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

(i) 15 ppm means that 15 parts of chloroform is present in `10^(6)` parts.
`therefore %` of `CHCl_(3) = 15/10^(6) xx 100 = 1.5 xx 10^(-3)%`
(ii) Molar mass of `CHCl_(3) = 12 +1+3 xx 35.5 = 119.5`
Moles of `CHCl_(3) = (1.5 xx 10^(-3)g)/(119.5 g mol^(-1))`
`=1.255 xx 10^(-5)`
Mass of water =100 g
`therefore` Molality `=(1.225 xx 10^(-5))/100 xx 1000`
`=1.255 xx 10^(-4)`m