Question

A mixture of ethyl alcohol and propyl alcohol has a vapour pressure of $290mm$ at $300K$. The vapour pressure of propyl alcohol is $200mm$. If the mole fraction of ethyl alcohol is $0.6$, its vapour pressure (in $mm$) at the same temperature will be:

• A. $300$
• B. $700$
• C. $360$
• D. $350$

Answer 1

The correct option is D $350$

By Roult's law,

$P_T=P_A^{\circ }{x}_A+P_B^{\circ }{x}_B$

$\therefore 290mm=P_A^{\circ }\times 0.6+200\times (1-0.6)$

$=P_A^{\circ }\times 0.6+80$

$\therefore P_A^{\circ }=350mm$

Hence, the correct option is $D$