Question

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

• A. 2
• B. 3
• C. 4
• D. 6

Answer 1

The correct option is C

4

The correct option is (C):-

The explanation for the correct answer:

Step 1: Finding the mass of nitrogen in caffeine:

The molecular weight of caffeine is = $194$.

Therefore, the mass of Nitrogen in the $194\mathrm{amu}$ caffeine will be:

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

$=0.289\times 194$

$=56\mathrm{amu}$.

Step 2: Finding the number of nitrogen atoms in the one molecule of caffeine:

So, $\frac{\mathrm{Weight}\mathrm{of}\mathrm{Nitrogen}\mathrm{in}\mathrm{caffeine}}{\mathrm{Atomic}\mathrm{weight}\mathrm{of}\mathrm{Nitrogen}}$

$=\frac{56}{14}=4$

Therefore, the number of nitrogen atoms which are present in the one molecule of caffeine is 4.

Hence, the correct answer is (C):- 4