A sample of drinking water was found to be severely contaminated with chloroform \((CHCl_3)\) supposed to be a carcinogen. 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.
(i) express this in percent by mass
(ii) determine the molality of chloroform in the water sample.
Solution and Explanation
Therefore, percent by mass \(=(\frac{15}{10^6})\times100\% \)
\(= 1.5 \times 10^{ - 3} \%\)
(ii) Molar mass of chloroform \((CHCl_3) = 1\times12 + 1\times1 + 3\times35.5\)
\(= 119.5 \,g mol^{ - 1}\)
Now, according to the question,
15 g of chloroform is present in \(10^6 g\) of the solution.
i.e., 15 g of chloroform is present in \((10^6 - 15) ≈ 10^6 g\) of water.
Molality of the solution \(=\frac{(\frac{15}{119.6} mol) }{(10^6\times10^{-3})}\)
\(=1.26 × 10^ {- 4} m\)
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Concepts Used:
Concentration of Solutions
It is the amount of solute present in one liter of solution.
Concentration in Parts Per Million - The parts of a component per million parts (106) of the solution.
Mass Percentage - When the concentration is expressed as the percent of one component in the solution by mass it is called mass percentage (w/w).
Volume Percentage - Sometimes we express the concentration as a percent of one component in the solution by volume, it is then called as volume percentage
Mass by Volume Percentage - It is defined as the mass of a solute dissolved per 100mL of the solution.
Molarity - One of the most commonly used methods for expressing the concentrations is molarity. It is the number of moles of solute dissolved in one litre of a solution.
Molality - Molality represents the concentration regarding moles of solute and the mass of solvent.
Normality - It is the number of gram equivalents of solute present in one liter of the solution and it is denoted by N.
Formality - It is the number of gram formula present in one litre of solution.