Question

A sample of ferrous oxide has actual formula Fe_0.93O_1.00. In this sample what fraction of metal ions are Fe²⁺ ions? What type of nonstoichiometric defect is present in this sample?

Answer 1

Let the formula of sample be (Fe²⁺) × (Fe³⁺)_yO.

On looking at the given formula of the compound

x + y = 0.93   ......(1)

Total positive charge on ferrous and ferric ions should balance the two units of negative charge on oxygen. Therefore,

2x + 3y = 2   .....(2)

⇒ `x + 3/2 y` = 1  ......(3)

On subtracting equation (1) from equation (3) we have

`3/2 y - y` = 1 – 0.93

⇒ `1/2 y` = 0.07

⇒ y = 0.14

On putting the value of y in equation (1) we get,

x + 0.14 = 0.93

⇒ x = 0.93 – 0.14

x = 0.79

Fraction of Fe²⁺ ions present in the sample = `0.79/0.93` = 0.81

Metal deficiency defect is present in the sample because iron is less in amount than that required for stoichiometric composition.