Question

A compound contains $20\%$ of nitrogen by weight. If one molecule of the compound contains $2$ nitrogen atoms, then the molecular weight (in ${\text{g mol}}^{-1}$) of the compound is:

• A. $35$
• B. $70$
• C. $140$
• D. $280$

Answer 1

Answer: (C)

$140$

Given, $1$ molecule of the compound contains $2$ nitrogen atoms.

So, $1$ mol of compound will have $2$ mol of nitrogen atoms $=2\times 14=$ $28$ g of nitrogen.

However, the compound contains $20\%$ of nitrogen by weight.

Therefore, the weight of the compound corresponding to $28$ g of nitrogen is $=\frac{100\times 28}{20}=140$ g.

Hence, the molecular weight of the compound is $140$ g/mol.

Option C is correct.