2.8×10⁻³ mol of CO₂ is left after removing 10²¹ molecules from its ‘x’ mg sample. Find x. (Nₐ = 6.02×10²³ mol⁻¹)

Molar mass of CO₂ = 44 g/mol

Step 1 — Convert removed molecules to moles:

Moles removed = 10²¹ / (6.02×10²³) = 1.661×10⁻³ mol

Step 2 — Initial moles = remaining + removed:

n_initial = 2.8×10⁻³ + 1.661×10⁻³ = 4.461×10⁻³ mol

Step 3 — Initial mass:

mass = 4.461×10⁻³ × 44 = 0.19629 g = 196.3 mg

Answer: x ≈ 196.3 mg

Dividing molecule count by Nₐ gives moles; multiplying by molar mass gives mass the two-step core of all JEE mole-concept problems.