Iron pyrite, ${FeS}_{2}$ ( molar mass of $Fe: 56 g {mol}^{- 1}$ ), is converted into ferric oxide and sulphur dioxide. The mass of residue left behind from 1.08 g iron pyrite ignited to a constant mass in air would be

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0.46 g

0.72 g

0.54 g

0.85 g

answer is A.

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Detailed Solution

Molar mass of ${FeS}_{2} = (56 + 2 \times 32) g {mol}^{- 1} = 120 g {mol}^{- 1}$

Molar mass of ${Fe}_{2} O_{3} = (2 \times 56 + 3 \times 16) g {mol}^{- 1} = 160 g {mol}^{- 1}$

The reaction is $2 {FeS}_{2} + \frac{11}{2} O_{2} ⟶ {Fe}_{2} O_{3} + 4 {SO}_{2}$

Hence, the mass of residue will be $\frac{160}{2 \times 120} \times 1.08 g = 0.72 g$

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