Chemistry 5070 · O Level · Acid–base titrations

Acid–base titrations — practice question

J is bleach. The student carries out an experiment to find the concentration of sodium chlorate(I), NaClO, in J.
(a)[1]

Why is a pipette chosen instead of a measuring cylinder when measuring $25.0\,\text{cm}^3$ of J?

(b)[1]

Which additional piece of apparatus could be used to measure $25.0\,\text{cm}^3$ of J?

(c)[1]

Why is it incorrect to wash the burette with water immediately before filling it with L?

(d)[4]

Use the diagrams to fill in the table. Tick (✓) the best titration results and work out the average volume of L.

(e)[1]

Calculate the number of moles of Na$_2$S$_2$O$_3$ in the mean volume of L, $0.0500\,\text{mol dm}^{-3}$ Na$_2$S$_2$O$_3$.

(f)[1]

Use the equation $2\text{Na}_2\text{S}_2\text{O}_3 + \text{I}_2 \rightarrow \text{Na}_2\text{S}_4\text{O}_6 + 2\text{NaI}$ to work out the number of moles of $\text{I}_2$ in $25.0\,\text{cm}^3$ of K.

(g)[1]

Use the equation $\text{NaClO} + 2\text{NaI} + 2\text{HCl} \rightarrow \text{I}_2 + 3\text{NaCl} + \text{H}_2\text{O}$ to find the number of moles of NaClO that produces the number of moles of $\text{I}_2$ found in (f).

(h)[1]

Calculate the $M_r$ of NaClO using [$A_r$: Na, 23; Cl, 35.5; O, 16].

(i)[1]

Use your answers to (g) and (h) to work out the mass of NaClO in $25.0\,\text{cm}^3$ of J.

(j(i))[1]

Calculate the concentration, in $\text{g dm}^{-3}$, of NaClO in J.

(j(ii))[1]

Calculate the concentration, in $\text{mol dm}^{-3}$, of NaClO in J.

Worked solution & mark scheme

This 14-mark question has a full step-by-step worked solution and mark scheme. One marking point: A measuring cylinder is less accurate / a pipette is more accurate

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