The molality of a urea solution in which 0.0100 g of urea, $\left[{\left({\mathrm{NH}}_2\right)}_2\mathrm{CO}\right]\mathrm{is}\mathrm{added}\mathrm{to}0.3000{\mathrm{dm}}^3$ of water at STP is

$\begin{array}{l}\mathrm{Molality}=\frac{\mathrm{moles}\mathrm{of}\mathrm{the}\mathrm{solute}}{\mathrm{mass}\mathrm{of}\mathrm{the}\mathrm{solvent}\mathrm{in}\mathrm{kg}}\\ \mathrm{Moles}\mathrm{of}\mathrm{urea}\left({\mathrm{n}}_{\mathrm{urea}}\right)=\frac{0.0100\mathrm{g}}{60\mathrm{g}{\mathrm{mol}}^{-1}}\\ \mathrm{Mass}\mathrm{of}1000\mathrm{mL}\mathrm{of}\mathrm{solution}=\mathrm{volume}\times \mathrm{density}\\ =300\mathrm{mL}\times \frac{1\mathrm{g}}{\mathrm{mL}}\left[\because 1{\mathrm{dm}}^3=1000\mathrm{mL}\right]\\ =300\mathrm{g}\\ \mathrm{Mass}\mathrm{of}\mathrm{solvent}=300\mathrm{g}-0.0100\mathrm{g}\\ =299.9\mathrm{g}=0.2999\mathrm{kg}\\ \mathrm{Molality}=\frac{0.0100}{60\times 0.2999}=5.55\times {10}^{-4}\mathrm{mol}{\mathrm{kg}}^{-1}\end{array}$