Chemistry 9701 · AS & A Level · Analytical techniques

Analytical techniques — practice question

Compound $R$ is a weak diprotic (dibasic) acid that dissolves very readily in water.
(a(i))[1]

Using $\text{H}_2\text{X}$ as the formula for $R$, write a balanced equation for the reaction of $\text{H}_2\text{X}$ with NaOH.

(a(ii))[1]

From the information given above, calculate the amount, in moles, of $\text{OH}^-$ ions used in the titration.

(a(iii))[1]

Using your answers from (i) and (ii), calculate the amount, in moles, of $R$ in $25.0\,\text{cm}^3$ of solution.

(a(iv))[1]

Calculate the amount, in moles, of $R$ in $250\,\text{cm}^3$ of solution.

(a(v))[1]

Calculate $M_r$ for $R$.

(b(i))[1]

Three candidate structures for $R$ are labelled $S$, $T$ and $U$. Calculate the $M_r$ of each of these acids.

(b(ii))[2]

Deduce which of the structures, $S$, $T$ or $U$, is the correct representation of acid $R$.

(c)[5]

State the reagent(s) and the essential conditions that would be used for these conversions: $S$ into $T$; $S$ into $U$; $T$ into $S$.

(d)[2]

Give the structural formula of the organic product formed in each case: $T$ with excess Na; $U$ with excess $\text{Na}_2\text{CO}_3$.

(e)[2]

Acid $S$ displays stereoisomerism. Draw structures to show this isomerism, and label each isomer.

(f)[2]

When one isomer of $S$ is heated at $110^\circ\text{C}$ in the absence of air, a cyclic compound $V$, with molecular formula $\text{C}_4\text{H}_2\text{O}_3$, is produced. The other isomer of $S$ does not react at this temperature. Suggest the displayed formula of $V$.

Worked solution & mark scheme

This 19-mark question has a full step-by-step worked solution and mark scheme. One marking point: Balanced equation: $H_2X + 2NaOH \rightarrow Na_2X + 2H_2O$

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