(a)     A student added \({\text{27.20 c}}{{\text{m}}^{\text{3}}}\) of \({\text{0.200 mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\) HCl to 0.188 g of eggshell. Calculate the amount, in mol, of HCl added.

(b)     The excess acid requires \({\text{23.80 c}}{{\text{m}}^{\text{3}}}\) of \({\text{0.100 mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\) NaOH for neutralization. Calculate the amount, in mol, of acid that is in excess.

(c)     Determine the amount, in mol, of HCl that reacted with the calcium carbonate in the eggshell.

(d)     State the equation for the reaction of HCl with the calcium carbonate in the eggshell.

(e)     Determine the amount, in mol, of calcium carbonate in the sample of the eggshell.

(f)     Calculate the mass and the percentage by mass of calcium carbonate in the eggshell sample.

(g)     Deduce one assumption made in arriving at the percentage of calcium carbonate in the eggshell sample.

(a)     \(n{\text{(HCl) (}} = 0.200{\text{ mol}}\,{\text{d}}{{\text{m}}^{ - 3}} \times {\text{0.02720 d}}{{\text{m}}^{\text{3}}}{\text{)}} = 0.00544/5.44 \times {10^{ - 3}}{\text{ (mol)}}\);

(b)     \(n{\text{(HCl) excess (}} = 0.100{\text{ mol}}\,{\text{d}}{{\text{m}}^{ - 3}} \times 0.02380{\text{ d}}{{\text{m}}^{\text{3}}}{\text{)}} = 0.00238/2.38 \times {10^{ - 3}}{\text{ (mol)}}\);

Penalize not dividing by 1000 once only in (a) and (b).

(c)     \(n{\text{(HCl) reacted }}( = 0.00544 - 0.00238) = 0.00306/3.06 \times {10^{ - 3}}{\text{ (mol)}}\);

(d)     \({\text{2HCl(aq)}} + {\text{CaC}}{{\text{O}}_3}{\text{(s)}} \to {\text{CaC}}{{\text{l}}_2}{\text{(aq)}} + {{\text{H}}_2}{\text{O(l)}} + {\text{C}}{{\text{O}}_2}{\text{(g)}}/\)

\({\text{2}}{{\text{H}}^ + }{\text{(aq)}} + {\text{CaC}}{{\text{O}}_3}{\text{(s)}} \to {\text{C}}{{\text{a}}^{2 + }}{\text{(aq)}} + {{\text{H}}_2}{\text{O(l)}} + {\text{C}}{{\text{O}}_2}{\text{(g)}}\);

Award [1] for correct reactants and products.

Award [1] if this equation correctly balanced.

Award [1 max] for the following equations:

2HCl(aq) + CaCO₃(s) \( \to \) CaCl₂(aq) + H₂CO₃(aq)

2H⁺(aq) + CaCO₃(s) \( \to \) Ca₂+(aq) + H₂CO₃(aq)

Ignore state symbols.

(e)     \({n{\text{(CaC}}{{\text{O}}_3}{\text{)}} = \left( {\frac{1}{2}n{\text{(HCl)}}} \right) = \frac{1}{2} \times 0.00306}\);

\({ = 0.00153/1.53 \times {{10}^{ - 3}}{\text{ (mol)}}}\);

Award [2] for correct final answer.

(f)     \({M_{\text{r}}}{\text{(CaC}}{{\text{O}}_3}{\text{) (}} = 40.08 + 12.01 + 3 \times 16.00) = 100.09/100.1/M = 100.09/100.1{\text{ (g}}\,{\text{mo}}{{\text{l}}^{ - 1}}{\text{)}}\);

Accept 100.

\(m{\text{(CaC}}{{\text{O}}_3}{\text{) }}( = nM) = 0.00153{\text{ (mol)}} \times 100.09{\text{ (g}}\,{\text{mo}}{{\text{l}}^{ - 1}}{\text{)}} = 0.153{\text{ (g)}}\);

\({\text{\%  CaC}}{{\text{O}}_3}{\text{ }}\left( { = \frac{{0.153}}{{0.188}} \times 100} \right) = 81.4\% /81.5\% \);

Accept answers in the range 79.8 to 81.5%.

Award [3] for correct final answer.

(g)     only \({\text{CaC}}{{\text{O}}_3}\) reacts with acid / impurities are inert/non-basic / impurities do not react with the acid / nothing else in the eggshell reacts with acid / no other carbonates;

Do not accept “all calcium carbonate reacts with acid”.

Responses to this question were mixed. Many candidates were able to calculate the amount of HCl given its volume and concentration; however some failed to convert the volume from \({\text{c}}{{\text{m}}^{\text{3}}}\) to \({\text{d}}{{\text{m}}^{\text{3}}}\). Some calculated the amount of acid which had reacted rather than the excess asked for. A significant number of candidates gave carbonic acid as a product of the reaction and some were not able to write the formula of calcium carbonate. Although candidates correctly determined the amount and percentage of \({\text{CaC}}{{\text{O}}_{\text{3}}}\) in the egg sample; many struggled with the assumption made. Only a small number realising that one had to assume that only the \({\text{CaC}}{{\text{O}}_{\text{3}}}\) reacted with the acid, nothing else in the sample would react. Some of the incorrect answers were: “it contained no contaminants”, “it is 100% calcium carbonate” or “the eggshell was pure”. There were a significant number of candidates however who received 0 marks for the whole question. Clearly, as was pointed out in the most recent November 2009 subject report, it appears that many schools are not covering core laboratory areas such as volumetric chemistry.