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Amount of substance

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Question 5

A sample of rose hip seed oil is mainly composed of a triacylglycerol derivative XXX mixed with a small amount of inert impurity. The compound XXX has the molecular formula C39H72O6\text{C}_{39}\text{H}_{72}\text{O}_6C39​H72​O6​ (Mr=636M_r = 636Mr​=636) and contains exactly 3 carbon-carbon double bonds per molecule.

The amount of XXX in the oil is determined using the following laboratory procedure:

  • Transfer a weighed sample of the rose hip seed oil to a 250 cm3250\text{ cm}^3250 cm3 volumetric flask and make up to the mark with an inert organic solvent.
  • Titrate 25.0 cm325.0\text{ cm}^325.0 cm3 samples of the resulting solution with 0.015 mol dm−30.015\text{ mol dm}^{-3}0.015 mol dm−3 Br2(aq)\text{Br}_2(\text{aq})Br2​(aq).

Part 1: A suitable target titre for this titration is 24.0 cm324.0\text{ cm}^324.0 cm3 of 0.015 mol dm−30.015\text{ mol dm}^{-3}0.015 mol dm−3 Br2(aq)\text{Br}_2(\text{aq})Br2​(aq).

a.

Justify why a much smaller target titre would not be appropriate.

[1]
b.

Calculate the amount, in moles, of bromine in this target titre.

[2]
c.

Part 2: Calculate a suitable mass of rose hip seed oil to transfer to the volumetric flask using your answer to Part 1 and the structure of XXX. Assume that the rose hip seed oil contains 75.0%75.0\%75.0% of XXX by mass. (If you were unable to calculate the amount of bromine in the target titre, you should assume it is 4.5×10−4 mol4.5 \times 10^{-4}\text{ mol}4.5×10−4 mol. This is not the correct amount.)

[5]
d.

Part 3: The solution is prepared using this method:

  • Place a weighing boat on a balance and record the mass, in g, to 2 decimal places.
  • Add the seed oil to the weighing boat until a suitable mass is obtained, and record the mass.
  • Pour the oil into a 250 cm3250\text{ cm}^3250 cm3 volumetric flask.
  • Add organic solvent to the volumetric flask until it is made up to the graduation mark.
  • Place a stopper in the flask and invert the flask several times.

Suggest an extra step to ensure that the mass of oil in the solution is recorded accurately. Justify your suggestion.

[2]
e.

State the reason for inverting the flask several times.

[1]
f.

Part 4: A sample of the seed oil 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=343m/z = 343m/z=343 formed from an impurity in the seed oil. This ion was formed from a compound with the empirical formula C6H10O2\text{C}_6\text{H}_{10}\text{O}_2C6​H10​O2​. Deduce the molecular formula of this compound, showing your working.

[4]
Markscheme

Amount of substance Questions

  1. A Level
  2. /Chemistry
  3. /Amount of substance

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.

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