A sample of drinking water was found to be severely contaminated with chloroform `(CHCl_3)`, supposed to be carcinogen. The level of contamination was 15 ppm (by mass) : (i) Express this in per cent by mass. (ii) Determine the molality of chloroform in the water sample.

Explanation

15 ppm means 15 parts in one million` (10^6) ` parts by mass in the solution
` therefore ` Percentage by mass = `(15)/(10^6) xx 100 = 15 xx 10^(-4)`
Taking 15 g chloroform in `10^6` g of the solution, mass of solvent `~~10^6 g` [Neglecting 15 in comparison with `10^6`]
Molar mass of `CHCl_3 = 12 + 1 + 3 xx 35.5 = 119.5 g "mol"^(-1)`
molality ` = (15// 119.5)/(10^6) xx 1000 = 1.25 xx 10^(-4) m`