Iron (II) oxide has a cubic structure and each unit cell has side 5A. If the density of the oxide is 4 g cm–3, calculate the number of Fe2+ and O2– ions present in each unit cell. (Molar mass of FeO = 72 g mol–1, NA = 6.02 x 1023 mol–1).
Solution: We have given volume of the unit cell = (5 x 10-8cm)3 = 1.25 x 10-22cm3
Density of FeO = 4g cm-3
Each unit cell has four units of FeO. So it has four Fe2+ and four O2– ions.
Why do solids have a definite volume?
A solid have a definite volume because the inter molecular distance between its molecules are fixed.