| Date | May 2011 | Marks available | 2 | Reference code | 11M.2.sl.TZ2.3 |
| Level | SL | Paper | 2 | Time zone | TZ2 |
| Command term | State | Question number | 3 | Adapted from | N/A |
Question
Rubidium contains two stable isotopes, \(^{{\text{85}}}{\text{Rb}}\) and \(^{{\text{87}}}{\text{Rb}}\). The relative atomic mass of rubidium is given in Table 5 of the Data Booklet.
Calculate the percentage of each isotope in pure rubidium. State your answers to three significant figures.
State the number of electrons and the number of neutrons present in an atom of \(^{{\text{87}}}{\text{Rb}}\).
Number of electrons:
Number of neutrons:
Markscheme
(let \(x = {\text{fraction of}}{{\text{ }}^{{\text{85}}}}{\text{Rb}}\))
\(\frac{{(x \times 85) + [(100 - x) \times 87]}}{{100}} = 85.47\);
\(^{{\text{85}}}{\text{Rb}} = 76.5\% \) and \(^{{\text{87}}}{\text{Rb}} = 23.5\% \);
Award [2] for correct final answer.
37 (electrons);
50 (neutrons);
Examiners report
This question was answered very well by those that knew the correct mathematical technique; however some candidates did not have any idea how to tackle this problem.
The vast majority of candidates could correctly state the number of electrons and neutrons present in Rubidium- 87.
