Find the molecular mass (in kilogram) of an ammonia molecule $N{H}_3$.

The atomic masses of N and H are 14 g/mol and 1 g/mol respectively. The molecular weight of ammonia is $14+3\left(1\right)=17g/mol$.

Convert unit from g to kg

$17g/mol\times \frac{1kg}{1000g}=0.017kg/mol$

1 mole of ammonia weighs 0.017 kg and contains Avogadro's number ( $6.023\times {10}^{23}$ ) of molecules.

Molecular mass of an ammonia molecule $=\frac{0.017kg/mol}{6.023\times {10}^{23}molecules/mol}=2.82\times {10}^{-26}kg$