Question

The number of atoms present in 4.25 g of $\left({\mathrm{NH}}_3\right)$ is approximately.

a) $1\times {10}^{23}$

b) $1.5\times {10}^{23}$

c) $0.25\times {10}^{23}$

d) $6.02\times {10}^{23}$

• A. 1 X 10²³
• B. 1.5 X 10²³

Answer 1

The correct option is D

$6.02\times {10}^{23}$

Given mass of ammonia$\left({\mathrm{NH}}_3\right)$ = 4.25 g
Gram molecular mass of ammonia$\left({\mathrm{NH}}_3\right)$ = 17 g/mol
Number of moles of ammonia $=\frac{\mathrm{Mass}\mathrm{of}\mathrm{ammonia}}{\mathrm{Gram}\mathrm{molecular}\mathrm{mass}\mathrm{of}\mathrm{ammonia}}$
$$\begin{gathered} =\frac{4.25\mathrm{g}}{17\mathrm{g}/\mathrm{mol}} \\ =0.25\mathrm{mol} \end{gathered}$$
1 mole of ammonia contains $6.02\times {10}^{23}$ molecules of ammonia
∴ Number of molecules present in 0.25 mol of ammonia $=0.25\times 6.022\times {10}^{23}$
$=1.5\times {10}^{23}$ molecules
One molecule of ammonia has 4 atoms (1 of nitrogen and 3 of hydrogen).
∴ Number of atoms present in $1.5\times {10}^{23}$ ammonia molecules $=4\times 1.5\times {10}^{23}$ $=6.022\times {10}^{23}$ atoms