An unknown solution (mol. wt. of solute = 250] is 20% (w/w). Molarity of solution is : (Given : ${\mathrm{d}}_{\mathrm{solution}} \mathrm{is} = 1.25 \mathrm{g}/\mathrm{ml}$]

Let mass of solution be 100g 

20% w/w mass of solute  20g

mass of solute $=\frac{20}{250}$

density $=1.25 \frac{\mathrm{g}}{\mathrm{ml}}=\frac{\mathrm{mass} \mathrm{of} \mathrm{solution} }{\mathrm{volume} \mathrm{of} \mathrm{solution}}$

$1.25=\frac{100}{\mathrm{v}}$

$\mathrm{v}=\frac{100}{1.25}\mathrm{ml}$

Molarity of solute $=\frac{\mathrm{mass} \mathrm{of} \mathrm{solute} }{\mathrm{volume} \mathrm{of} \mathrm{solution} \mathrm{in} \mathrm{litre}}$

$\mathrm{M}=\left[\frac{\frac{20}{250}}{\frac{100}{1.25}}\right]\times 1000$

$\mathrm{M}=1$