11.65 g of BaSO4 was obtained as a dry precipitate when 10 g of an impure sample of Na2SO4 was dissolved in water and treated with an excess of BaCl2. Calculate the percentage purity of the Na2SO4 sample. (Molar mass of BaSO4=233gandNa2SO4=142g)
A
38 %
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B
71 %
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C
22 %
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D
88 %
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Solution
The correct option is B 71 % Na2SO4+BaCl2→BaSO4↓+2NaCl Moles of BaSO4=given massmolar mass=11.65233=0.05 moles According to the reaction, 1 mole of BaSO4 precipitate is obtained from 1 mole of Na2SO4 0.05 moles of BaSO4 will be produced by 0.05 mole of Na2SO4 Amount of pure Na2SO4 required = 0.05 moles ×142 (gmole)=7.1 g % purity = mass of pure compound in sampletotal mass of impure sample×100=7.110×100 = 71 %