Question

Vapour pressure of water at certain temperature is $155$mm Hg and that of the another solvent 'X' is 'p' mm Hg. Molecular weight of 'X' is $128$. An aqueous solution of 'X'($64\%$ by wt) has a vapour pressure of $145$mm Hg. What is 'p'?

Answer 1

vapour pressure of water = $P_A^o=155$ mm Hg

vapour pressure of solvent $P_B=p$

Let $100$ g aqueous solution

Amount of solvent = $64g$

Amount of water = $36g$

number of mole of water $n_A=\frac{36}{18}=2$

number of mole of solvent $n_B=\frac{64}{128}=0$

mole fraction $X_A=\frac{2}{2+0.5}=0.8$

$X_B=0.2$

$P_A^0{x}_A+P_B^0{x}_B=145$

$155\times 0.8+P\times 0.2=145$

$0.2P=145-124$

$P=\frac{21}{0.2}=105mmHg$