(solved)Question 1.17 of NCERT Class XI Chemistry Chapter 1

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 15 ppm (by mass). (i) Express this in per cent by mass. (ii) Determine the molality of chloroform in the water sample.
(Rev. 20-Nov-2022)

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Question 1.17 NCERT Class XI Chemistry

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

Video Explanation(detailed solution given after this video)

Please watch this youtube video for a quick explanation of the solution:

Solution in Detail

\begin{aligned} &\underline{\underline{\text{(i) mass percentage}}}\\\\ &\text{Let water be } 10^6 \text{ (1 million) g}\\ &\therefore \text{CHCl}_3 \text{ is } 15 \text{ ppm} = 15 \text{ g}\\\\ & \text{mass of solvent water } = 1000000 \text{ g}\\ & \text{mass of solute CHCl}_3 = 15\text{ g}\\ & \text{total mass of solution } = 1000015\text{ g}\\\\ &\therefore \text{mass }\% = \frac{\text{mass of solute}}{\text{mass of solution}} \times 100\\\\ &\therefore \text{mass }\% = \frac{15}{1000015} \times 100\\\\ &\approx 0.0015 \%\:\underline{Ans}\\\\ &\underline{\underline{\text{(ii) molality}}}\\\\ &\text{molar mass of CHCl}_3 \\ &=12.00 + 1.008 + 35.5 \times 3 \\ &= 119.51 \text{ g/mol (2 decimal)}\\\\ &\text{mols in }15 \text{ g } = \frac{15}{119.51} = 0.125 \text{ mol}\\\\ &\text{mass of solvent } = 10^6 \text{ g} = 1000 \text{ kg}\\\\ &\therefore \text{molality} = \frac{\text{mols of solute}}{\text{mass of solvent in kg}}\\\\ &\therefore \text{molality} = \frac{0.125 \text{ mol}}{1000 \text{ kg}}\\\\ &= 1.25 \times 10^{-4} \text{ m}\:\underline{Ans} \end{aligned}

Concept in Brief(detailed solution after the video below)

PPM of chloroform are given as 15 ppm. This means for every 1 million grams of water there are 15 grams of chloroform.

Water is the solvent, and chloroform is the solute. The solution is made of solute + solvent. Mass percentage is the percent ratio of mass of solute to the mass of solution. Please take note that mass of solution is the combined mass of solute and solvent = 1000000 + 15.

Molality is the ratio of moles of solute to the mass of the solvent. As you can see that there is a fine difference in the two definitions.