Question

The vapour pressure of an aqueous solution is found to be 750 torrs at certain temperature 'T'. If 'T' is the temperature at which pure water boils under atmospheric pressure and same solution show elevation in the boiling point $\Delta T_b=1.04K$, find the atmospheric pressure?

$[K_b=0.52Kkgmol-1]$

• A. $777$
• B. $779$
• C. $782$
• D. $746$

Answer 1

The correct option is A $777$

The pressure of the aquous solution is $P=750torr$
The depression in boiling point is $\Delta T_b=1.04$
$\Delta T_b=k_{b}m$
$1.04=0.52\times m$
Hence, $m=2molk{g}^{-1}$

$N=\frac{1000}{18}=55.56$ for water

The expression for the relative lowering in vapour pressure is $\frac{P^o-P}{P^o}=\frac{n}{N}$

Substitute values in the above expression.

$\frac{P^o-750torr}{P^o}=\frac{2}{55.56}$

Hence, $P^o=777torr$

Hence, the correct option is $A$