Deduce the change in hydrogen ion concentration, \({\text{[}}{{\text{H}}^ + }{\text{]}}\).

State the name of the compound formed that is responsible for this decreased pH value.

Sulfuric acid present in acid rain can damage buildings made of limestone. Predict the balanced chemical equation for the reaction between limestone and sulfuric acid including state symbols.

\({\text{[}}{{\text{H}}^ + }{\text{]}}\) increased by factor of 10;

Allow a difference of 1.426 \( \times \) 10^–3.

ethanoic acid;

Allow acetic acid.

\({\text{CaC}}{{\text{O}}_3}{\text{(s)}} + {{\text{H}}_2}{\text{S}}{{\text{O}}_4}{\text{(aq)}} \to {\text{CaS}}{{\text{O}}_4}{\text{(s)}} + {{\text{H}}_2}{\text{O(l)}} + {\text{C}}{{\text{O}}_2}{\text{(g)}}\)

correct chemical equation;

correct state symbols;

Allow CaSO₄(aq) instead of CaSO₄(s).

M2 can only be scored if M1 is correct.

Award [1max] if H₂CO₃(aq) is given instead of H₂O(l) + CO₂(g).

Question 3 a)(i) presented difficulties to some candidates who attempted to calculate the concentration of \({\text{[}}{{\text{H}}^ + }{\text{]}}\) ions even though this is not on the SL course. Simply recognizing that a decrease in pH of 1 unit is equivalent to an increase in \({\text{[}}{{\text{H}}^ + }{\text{]}}\) by a factor of 10 was sufficient here (A.S. 8.4.3).

In a) (ii) many candidates correctly identified ethanoic acid as the cause of the decrease in pH. Some simply stated carboxylic acid, which is a class of compound and not a name of a compound.

Part b) was a challenge to candidates who did not know the formula of limestone. This reaction is mentioned in teachers’ notes in 8.3.1. State symbols were also required. Some candidates mistakenly identified sulfuric acid in acid rain as H₂SO₄(l) and did not score the second mark.