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 15ppm (by mass). (i) Express this in percent by mass. (ii) Determine the molality of chloroform in the water sample.

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 15ppm (by mass).
(i) Express this in percent by mass.
(ii) Determine the molality of chloroform in the water sample.

Answer : - 

1)   15 ppm means 15 parts in 1 millions (10^6)

                  and percentage always for 100 

           hence  15 ppm = (15/10^6 )*100 

                                   = 15*10^-4

                                   = 1.5*10^-3 

2) molar mass of chloroform  = 118.5g/ mol 

  from percentage by mass   100 gram of sample has 1.5*10^-3 g of chloroform 

then 1000 gram of sample has = 1.5*10^-3 g * 1000 gram/ 100 gram 

                                                = 1.5*10^-2 g

molality = moles of solute / mass of solvent in Kg

          

moles of solute = mass of solute / molar mass 

                         = 1.5*10^-2/ 118.5  

                         =1.266*10^-4 moles

molality  =     1.266*10^-4 moles/ 1 Kg  = 1.266*10^-4 m