A sample of drinking water was found to be severely contaminated with chloroform, `CHCl_(3)`, supposed to be carcinogenic in nature. 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 (by mass) means 15 parts by mass of `CHCI_3` are present in `10^6` parts by mass of water.
`therefore` Per cent by mass `=15/10^(6) xx 100 = 1.5 xx 10^(-3) %`
(ii) Molar mass of `CHCl_(3) = 12 +1 + (3 xx 35.5) = 119.5 g "mol"^(-1)`
`therefore 10^(6)` g of sample contain chloroform = 15 g
`therefore 10^(3)` g (1 kg) of sample will contain chloroform `=15/10^(6) xx 10^(3) = 1.5 xx 10^(-2) g`
Number of moles of chloroform present in 1 kg of the sample
`=(1.5 xx 10^(-2))/119.5`
`=1.255 xx 10^(-4)` mole
`therefore` Molality `=1.255 xx 10^(-4)` m