Question

The density of 2 M aqueous solution of NaOH is 1.28 g/cm$^3$. The molality of the solution is [Given that molecular mass of NaOH = 40 g mol$^{-1}$]

• A. 1.20 m
• B. 1.56 m
• C. 1.67 m
• D. 1.32 m

Answer 1

Answer: (C)

1.67 m

Answer 2

$2 \,M$ solution of $NaOH$ means $2$ mole $NaOH$ is present in $1\, L$ solution;
density $= 1.28 \,g/ml$
mass of solution = volume of solution $\times$ density
$= 1200 \times 1.28$
$= 1280\, g$
mass of solvent = mass of solution - mass of solute
$= 1280 - 80$
$= 1200 \,g$
molality $= \frac{2}{1200} \times 1000$
$= \frac{20}{12} = \frac{10}{6}$
$= \frac{5}{3} = 1.67\,m$