Question

What will be the molarity of chloroform in the water sample which contains 15 ppm chloroform by mass?

Answer 1

To answer this question first we have to find the weight of chloroform present in water using the density of water which is the ratio of mass to volume and then we can easily convert this value to the molarity of water.

Complete step by step solution:
Molarity is defined as the number of moles of solutes present in per mass of the solvent in kilograms.
Given in the question:
The concentration of chloroform = 15ppm
We can say that ${{10}^{-6}}$of the solution of chloroform contains 15 g of chloroform
Thus 1000 g of the solution contains $\dfrac{15}{{{10}^{6}}}(1000)$= 15 $({{10}^{-3}})g$ chloroform
Molarity is defined as the number of moles of solutes present in per mass of the solvent in kilograms.
And the number of moles is calculated by the ratio of given mass to the molar mass
Given mass of chloroform =15 $({{10}^{-3}})g$
Molar mass of chloroform will be equal to the sum of atomic mass of carbon, atomic mass of hydrogen and atomic mass of chlorine.
Molar mass of chloroform = 119.5 g/mol
Now the weight of water = 1000g or 1 kg
The molarity of the solution will be =$\dfrac{15({{10}^{-3}})}{119.5(1)}$= 0.124$({{10}^{-3}})g$
The molarity of chloroform in the water sample which contains 15 ppm chloroform by mass 0.124$({{10}^{-3}})g$

Note:
We should know that the concentrations expressed in molality are used when studying the properties of the solutions related to the vapour pressure and the temperature changes. Molarity is used because the value of molarity does not changes with change in temperature.$$