How many molecules of water are present in 54 g of H2O? (Molecular mass of H2O = 18, NA = 6.023 x 10^23)

1. Number of moles $= \dfrac{54}{18} = 3$ moles. 2. Number of molecules $= \text{moles} \times N_A$. 3. $= 3 \times 6.023 \times 10^{23}$. 4. $= 18.069 \times 10^{23}$ molecules. Using 1 mole (forgetting to divide by 18 correctly) gives $6.023 \times 10^{23}$; using 2 moles gives $12.046 \times 10^{23}$. _Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 7 Atoms and Molecules "Solved Problems", p.108_