(i)     Give the equations for the reactions of sodium hydrogencarbonate and magnesium hydroxide with hydrochloric acid.

(ii)     Compare the effectiveness of 1.00 g of sodium hydrogencarbonate to 0.50 g of magnesium hydroxide in combating acidity in the stomach.

(i)     \({\text{NaHC}}{{\text{O}}_{\text{3}}} + {\text{HCl}} \to {\text{NaCl}} + {{\text{H}}_{\text{2}}}{\text{O}} + {\text{C}}{{\text{O}}_{\text{2}}}\);

Accept H₂ CO₃ instead of H₂O and CO₂.

\({\text{Mg(OH}}{{\text{)}}_2} + {\text{2HCl}} \to {\text{MgC}}{{\text{l}}_2} + {\text{2}}{{\text{H}}_2}{\text{O}}\);

(ii)     \({\text{n(NaHC}}{{\text{O}}_{\text{3}}}{\text{)}} = 1.19 \times {10^{ - 2}}{\text{ mol}}\);

\({\text{n}}\left( {{\text{Mg(OH}}{{\text{)}}_{\text{2}}}} \right) = 8.37 \times {10^{ - 3}}{\text{ mol}}\);

\({\text{Mg(OH}}{{\text{)}}_{\text{2}}}\) reacts with twice the number of moles of acid / is more effective than \({\text{NaHC}}{{\text{O}}_{\text{3}}}\) / OWTTE;

Most candidates successfully wrote balanced equations for antacid reactions in part (a), although a few didn’t know the products, some didn’t balance the equations and many candidates incorrectly wrote the formula for magnesium chloride as MgCl. This led to difficulties in comparing the effectiveness of two antacids, with several candidates not even attempting to answer the question. Some candidates interpreted the coefficients in the equations as representing the mass ratio rather than a mole ratio.