A sample of drinking water was found to be contaminated with chloroform level of 15 ppm by mass.
Calculate the percentage by mass and molality (Given molecular mass of chloroform 119.5 g/mol)

Dear student
The level of contamination is 15 ppm. So 15 g of chloroform is there in 10⁶ g of the water.
And to calculate the mass % , we can use the following formula:
mass % = (mass of the component / total mass of the solution )$\times$ 100
mass % = (15g / 10⁶ g)$\times$ 100
mass % = 15$\times$ 10^-4 %

Now, molarity is defined as the number of moles of solute dissolved in 1 L solution.
Chloroform is CHCl₃ having molar mass 119.5 gmol^-1.
So, number of moles =  mass of the solute/molar mass
number of moles = 15/ 119.5 = 0.1255 moles.
To get the volume of water, we should know the density of water.
Assuming density of water  = 1 g/ mL
density = mass/ volume
volume = 10⁶ / 1
Volume = 10⁶ mL or 10³L
Thus , molarity = 0.1255 / 10³ L
Molarity = 1.25 $\times$10^-4 M
Regards