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Question Q163) 20.0 g of a magnesium carbonate sample decomposes on heating to give carbon dioxide and 8.0 g magnesiumoxide. What will be the percentage purity of magnesium carbonate in the sample? (At. wt. : Mg = 24) [Re-AIPMT-2015]



SOLUTION

MgCO3 → MgO + CO2
One mole of MgCO3 gives one mole each of MgO and CO2
Molecular weight of MgCO3 = 24+12+3(16) = 84g
Molecular weight of MgO = 24+16 = 40g
40g of MgO = 84 MgCO3
∴ 8 g of MgO = (84/5) g MgCO3 = 16.8 g
But sample is 20g
20g sample = 16.8 MgCO3
100g same = 16.8 × 5 = 84 MgCO3
Answer : (b)