Question

The mole fraction of a solution of acetone $\left(C_3{H}_{6}O\right)$ in heptane $\left(C_7{H}_{16}\right)$ is 0.2. The molality is:

• A. 2 m
• B. 2.5 m
• C. 0.2 m
• D. 0.25 m

Answer 1

The correct option is B 2.5 m

Let's assume the total moles of acetone and heptane to be 1 mole.
Number of moles of acetone = 0.2 mol
Number of moles of heptane = 0.8 mol
Molar mass of heptane = 100 g/mol
Mass of heptane $=0.8\times 100=80g$
$Molality=\frac{numberofmolesofthesolute}{massofthesolventinkg}$
Molality $=\frac{0.2}{0.08}=2.5m$