Chemistry 5070 · O Level · Acid–base titrations

Acid–base titrations — practice question

A student finds the concentration of aqueous sodium hydroxide, $\text{NaOH(aq)}$, by titrating it with aqueous citric acid, $\text{C}_6\text{H}_8\text{O}_7\text{(aq)}$.
(a)[2]

The student measures a volume with the wrong apparatus. Identify the incorrect apparatus and suggest a more precise alternative.

(b)[1]

Explain why the mixture in the volumetric flask is shaken at step 5.

(c)[1]

Explain the purpose of the indicator in step 9.

(d)[1]

Calculate the mass of citric acid put into the beaker.

(e)[1]

Calculate the number of moles of citric acid put into the beaker. Give your answer to three significant figures. $[M_r\text{ citric acid} = 192]$

(f)[1]

Calculate the concentration, in $\text{mol dm}^{-3}$, of the citric acid solution in the volumetric flask.

(g)[2]

Use the diagrams to fill in the values for titration 1 in the results table shown.

(h)[2]

Complete the table by ticking the most reliable titration results. Explain why these values have been ticked.

(i)[1]

Use the most reliable titration results to work out the mean volume of $\text{NaOH(aq)}$ used.

(j)[1]

Use your result from (f) to calculate the number of moles of citric acid present in $25.0\text{ cm}^3$ of solution P.

(k)[1]

Calculate the number of moles of $\text{NaOH}$ that react with the citric acid in $25.0\text{ cm}^3$ of solution P, using $\text{C}_6\text{H}_8\text{O}_7 + 3\text{NaOH} \rightarrow \text{Na}_3\text{C}_6\text{H}_5\text{O}_7 + 3\text{H}_2\text{O}$.

(l)[1]

Calculate the concentration, in $\text{mol dm}^{-3}$, of $\text{NaOH(aq)}$.

(m(i))[2]

Use the equation $2\text{NaOH(aq)} + \text{FeSO}_4\text{(aq)} \rightarrow \text{Na}_2\text{SO}_4\text{(aq)} + \text{Fe(OH)}_2\text{(s)}$ together with your answer from (l) to work out the number of moles of $\text{Fe(OH)}_2\text{(s)}$ formed.

(m(ii))[1]

State the colour seen for the precipitate.

Worked solution & mark scheme

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