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 0.1 , Its molality is:

• A. $11.1m$
• B. $1.11m$
• C. $12.5m$
• D. $1.25m$

Answer 1

The correct option is B $1.11m$

Let's assume the total moles of acetone and heptane to be 1 mole.
Number of moles of acetone = 0.1 mol
Number of moles of heptane = 0.9 mol
Molar mass of heptane = 100 g/mol
Mass of heptane $=0.9\times 100=90g=0.09kg$
$Molality=\frac{numberofmolesofthesolute}{massofthesolventinkg}$
Molality $=\frac{0.1}{0.09}=1.11m$