An organic compound made of C, H and N contains 20% nitrogen by mass. Its molecular weight could be:

Assume that the molecular weight of compound is x. Organic compound contains 20% of Nitrogen, hence it can express as:

$20\% \mathrm{of} \mathrm{x}=14$
Solving for x;

$$\begin{gathered} \frac{20}{100}\times \mathrm{x}=14 \\ \mathrm{x}=\frac{14\times 100}{20}=70 \end{gathered}$$

X=70, means that the minimum molecular weight of compound is 70 and expected molecular weight are the multiple of 70. Hence, 140 can be the molecular weight of the compound.