Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: devvaibhav on June 11, 2011, 10:08:09 AM
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Q:- An impure sample of NaCl, which weighed 0.50 g, gave on treatment with excess of AgNO3 solution, 0.90 g of AgCl as precipitate. Calculate the percentage purity of the sample.
Reaction: NaCl + AgNO3 :rarrow: NaNO3 + AgCl
The formula for percentage purity is : (Pure/Total wt.) * 100
Is this correct?
My attempt to the solution: 143.5 g of AgCl will be given by = 58.5 g of pure NaCl
Therefore, 0.9 g of Agcl will be given by = (58.5/143.5)*0.9 = 0.36 g of pure NaCl
We have 0.36 g pure NaCl(by calculation) and total wt. = 0.50 g.
Therefore, percentage purity= (0.36/0.50)*100 = 72 %
But it is incorrect. The correct answer is 73.4 %
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Your approach is correct, but don't round down intermediate values.
Technically correct answer should be 73%, as it will have two significant figures, like data. But significant figures are not that important.