Calculate the number of molecules present in 11.2 litre of CO2 measured at STP. (NA = 6.023 x 10^23)

1. Number of moles $= \dfrac{11.2}{22.4} = 0.5$ mole. 2. Number of molecules $= \text{moles} \times N_A$. 3. $= 0.5 \times 6.023 \times 10^{23}$. 4. $= 3.011 \times 10^{23}$ molecules. Forgetting to halve gives $6.023 \times 10^{23}$ (one full mole); halving again wrongly gives $1.506 \times 10^{23}$ (0.25 mole). _Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 7 Atoms and Molecules "Solved Problems", p.107_