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)

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