Chemistry 9701 · AS & A Level · Analytical techniques

Analytical techniques — practice question

A blend of volatile organic compounds X, Y and Z may be separated using a gas chromatograph. Their identities can be checked by comparing their different retention times with known values. A gas chromatogram is shown.
(a(i))[1]

Suggest the meaning of the term retention time.

(a(ii))[1]

Give one example of a carrier gas employed in gas chromatography.

(a(iii))[1]

Z remains in the chromatography column for the longest time. Suggest why this may be the case.

(a(iv))[1]

Explain one possible limitation of gas/liquid chromatography when trying to separate two esters such as ethyl methanoate, $\text{HCO}_2\text{CH}_2\text{CH}_3$, and methyl ethanoate, $\text{CH}_3\text{CO}_2\text{CH}_3$.

(a(v))[1]

A student calculates the areas under the three peaks in the chromatogram. If the areas beneath the peaks are proportional to the masses of the corresponding components, what percentage of the original mixture consisted of organic compound X?

(b(i))[1]

The NMR spectrum of Y shows four absorptions. Identify the compound responsible for the absorption at $\delta = 0$.

(b(ii))[4]

Compound Y is an ester with molecular formula $\text{C}_4\text{H}_8\text{O}_2$. Finish the table for the NMR spectrum of Y. The three chemical shifts for absorptions in Y, together with the splitting pattern for the resonance at $\delta = 3.7\,\text{ppm}$, are already provided.

(b(iii))[1]

Use your deductions to suggest a structure for ester Y.

Worked solution & mark scheme

This 11-mark question has a full step-by-step worked solution and mark scheme. One marking point: the time taken for a compound to pass through the column

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