If 10²¹ molecules are removed from 200 mg of CO₂, then the number of moles of CO₂ left are

Molar mass of CO₂​ = 44 g

Given mass = 200 mg = 0.2 g

Number of moles = $\frac{0.2}{44}$​= $\frac{1}{220}$​

Number of moles = $\frac{\mathrm{number} \mathrm{of} \mathrm{molecules}}{\mathrm{Avogadro}\text{'}\mathrm{s} \mathrm{number}}$​

⇒ Number of molecules = Avogadro’s number x number of moles

⇒ No. of molecules = 6.022 × 10²³ × $\frac{1}{220}$​ = 2.73 × 10²¹ molecules in $\frac{1}{220}$ ​moles

As 10²¹ molecules  are removed,

∴ No. of molecules left = 2.73×10²¹ − 10²¹ = 1.73 × 10²¹

∴ No. of moles = $\frac{\mathrm{No}. \mathrm{of} \mathrm{molecules}}{\mathrm{Avogadro}\text{'}\mathrm{s} \mathrm{number}}$​ = $\frac{1.73 \times {10}^{21}}{6.023 \times {10}^{23}}$​

                             = 2.87 × 10⁻³