A sample of cosmetic jojoba-like wax is composed mainly of an unsaturated ester Y\text{Y}Y mixed with a small amount of an inert organic impurity. The unsaturated ester Y\text{Y}Y has the molecular formula C39H68O5\text{C}_{39}\text{H}_{68}\text{O}_5C39H68O5 (Mr=616M_r = 616Mr=616) and contains 3 carbon-carbon double bonds per molecule.
The amount of Y\text{Y}Y is determined using the following procedure:
A suitable target titre for the titration is 24.0 cm3 of 0.015 mol dm-3 Br2\text{Br}_2Br2.
Justify why a much smaller target titre would not be appropriate.
Calculate the amount, in moles, of bromine in this target titre.
Calculate a suitable mass of the cosmetic wax to transfer to the volumetric flask using your answer to Part 1 and the chemical structure of Y\text{Y}Y. Assume that the wax contains 75.0% of Y\text{Y}Y by mass. (If you were unable to calculate the amount of bromine in the target titre, you should assume it is 3.0 × 10-4 mol. This is not the correct amount.)
The wax solution is prepared using this method:
Suggest an extra step to ensure that the mass of wax in the solution is recorded accurately. Justify your suggestion.
State the reason for inverting the flask several times.
A sample of the cosmetic wax was dissolved in methanol and analysed using electrospray ionisation mass spectrometry. Each molecule gained a hydrogen ion (H+\text{H}^+H+) during ionisation. The spectrum showed a peak for an ion with m/z=281m/z = 281m/z=281 formed from an impurity in the wax. This ion was formed from a compound with the empirical formula C4H6O\text{C}_4\text{H}_6\text{O}C4H6O. Deduce the molecular formula of this compound, showing your working.
79 exam-style questions on AQA A Level Chemistry Amount of substance, covering Relative atomic mass and relative molecular mass, The mole and the Avogadro constant, The ideal gas equation, Empirical and molecular formula, and Balanced equations and associated calculations. Each one has a worked solution and a mark scheme showing where the marks go.