3 molecules of ${\mathrm{CO}}_2 \mathrm{weighs}$

1 Mole ${\mathrm{CO}}_2=6\times {10}^{23} \mathrm{molecles} \mathrm{of} {\mathrm{CO}}_2$

 We know that  $6\times {10}^{23} \mathrm{molecles} \mathrm{of} {\mathrm{CO}}_2 \mathrm{has} 44\mathrm{g} \mathrm{of} {\mathrm{CO}}_2$

3 molecules  ${\mathrm{CO}}_2\to ? \mathrm{g} \mathrm{of} {\mathrm{CO}}_2$

Then weigh of ${\mathrm{CO}}_2$ in 3 molecules,

$\frac{44\mathrm{g}}{6\times {10}^{23} \mathrm{molecules} \mathrm{of} {\mathrm{CO}}_2}\times 3 molecules C{O}_2=22\times {10}^{-23}g$