CBSE Class 11-science Answered

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