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Question
A sample of drinking water was found to be severely contaminated with chloroform (CHCl3) supposed to be a carcinogen. The level of contamination was 15 ppm (by mass):
- express this in percent by mass.
- determine the molality of chloroform in the water sample.
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Solution
(i) 15 ppm (by mass) means 15 parts per million (106) of the solution.
∴ Percent by mass = `15/10^6 xx 100`
= 1.5 × 10−3
(ii) Molar mass of chloroform (CHCl3) = 1 × 12 + 1 × 1 + 3 × 35.5
= 119.5 g mol−1
Now, according to the question,
15 g of chloroform is present in 106 g of the solution.
i.e., 15 g of chloroform is present in (106 − 15) ≈ 106 g of water.
Number of moles of CHCl3 = `"Mass (in g)"/"Molar mass"`
= `15/119.5`
∴ Molality of the solution = `(15/119.5 "mol")/(10^6) xx 1000`
= 1.25 × 10−4 m
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