Question

Caffine contains $28.9$% by mass of nitrogen. If molecular mass of caffine is $194$, then the number of $N$ atoms present in one molecule of caffine is:

• A. $3$
• B. $4$
• C. $5$
• D. $6$

Answer 1

Answer: (B)

$4$

Molecular mass of caffeine = $194u$

$N$ present in one molecule of caffeine = 28.9 %

Therefore amount of $N$ atoms present= $28.9/100\times 194=56u$

Mass of one atom of $N$ = 14 u

Number of $N$ atoms present in 56 u are: $\frac{56}{14}=4$

Therefore there are 4 $N$ atoms present in one molecule of caffeine.

Hence, option B is correct.