Question

Caffeine has a molecular weight of $194u$. If it contains $28.9$% by mass of nitrogen, number of atom of nitrogen in one molecule of caffeine is:

• A. $4$
• B. $6$
• C. $2$
• D. $3$

Answer 1

The correct option is A $4$

M.wt. of caffeine $=194u$

% of $N$ present in one molecular of caffeine is $28.9$% of

$194u=\frac{28.9}{100}\times 194=56u$

Mass of one $N$ atom $=14u=1$ atom $(\because 14=14u)$

$\therefore 56u=\frac{56}{14}$ atoms $=4-$ atoms