Question

The vapour pressure of a dilute aqueous solution of a non-volatile solute (molar mass $180$ g $mo{l}^{-1}$) at $373$K is $750$ mm $Hg$. Calculate the mole fraction of the solute.

• A. $0.013$
• B. $0.13$
• C. $0.3$
• D. $0.34$

Answer 1

Answer: (A)

$0.013$

Relative lowering of vapour pressure of solution is given as:

$\frac{P_0-P_{soln}}{P_0}={\chi }_{solute}$

where,

$p_{solvent}=$ vapour pressure of solution

$P_o=$ vapour pressure of pure solvent

${\chi }_{solute}=$ mole fraction of non-volatile solute

For water as solvent at 373 K, $P_o=1atm=760$ mm of Hg

Given that

$p_{solution}=750$ mm of Hg

Thus upon substitution we get:

$\frac{760-750}{760}={\chi }_{solute}$

or ${\chi }_{solute}=\frac{10}{760}$

${\chi }_{solute}=0.013$

Mole fraction of solute $=0.013$.