A sample of olive oil is mainly the unsaturated fat Y mixed with a small amount of inert impurity - AQA - A-Level Chemistry - Question 9 - 2022 - Paper 2
Question 9
A sample of olive oil is mainly the unsaturated fat Y mixed with a small amount of inert impurity.
The structure of Y in the olive oil is shown.
Y has the molecular... show full transcript
Worked Solution & Example Answer:A sample of olive oil is mainly the unsaturated fat Y mixed with a small amount of inert impurity - AQA - A-Level Chemistry - Question 9 - 2022 - Paper 2
Step 1
A suitable target titre for the titration is 30.0 cm³ of 0.205 mol dm⁻³ Br2(aq).
Justify why a much smaller target titre would not be acceptable.
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Answer
A smaller target titre would increase the uncertainty in the titration results. This is because titration readings are more prone to error when the volume of titrant used is small. Larger volumes provide more accurate measurements due to the reduction in relative error.
Step 2
Calculate the amount, in moles, of bromine in the target titre.
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Answer
To find the amount of bromine, we can use the concentration and volume:
Calculate a suitable mass of olive oil to transfer to the volumetric flask using your answer to Question 09.1 and the structure of Y.
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Given that the amount of bromine reacting with Y is 6.15 × 10⁻³ mol and considering that Y contains one double bond reactive with Br2, the molar mass of Y (C18H34O2) is 880 g/mol. The mass of olive oil is calculated using:
Since only 85% of the olive oil is Y, the mass of olive oil needed is:
ext{Mass of olive oil} = rac{5.43 ext{ g}}{0.85} ext{ g} ext{ (to account for purity)} \ = 6.39 ext{ g}
Step 4
Suggest an extra step to ensure that the mass of olive oil in the solution is recorded accurately.
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An extra step could be to rinse the bottle with solvent after transferring the mass. This ensures that all the oil is transferred into the volumetric flask, reducing loss due to residue.
Justification: This step guarantees that the entirety of the olive oil sample is measured, thereby improving the accuracy of the volume concentration in the solution.
Step 5
State the reason for inverting the flask several times.
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Inverting the flask ensures thorough mixing of the olive oil solution with the inert liquid, leading to a homogeneous solution that provides accurate titration results.
Step 6
Deduce the molecular formula of this compound.
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This compound can consist of 12 Carbon atoms, 24 Hydrogens, and 4 Oxygens based on the calculations, leading to the molecular formula C₁₂H₂₄O₄.
