Question

Mole fraction of $A$ in $H_{2}O$ is $\mathrm{0.2.}$ The molality of $A$ in $H_{2}O$ is

• A. $13.9$
• B. $15.5$
• C. $14.5$
• D. $16.8$

Answer 1

Answer: (A)

Mole fraction of $A$ i.e. $X_A=\frac{n_A}{Totalmoles}$
So $X_{H_{2}O}=\frac{n_{H_{2}O}}{Totalmoles}$
Now $\frac{X_A}{X_{H_{2}O}}=\frac{n_A}{n_{H_{2}O}}$
and molality $=\frac{n_A\times 1000}{n_{H_{2}O}\times 18}=\frac{X_A\times 1000}{X_{H_{2}O}\times 18}$
$=\frac{0.2\times 1000}{0.8\times 18}13.9$