Ice crystallises in a hexagonal lattice having a volume of the unit cell as 132 × 10–24 cm3 . If density of ice at the given temperature is 0.92 g cm–3, then number of H2O molecules per unit cell is

Volume =132×10-24cm3   Density  =0.92 g cm-3  Mass of the unit cell = Volume × Density =132×10-24×0.92 =121.44×10−24 g   Molar mass of water =18.02 g mol-1   Number of moles of water =121.44 x 10-2418.02 =6.739×10-24mol   Number of molecules =6.739×10-24×6.023×1023 molecules per mole                                           =4.05 molecules =4 molecules