Question

A grounwate has been contaminated with carbon tetrachloride at a level of 250 ppm (by mass). Assuming equilibrium with the air in the soil right above the groundwater table, calculate the concentration of carbon tetrachloride in the air space. Give your answer in ppm (by volume). The Henry's Law constant for carbon tetrachloride is 3.0 kPa/ (gmol/m^3) and the total air pressure in the soild is 1,013 kPa.

Answer 1

Molecular weight of carbon tetrachloride= 153.82 g/mol

Given, carbon tetrachloride level in ground water = 250 ppm (by mass)= 250mg/lit

                                                                                                                 =250/153.82= 1.625 gmol/m^3

Henry's Law constant for carbon tetrachloride=3.0kPa/(gmol/m^3)

Henry's law:   p=H*c

Where p= partial pressure of carbon tetrachloride in air

            H= Henry's law constant

            c= concentration of carbon tetrachloride in water

            p= 3*1.625=4.875 kPa

             Given, total air pressure in the solid= P_t= 1,013 kPa

             Mole fraction of carbon tetrachloride in air= p/ P_t

                                                                               = 3/1013= 0.0048124

             The above value in ppm (by volume)= 0.0048124*10^6=4812.43