Question

Calculate the mass of urea $\left(N{H}_{2}CON{H}_2\right)$ required in making $2.5kg$ of $0.25$ molal aqueous solution.

• A. 37.5 g
• B. 73.5 g
• C. 53.7 g
• D. 75.3 g

Answer 1

Answer: (A)

We know that, Molality $=\frac{\text{ moles of solute }}{\text{ mass of solvent }(\text{ in }kg)}$
$\because$ mass of solvent $=\frac{\text{ mass }}{\text{ molar mass }}$
So, Molality $=\frac{\frac{\text{ mass }}{\text{ molar mass }}}{\text{ mass of solvent }(\text{ in }kg)}$
$0.25=\frac{60}{2.5}$
( $\because$ Molar mass of urea $=60$ )
or Mass $=0.25\times 2.5\times 60$
$=37.5g$