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Q.
Caffeine has a molecular weight of $194\, u$. If it contains $28.9\%$ by mass of nitrogen, number of atom of nitrogen in one molecule of caffeine is
Molecular weight of caffeine $=194 \, u$ N
present in one molecule of caffeine $=28.9 \%$ of
$194 u =\frac{28.9}{100} \times 194=56 \,u$
Mass of one $N$ atom $=14 \,m$
$\therefore $ $14 u=1 \,N$ atom
$\therefore 56 u=\frac{56}{14} N$ atom $=4 \,N$ atom