Q.17:- 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.
(i) 15 ppm means 5 parts in million(106) parts.
∴ % by mass = 15/106 × 100 = 15×10-4 = 1.5×10-3 %
(ii) Molar mass of chloroform(CHCl3) = 12+1+(3×35.5) = 118.5 g mol-1
100g of the sample contain chloroform = 1.5×10-3g
∴ 1000 g(1 kg) of the sample will contain chloroform = 1.5×10-2 g
= 1.5×10-2/118.65 mole = 1.266×10-4 mole
∴ Molality = 1.266×10-4 m.