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):

(i) express this in percent by mass

(ii) determine the molality of chloroform in the water sample.

Answer 1

(i) 15 ppm (by mass) means 15 parts per million (10⁶) of the solution.

Therefore, percent by mass

= 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.

Molality of the solution

= 1.26 × 10⁻⁴ m