Ice crystallises in a hexagonal lattice. At low temperature at which the structure was determined, the lattice constants were a = 4.53 Ao  and c = 7.41 Ao. How many H2O molecules are contained in a unit cell? (dice = 0.92 g /cm3)?

Density of ice = 0.92 g/cm3Volume of unit cell = a × a sin 60° × c = a2 sin 60° × c=4.53×10−8232×7.41×10−8cm3=131.688×10−24cm3d=Ma3N0∴M (unit cell )=da3N0 Molar mass of H2O=18gmol−1Thus, number of H2O molecules in one unit cell =72.9718≈4