A sample of urea, ${\mathrm{NH}}_2{\mathrm{CONH}}_2$ , contains $42\%$ nitrogen, by mass. Percentage purity of the sample is :

Let the mass of sample $= 100 \mathrm{g}$

Mass of nitrogen in the sample $= 42 \mathrm{g} = \frac{42 }{14}= 3$ mole

 Mass of pure urea $= \frac{3}{2}\times 60= 90 \mathrm{g}$                                 $\therefore \mathrm{Purity} = 90\%$

$\left[\begin{array}{c}\because \frac{\mathrm{Urea}}{1\mathrm{mole}} \frac{\mathrm{N}}{2 \mathrm{mole}}\\ \frac{3}{2}\mathrm{mole}\leftarrow 3 \mathrm{mole}\end{array}\right]$