this antacid.
Calcium carbonate can also neutralize stomach acid. The same amounts (in moles) of sodium hydrogencarbonate and calcium carbonate are available. Deduce which antacid will neutralize the greater amount of acid present in the stomach and explain your reasoning.
Markscheme
\({\text{NaHC}}{{\text{O}}_{\text{3}}} + {\text{HCl}} \to {\text{NaCl}} + {{\text{H}}_{\text{2}}}{\text{O}} + {\text{C}}{{\text{O}}_{\text{2}}}/{\text{HCO}}_3^-- + {{\text{H}}^ + } \to {{\text{H}}_{\text{2}}}{\text{O}} + {\text{C}}{{\text{O}}_{\text{2}}}\);
States not required for mark.
\({\text{CaC}}{{\text{O}}_{\text{3}}}\);
1 mol \({\text{NaHC}}{{\text{O}}_{\text{3}}}\) neutralizes 1 mol HCl and 1 mol \({\text{CaC}}{{\text{O}}_{\text{3}}}\) neutralizes 2 mol HCl / \({\text{CaC}}{{\text{O}}_{\text{3}}} + {\text{2HCl}} \to {\text{CaC}}{{\text{l}}_{\text{2}}} + {{\text{H}}_{\text{2}}}{\text{O}} + {\text{C}}{{\text{O}}_{\text{2}}}\);
Examiners report
Most candidates gave correct equations.
Most candidates identified \({\text{CaC}}{{\text{O}}_{\text{3}}}\) as the antiacid that neutralizes more acid.
