question_answer 25)

A sample of drinking water was found to be severely contaminated with chloroform, \[CHC{{l}_{3}}\], 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.  

Answer:

15 ppm contamination means; 15 g chloroform is dissolved in \[{{10}^{6}}g\] of solution, i.e., \[{{w}_{B}}=15g\] \[{{w}_{A}}={{10}^{6}}-15\approx {{10}^{6}}g\] Molar mass of solute \[\left( {{m}_{B}} \right)=119.5\] (i) Percentage by mass = \[\frac{\text{mass}\,\text{of}\,\text{solute}}{\text{Mass}\,\text{of}\,\text{solution}}\text{ }\!\!\times\!\!\text{ 100}\] \[=\frac{15}{{{10}^{6}}}\times 100=15\times {{10}^{-4}}\] (ii) Molality, \[m=\frac{{{w}_{B}}\times 1000}{{{m}_{B}}\times {{w}_{A}}}\] \[=\frac{15\times 1000}{119.5\times {{10}^{6}}}\] \[=1.255\times {{10}^{-4}}\]