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Question:

A sample of drinking water was found to be severely contaminated with chloroform, CHCl3, supposed to be carcinogenic in nature. The level of contamination was 15 ppm (by mass). (i) Express this in percent by mass. (ii) Determine the molality of chloroform in the water sample.

Solution:

(1) 15 parts of chloroform in 1,000,000 parts of water corresponds to 15 ppm.
Percentage by mass of chloroform = 15 × 100 / 1,000,000 = 1.5 × 10⁻³ %