Question

In Ammonia ${\mathrm{NH}}_3$ there are three hydrogen atoms and one nitrogen atom but still the percentage composition of nitrogen is higher. Discuss.

Answer 1

Part 1: Percentage composition:-

• Percentage composition of an element in a compound is the percentage of the mass contributed by the element to the total mass of the compound.
• It is obtained by diving mass of that element in the compound by the total mass of the compound and multiplying by 100:$\mathrm{Percentage}\mathrm{composition}=\frac{\mathrm{Mass}\mathrm{contributed}\mathrm{by}\mathrm{the}\mathrm{element}}{\mathrm{Total}\mathrm{molar}\mathrm{mass}\mathrm{of}\mathrm{compound}}\times 100$

Part 2: Atomic mass of nitrogen and hydrogen:-

• In ammonia(${\mathrm{NH}}_3$), there are 3 hydrogen atoms and 1 nitrogen atom.
• But the atomic mass of nitrogen which is 14u, is more than that of hydrogen which is 1u

Part 3: Percentage composition of nitrogen and hydrogen :-

Percentage composition of nitrogen is

$$\begin{gathered} =\frac{\mathrm{Mass}\mathrm{of}\mathrm{nitrogen}}{\mathrm{Total}\mathrm{molar}\mathrm{mass}\mathrm{of}\mathrm{the}\mathrm{ammonia}\left({\mathrm{NH}}_3\right)}\times 100 \\ =\frac{14}{14+(1\times 3)}\times 100 \\ =\frac{14}{17}\times 100 \\ =0.8235\times 100 \\ =82.35\% \end{gathered}$$

Percentage composition of hydrogen is

$$\begin{gathered} =\frac{\mathrm{Mass}\mathrm{of}\mathrm{hydrogen}}{\mathrm{Total}\mathrm{molar}\mathrm{mass}\mathrm{of}\mathrm{ammonia}\left({\mathrm{NH}}_3\right)}\times 100 \\ =\frac{3\times 1}{14+(1\times 3)}\times 100 \\ =\frac{3}{17}\times 100 \\ =17.64\% \end{gathered}$$

Therefore, since the atomic mass of nitrogen is greater than that of hydrogen, in spite of being 3 hydrogen atoms, the percentage composition of nitrogen is higher than hydrogen.