Question

If mole fraction of a solute in $1kg$ benzene is $0.2$ then molality of solute is

• A. $3.2$
• B. $2$
• C. $4$
• D. $3.6$

Answer 1

Answer: (A)

Let number of moles of solute in solution $=x$
Moles of benzene in solution $=\frac{1000g}{78gmo{l}^{-1}}$
$=12.82$ moles
Mole fraction of solute $=\frac{x}{x+12.82}$
$\Rightarrow 0.2=\frac{x}{x+12.82}$
On solving, $x=3.2$
$\therefore$ Molality $(m)=\frac{\text{ Number of moles of solute }}{\text{ Mass of solvent (in kg) }}=\frac{3.2}{1}=3.2$