Calculate the number of molecules in 1½ litres of water. [density of water 1.0 g./cc. ∴ mass of water = volume × density]

Vol of water = 1.5 lit = 1500 mL =1500 cc

Density of water 1.0 g./cc..

∴ Mass of water = volume × density

Hence, mass of water = 1500 × 1 = 1500 g

Gram molecular mass of water

= 2H + O

= (2 × 1) + 16 = 18 g

So, 18 g of water = 6.023 × 10²³ molecules

Therefore, 1500 g of water will have

= 6.023 × 10^{23}\over 18×1500

= 1500\over 18×6.023 × 10²³

= 83.33 × 6.023 × 10²³

Hence, the number of molecules in 1½ litres of water is 83.33 × 6.023 × 10²³ molecules.