A certain quantity of a gas occupies 100ml when collected over water at 15°C and 750mm pressure. If it occupies 91.9ml in dry state at STP, the vapour pressure of water at 15°C is

$$\begin{gathered} {\mathrm{P}}_{\mathrm{dry} \mathrm{gas}}={\mathrm{P}}_{\mathrm{wet} \mathrm{gas}}-\mathrm{Aqueous} \mathrm{tension} \\ {\mathrm{P}}_{\mathrm{wet} \mathrm{gas}}=750\mathrm{mm} \\ \mathrm{We} \mathrm{know} \mathrm{that}, \underset{\left(\mathrm{STP}\right)}{\frac{{\mathrm{P}}_1{\mathrm{V}}_1}{{\mathrm{T}}_1}}=\frac{{\mathrm{P}}_2{\mathrm{V}}_2}{{\mathrm{T}}_2} \\ \frac{1\times 91.9}{273}=\frac{{\mathrm{P}}_2\times 100}{288} \\ {\mathrm{P}}_2=736.8\mathrm{mm} \\ \mathrm{Aqueous} \mathrm{tension}=750-736.8 \\ =13.184 \\ =13.2\mathrm{mm} \end{gathered}$$