Question

Find the lowering in vapour pressure (in mm of Hg) of a binary aqueous solution of a non-volatile solute at ${100}^{o}C$, if the mole fraction of the solute is $0.01$.

• A. $76.0$
• B. $7.6$
• C. $0.76$
• D. $0.076$

Answer 1

The correct option is B $7.6$

As we know, the vapour pressure of water $\left(P_{H_{2}O}^o\right)$ at ${100}^{o}C$ is $760$ mm of Hg.

According to Raoult's law, we have

$\Delta P=x_{solute}{P}_{H_{2}O}^o$,

where $x_{solute}$ is the mole fraction of solute and $\Delta P$ is the lowering in vapour pressure.

Therefore, $\Delta P$ $=0.01\times 760$ $=7.6$ mm of Hg

Hence, the lowering in vapour pressure is $7.6$ mm of Hg.

Option B is correct.