question_answer 21)

A sample of drinking water was found to be severely contaminated with chloroform, CHCl3, supposed to be 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:

15 ppm means 15 parts in million (106) parts by mass in the solution. Percentage by mass                                         = 15 × 10-4 Taking 15 g chloroform in 106 g of the solution, mass of solvent  106 g = 1000 kg Molar mass CHCl3 = 12 + 1 + 3 x 35.5 = 119.5 g mol-1   Moles of solute  = 0.126 mol Molality   = 1.26 × 10?4 mol kg-1 = 1.26 × 10-4m