| Date | May 2012 | Marks available | 3 | Reference code | 12M.2.sl.TZ2.2 |
| Level | SL | Paper | 2 | Time zone | TZ2 |
| Command term | Calculate | Question number | 2 | Adapted from | N/A |
Question
A student added \(7.40 \times {10^{ - 2}}{\text{ g}}\) of magnesium ribbon to \({\text{15.0 c}}{{\text{m}}^{\text{3}}}\) of \({\text{2.00 mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\) hydrochloric acid. The hydrogen gas produced was collected using a gas syringe at 20.0 °C and \(1.01 \times {10^5}{\text{ Pa}}\).
Calculate the theoretical yield of hydrogen gas:
State the equation for the reaction between magnesium and hydrochloric acid.
Determine the limiting reactant.
(i) in mol.
(ii) in \({\text{c}}{{\text{m}}^{\text{3}}}\), under the stated conditions of temperature and pressure.
The actual volume of hydrogen measured was lower than the calculated theoretical volume.
Suggest two reasons why the volume of hydrogen gas obtained was less.
Markscheme
\({\text{Mg(s)}} + {\text{2HCl(aq)}} \to {{\text{H}}_2}{\text{(g)}} + {\text{MgC}}{{\text{l}}_2}{\text{(aq)}}\);
\(n{\text{(Mg)}} = \left( {\frac{{0.0740}}{{24.31}}} \right) = 3.04 \times {10^{ - 3}}{\text{ (mol)}}\);
Accept range 3.04 \( \times \) 10–3 to 3.08 \( \times \) 10–3.
\(n{\text{(HCl)}} = (2.00 \times 15.0 \times {10^{ - 3}}) = 3.00 \times {10^{ - 2}}{\text{ (mol)}}\);
Mg;
(i) \(n{\text{(}}{{\text{H}}_{\text{2}}}{\text{)}} = n{\text{(Mg)}} = 3.04 \times {10^{ - 3}}{\text{ (mol)}}\);
Accept same value as in 2(b).
Answer must be in range 3.04 x 10–3 to 3.08 x 10–3 and must have 2, 3 or 4 significant figures.
(ii) \(V\left( { = \frac{{nRT}}{P}} \right) = \frac{{3.04 \times {{10}^{ - 3}} \times 8.31 \times 293 \times {{10}^6}}}{{1.01 \times {{10}^5}}}\);
\( = 73.4{\text{ (c}}{{\text{m}}^{\text{3}}}{\text{)}}\);
Accept answers in the range 72.3 to 74.3 (cm3).
gas leaks from apparatus / gas escapes;
the syringe stuck;
Mg impure;
Examiners report
Part (a) was scored correctly about 50% of the time but many assumed magnesium chloride to be MgCl.
Many candidates were able to answer (b) correctly with ECF (error carried forward) taken into account as necessary.
In (c)(i), many following through directly from (b) weren’t careful enough with the significant figures of the answer and were penalized here. Part (c)(ii) required a careful calculation; most did not make the correct correction to \({\text{c}}{{\text{m}}^{\text{3}}}\).
In (d), candidates needed to think whether the answer they gave made sense in the context of the experiment and their previous answers. It is important that candidates are exposed to a wide range of practical experiences.
