Calculate the mass of urea (NH₂CONH₂) required in making 2.5 kg 0.25 of molal aqueous solution.

 
Mol. mass of urea ${\mathrm{NH}}_2{\mathrm{CONH}}_2$
                 = 14 + 2 + 12 + 16 + 14 + 2
                 = $60 \mathrm{g} {\mathrm{mol}}^{-1}$

Molality (m) = $\frac{\mathrm{Moles} \mathrm{of} \mathrm{solute}}{\mathrm{Mass} \mathrm{of} \mathrm{solvent} \mathrm{in} \mathrm{kg}}$

$25 = \frac{\mathrm{Moles} \mathrm{of} \mathrm{solute}}{2.5}$

or Moles of solute
                = 0.25 x 0.25 =  0.625

  Mass of urea
                   = Moles of solute x Molar mass

                   = 0.625 x 60 = 37.5 g