Question

4.0 g of a substance when dissolved in $36.0\text{g}$ of water lowered its vapour pressure by $0.5\text{mm Hg}$ at a given temperature. The vapour pressure of water at this temperature is $25.0\text{mm Hg}$. Calculate the molecular weight of solute.

• A. $100\text{g/mol}$
• B. $100\text{g/mol}$
• C. $98\text{g/mol}$
• D. $100\text{g/mol}$

Answer 1

The correct option is C $98\text{g/mol}$

$\frac{\Delta P}{P_B^o}=x_B=\frac{n_B}{n_A+n_B}=\frac{W_B/M{w}_B}{W_A/M{w}_A+W_B/M{w}_B}$
$W_{A}and{W}_B$ are mass of solvent and solute.
$M{w}_{A}andM{w}_B$ are the molar masses of solvent adn solute.
By subtituting the values we get,
$=\frac{0.5}{25}=\frac{4/M{w}_B}{36/18+4/M{w}_B}$
$M{w}_B=98g/mol$