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Question
A sample of ferrous oxide has actual formula Fe0.93O1.00. In this sample what fraction of metal ions are Fe2+ ions? What type of nonstoichiometric defect is present in this sample?
Solution
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.
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