Question

A cylinder contains $68\mathrm{g}$ of ammonia gas at s.t.p.
(1) What is the volume occupied by this gas?
(2) How many moles of ammonia are present in the cylinder?
(3) How many molecules of ammonia are present in the cylinder? [N-14, H-1]

Answer 1

Cylinder contains ammonia $=68\mathrm{g}$

(1) Molecular mass of ammonia $=\left[{\mathrm{NH}}_3\right]$ $=14+3=17\mathrm{g}$

• $17\mathrm{g}$ of ammonia at s.t.p. occupies volume $=22.4\mathrm{L}$
• $1\mathrm{g}$ of ammonia at s.t.p. occupies volume $=\frac{22.4}{17}\mathrm{L}$
• $68\mathrm{g}$ of ammonia at s.t.p. occupies volume $=\frac{22.4}{17}\times 68=89.6\mathrm{L}$
• Therefore, the volume occupied by the gas is $89.6\mathrm{L}$.

(2) $17\mathrm{g}$ of ammonia $=1$ mole

• $1\mathrm{g}$ of ammonia $=\frac{1}{17}$ mole
• $68\mathrm{g}$ of ammonia $=\frac{1}{17}\times 68$ mole $=4$ mole of ammonia

(3) $1$ mole contains $=6.022\times {10}^{23}$ molecules

• $4$ mole contains $=4\times 6.022\times {10}^{23}$ $=24.088\times {10}^{23}$ molecules