Question

A cylinder contains $68\text{g}$ of ammonia gas at S.T.P. How many moles of ammonia are present in the cylinder?

$\left[\mathrm{N}-14,\mathrm{H}-1\right]$

Answer 1

Step 1: Given information

$68\text{g}$ of ammonia $\left({\text{NH}}_3\right)$ at S.T.P

Step 2: S.T.P

• S.T.P indicates standard temperature and pressure
• At S.T.P, one mole of a gas has a volume of $22.4\text{L}$.

Step 3: Calculation of molecular weight of ${\mathrm{NH}}_3$

Molecular weight of ${\mathrm{NH}}_3$

$$\begin{gathered} \text{=N}+3\text{H} \\ =\left(14+3\times 1\right)\text{g} \\ =17\text{g} \end{gathered}$$

Step 4: Calculation of moles of ${\mathrm{NH}}_3$

Since, $17\text{g}$ of ammonia contains $1\text{mole}$ of ${\mathrm{NH}}_3$.

Therefore, $68\text{g}$ of ammonia will contain $\frac{1}{17}\times 68\text{=4 mole}$ of ${\mathrm{NH}}_3$.

Hence, $4\text{moles}$ of ammonia are present in the cylinder.