Chemistry 5070/31 — October/November 2014

Put $\mathbf{Q}$ into the burette.

Pipette a $25.0\text{ cm}^3$ (or $20.0\text{ cm}^3$) portion of $\mathbf{P}$ into a flask.

Add $\mathbf{Q}$ from the burette until the red-brown colour fades to pale yellow, then add a few drops of the starch indicator. This will give a dark blue solution. Continue adding $\mathbf{Q}$ slowly from the burette until one drop of $\mathbf{Q}$ causes the blue colour to disappear, leaving a colourless solution.

Record your results in the table, repeating the titration as many times as you consider necessary to achieve consistent results.

Results

Burette readings

Summary

Tick (✓) the best titration results.

Using these results, the average volume of $\mathbf{Q}$ required was ______ $\text{cm}^3$.

Volume of $\mathbf{P}$ used was ______ $\text{cm}^3$.

$\mathbf{Q}$ is $0.100\text{ mol / dm}^3$ sodium thiosulfate.

Using your results from (a), calculate the concentration, in $\text{mol / dm}^3$, of iodine in $\mathbf{P}$.

$$2\text{Na}_2\text{S}_2\text{O}_3 + \text{I}_2 \rightarrow \text{Na}_2\text{S}_4\text{O}_6 + 2\text{NaI}$$

concentration of iodine in $\mathbf{P}$ = ______ $\text{mol / dm}^3$

Using your answer from (b), deduce the number of moles of oxygen required to produce the iodine in $1.00\text{ dm}^3$ of $\mathbf{P}$.

$$\text{O}_2 + 4\text{I}^- + 4\text{H}^+ \rightarrow 2\text{H}_2\text{O} + 2\text{I}_2$$

moles of oxygen = ______

Given that the number of moles of oxygen in your answer from (c) were present in $3.00\text{ dm}^3$ of air measured at room temperature and pressure, calculate the percentage by volume of oxygen in this sample of air.

(One mole of gas occupies a volume of $24\text{ dm}^3$ at room temperature and pressure.)

percentage by volume of oxygen = ______