10²¹ molecules are removed from 200 mg of CO₂. The moles of CO₂ left is

$$\begin{gathered} {10}^{21}\text{ molecules }=\frac{{10}^{21}\text{ molecule }}{6.022\times {10}^{23}\text{ molecule }/\mathrm{mol}}=1.66\mathrm{x}{10}^{-3} \mathrm{mol} \\ 200 \mathrm{mg}\text{ of }{\mathrm{CO}}_2=\frac{200\times {10}^{-3}\mathrm{g}}{44\mathrm{g}/\mathrm{mol}}=4.54\times {10}^{-3}\mathrm{mol}. \end{gathered}$$
The number of moles of CO₂ remaining = (4.54 -1.66) x 10^-3 = 2.88 x 10^-3.