How many molecules of ammonia gas are present in $67.2 \mathrm{dm}^3$, measured at S.T.P.?

How many molecules of ammonia gas are present in $67.2 \mathrm{dm}^3$, measured at S.T.P.?
  1. $2.0 \times 10^{24}$
  2. $1.0 \times 10^{23}$
  3. $1.8 \times 10^{24}$
  4. $5.0 \times 10^{24}$

Solution

$\begin{aligned} & \text { Number of moles of gas }=\frac{\text { Given volume }}{22.4} \\ & =\frac{67.2}{22.4}=3 \mathrm{moles} \end{aligned}$ Number of molecules $=$ moles $\times \mathrm{N}_{\mathrm{A}}$ $\begin{aligned} & =3 \times 6 \times 10^{23} \\ & =18 \times 10^{23} \\ & =1.8 \times 10^{24} \end{aligned}$

Asked in: MHT CET 2021 (24 Sep Shift 1)

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