Question

20g of NaOH (Molar mass = 40 g mol⁻¹) is dissolved in 500 cm³ of water. Molality of resulting solution is
a) 0.1m
b) 0.5m
c) 1.5m
d) 1.0m

Answer 1

Density of water is 1g/cc. Hence mass of water of 500cc will be 500g= 0.5 litres.
$molality = \frac{moles \: of \: solute}{mass \: of \: solvent}$
Moles are $\frac{20}{40} = 0.5$

Hence
$\\ molality=\frac{0.5}{0.5} \\ = 1m$