Question

When $25g$ of a non-volatile solute is dissolved in $100g$ of water, the vapour pressure is lowered by $2.25\times {10}^{-1}mm$. If the vapour pressure of water at $20$ is $17.5mm$, what is the molecular weight of the solute?

• A. $206$
• B. $302$
• C. $350$
• D. $276$

Answer 1

Answer: (C)

$350$

Weight of non voltalite solute, $w=25$g

Weight of solvent,$W=100$g

Lowering of vapour pressure,

$P^o-P_s=0.225$

Vapour pressure of pure solvent ,

$P^o=17.5$mm

molecular weight of solvent $\left(H_{2}O\right),M=18$g

molecular weight of solute,

According to raoult's law,

$\frac{P^0-P_s}{p°}$$=\frac{W\left(M\right)}{M\times w}$

$\frac{0.225}{17.5}=$$\frac{25\times 18}{m\times 100}$

$m=350$g