Chemistry 5070/31 — May/June 2021

Put P into the burette.

Pipette $25.0\text{ cm}^3$ of Q into a flask and titrate with P using three drops of thymolphthalein as the indicator.

The end-point is when the solution remains colourless for 30 seconds.

Record your results in the table.

Repeat the titration as many times as necessary to achieve consistent results.

Results

Summary

Tick ($\checkmark$) the best titration results.

Use the best titration results to calculate the average volume of P required.

______ $\text{cm}^3$

Q is $0.100\text{ mol / dm}^3\text{ KOH(aq)}$.

Calculate the number of moles of $\text{KOH}$ in $25.0\text{ cm}^3$ of Q.

______ $\text{mol}$

Use your answer from (b) to calculate the number of moles of malic acid, $\text{H}_2\text{C}_4\text{H}_4\text{O}_5$, in the average volume of P used.

$$2\text{KOH} + \text{H}_2\text{C}_4\text{H}_4\text{O}_5 \rightarrow \text{K}_2\text{C}_4\text{H}_4\text{O}_5 + 2\text{H}_2\text{O}$$

______ $\text{mol}$

Use your answer from (c) to calculate the concentration of malic acid in P.

Give your answer to three significant figures.

______ $\text{mol / dm}^3$

The average concentration of malic acid in apple juice is $4.5\text{ g / dm}^3$.

Calculate the average concentration of malic acid, $\text{H}_2\text{C}_4\text{H}_4\text{O}_5$, in apple juice in $\text{mol / dm}^3$.

[$A_r$: H, 1; C, 12; O, 16]

______ $\text{mol / dm}^3$

Use your answers from (d) and (e) to calculate the average volume of apple juice which contains the same mass of malic acid as $200\text{ cm}^3$ of P.

Give your answer in $\text{dm}^3$.

______ $\text{dm}^3$

Suggest why accurate titration of apple juice with $\text{KOH}$ usually produces an average value higher than your answer from (e).

______