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# In the 17th question of 1 st chapter :A sample of drinking water.......Contaminated by CHCl3....The level of contaminaton is 15 ppm..For (ii) I have a doubt that if molality = No. of moles of solute/Mass of solvent in kg. Then by which logic in the Answers the mass prcent is dvided by molar mass of CHCl3 aithough knowing that CHCl3 is not a solvent here because here the solvent is water...

Asked by nitishkrnehu09 3rd July 2016, 10:14 PM
Answered by Expert
Answer:

Molality = Number of mole of solute / mass of solvent in Kg -----(1)

We already have the mass % is 15x10-4

Total mass of solution = 100 gram

Mass of solute (CHCl3)   = 15x10-4  g

Now Mass of solvent = Mass of solution – Mass of solute

(Since the mass of solute is very small compared to total mass of solution so neglecting we get)

Mass of solvent = Mass of solution

Mass of solvent = 100 g

Now let us find the number of moles of solute

Number of moles of solute = mass of solute / molar mass ---(2)

Let us calculate the molar mass of solute (CHCl3)

Molar mass of CHCl3    =12+1+106.5

=119.5

Substitute the values in equation (2)

No of moles of chloroform              = 15x10-4 /119.5

=1.25x10-5

Now substitute the value of number of moles and mass of solvent in equation (1)

Mass of solvent = 100 g

Mass of solvent = 0.1Kg

Molality       = 1.25X10-5/0.1= 1.25X10-4 m

Answered by Expert 4th July 2016, 6:13 PM
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