Question

The mole fraction of methanol in water is $0.12$. Calculate its molality.

• A. $12.50\text{m}$
• B. $7.50\text{m}$
• C. $18.50\text{m}$
• D. $3.25\text{m}$

Answer 1

The correct option is B $7.50\text{m}$

Assuming the total number of moles to be $1$.
Moles of methanol $=0.12$
Moles of water $=0.88$
mass of water = $0.88\times 18\approx 16g$
Molality of methanol = $\frac{\text{moles of the solute}}{\text{mass of the solvent in kg}}=\frac{0.12}{0.016}=7.5\text{m}$