The number of chlorine atoms in $20\mathrm{mL}$ of chlorine gas at STP is ………${10}^{21}$.

Step 1: Given data

1. Volume$\left(\mathrm{V}\right)=20\mathrm{ml}$
2. Constant$\left(\mathrm{R}\right)=0·083\mathrm{L}\mathrm{bar}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-1}$
3. Temperature$\left(\mathrm{T}\right)=273\mathrm{K}$

Step 2: Formula Used

• The number of moles can be computed using the following formula:
• $\mathrm{PV}=\mathrm{nRT}$
• Where $\mathrm{P}=$Pressure, $\mathrm{V}=$Volume, $\mathrm{n}=$Number of moles, $\mathrm{R}=$constant and $\mathrm{T}=$Temperature

Step 3: Compute the number of moles

• Substitute the known values in the formula $\mathrm{n}=\frac{\mathrm{PV}}{\mathrm{RT}}$,
• $$\begin{gathered} \mathrm{n}=\frac{1\times 20\times {10}^{-3}}{0·083\times 273} \\ \mathrm{n}=0·882\times {10}^{-3} \end{gathered}$$

Step 4: Determine the number of atoms

• $\begin{array}{rcl}\mathrm{The}\mathrm{number}\mathrm{of}\mathrm{atoms}& =& 0·882\times {10}^{-3}\times 2\times 6·023\times {10}^{23}\\ & =& 1·06\times {10}^{21}\mathrm{atoms}\end{array}$