| Date | May 2011 | Marks available | 2 | Reference code | 11M.2.hl.TZ2.2 |
| Level | HL | Paper | 2 | Time zone | TZ2 |
| Command term | Calculate | Question number | 2 | Adapted from | N/A |
Question
The element antimony, Sb, is usually found in nature as its sulfide ore, stibnite, \({\text{S}}{{\text{b}}_{\text{2}}}{{\text{S}}_{\text{3}}}\). This ore was used two thousand years ago by ancient Egyptian women as a cosmetic to darken their eyes and eyelashes.
Antimony contains two stable isotopes, \(^{{\text{121}}}{\text{Sb}}\) and \(^{{\text{123}}}{\text{Sb}}\). The relative atomic mass of antimony is given in Table 5 of the Data Booklet.
Calculate the percentage by mass of antimony in a sample of pure stibnite. State your answer to four significant figures.
Calculate the percentage of each isotope in pure antimony. State your answers to three significant figures.
State the number of neutrons present in an atom of \(^{{\text{121}}}{\text{Sb}}\).
Markscheme
\({{\text{M}}_{\text{r}}}{\text{ for S}}{{\text{b}}_{\text{2}}}{{\text{S}}_{\text{3}}} = (2 \times 121.75) + (3 \times 32.06) = 339.68\);
% of Sb in \({\text{S}}{{\text{b}}_2}{{\text{S}}_3} = \frac{{(2 \times 121.75)}}{{339.68}} \times 100 = 71.69\% \);
Accept 71.68%.
Award [2] for correct final answer.
the (vaporized) ions are deflected by the (external) magnetic field;
the \(^{{\text{121}}}{\text{Sb}}\) /lighter ions are deflected more than the \(^{{\text{123}}}{\text{Sb}}\) /heavier ions / OWTTE;
Allow deflection depends on mass to charge ratio or momentum of ions.
the ions are detected by conversion into an electrical current / OWTTE;
the ratio of the intensity of the peaks in the spectrum is equal to the ratio of the ions in the sample / the ratio of the height of the peaks due to \(^{{\text{121}}}{\text{Sb}}\) and \(^{{\text{123}}}{\text{Sb}}\) will be 62.5:37.5 / OWTTE;
If atoms/elements used instead of ions, penalize only once.
Allow the use of sample or isotopes instead of ions.
70;
Examiners report
In (a) (i) most candidates calculated the molecular mass but many lost a mark as they forget to multiply by 2 when calculating the mass of antimony. A small number of candidates also lost the second mark as they failed to report the answer to four significant figures as directed by the question. Most candidates were able to deduce the oxidation state of antimony but many lost a mark as they used an incorrect format such as 3, or \(3 + \). It was expected that candidates would be able to deduce the other oxidation states from the position of the element in the periodic table but most candidates incorrectly gave \( + 2\) as an answer.
Most candidates were aware of some environmental concerns due to the production of pollutant gases but some incorrectly identified both carbon dioxide and sulfur dioxide as greenhouse gases. Most candidates were able to calculate the relative abundance of the two isotopes and the use of a magnetic field to deflect ions in a mass spectrometer was generally understood, although a significant number of responses were penalised as they referred to atoms rather than ions. Few students were able, however, to describe the deflection stage in sufficient detail.
Most candidates were able to deduce the number of neutrons in \(^{{\text{121}}}{\text{Sb}}\).
