Chemistry 5070 · O Level · Acid–base titrations

Acid–base titrations — practice question

A chemist intends to run an experiment to determine the acid concentration in lemon juice. The preparation of the solution and the titration method are outlined.
(a(i))[1]

In the titration, state the name of the apparatus used to deliver $25.0\text{ cm}^3$ of the diluted lemon juice into the conical flask.

(a(ii))[1]

In the titration, state the name of the apparatus used to add $R$ to the diluted lemon juice in the conical flask.

(b(i))[1]

Complete the table by working out the volumes of $R$ used.

(b(ii))[1]

Tick the best titration results and explain why these values were chosen.

(b(iii))[1]

Use these best titration results to work out the average volume of $R$ used.

(c)[1]

$R$ is $0.0500\text{ mol dm}^{-3}$ sodium hydroxide, $\text{NaOH}$. Calculate the number of moles of $\text{NaOH}$ present in the average volume of $R$ used.

(d)[1]

The acid in lemon juice is represented by the formula $HA$. The equation for the reaction of $HA$ with NaOH is shown: $\text{NaOH} + HA \rightarrow \text{NaA} + H_2O$. Calculate the number of moles of $HA$ that react with the NaOH in the average volume of $R$ used.

(e)[1]

$25.0\text{ cm}^3$ of diluted lemon juice is used for each titration. Calculate the concentration, in $\text{mol dm}^{-3}$, of $HA$ in the diluted lemon juice.

(f(i))[1]

At the start of the experiment, distilled water was added to $10\text{ cm}^3$ of lemon juice to make $100\text{ cm}^3$ of diluted lemon juice. Calculate the concentration, in $\text{mol dm}^{-3}$, of $HA$ in the original lemon juice.

(f(ii))[1]

The actual acid concentration in the lemon juice is different from your answer in f(i). Suggest the most likely error in the method used to make the solution described at the start of the question.

Worked solution & mark scheme

This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: 25.0 cm^3 volumetric pipette

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