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

15 ppm means 15 parts in million (10⁶) parts by mass in solution 

∴ % by mass = \(\frac{15}{10^6}\) × 100 = 15 × 10^-4 

Taking 15 g chloroform in 10⁶g 

Molar mass of CHCl₃ = 12 + 1 + 3 × 35.5 = 119.5 g mol^-1 

Molality = \(\frac{\frac{15}{119.5}}{10^6}\) × 1000 = 1.25 × 10^-4 m