Chloroform has a boiling point of 61.1°C and dichloromethane has a boiling point of 40.0°C.When 0.100 mol of dichloromethane is added to 0.900 mol of chloroform,the resulting solution will have a boiling point
A)between 40.0°C and 61.1°C,but closer to 61.1°C.
B)between 40.0°C and 61.1°C,but closer to 40.0°C.
C)greater than 61.1°C.
D)less than 40.0°C.

Question

Quizplus · Accepted Answer

The boiling point elevation, ΔTb, is given by the formula:

ΔTb = Kb x m

where Kb is the molal boiling point elevation constant (which is a characteristic of the solvent, in this case chloroform), and m is the molality of the solution (which is the number of moles of solute per kilogram of solvent).

Since we are adding 0.100 mol of dichloromethane to 0.900 mol of chloroform, the total number of moles of solute is:

n = 0.100 mol + 0.900 mol = 1.000 mol

The molality of the solution is:

m = n / (mass of solvent in kg)

We can assume that the mass of the solvent is equal to the sum of the masses of chloroform and dichloromethane, because the densities of the two solvents are similar (1.48 g/mL for chloroform and 1.33g/mL for dichloromethane). Therefore:

mass of solvent = (0.900 mol x 119.38 g/mol) + (0.100 mol x 84.93 g/mol) = 110.44 g

m = 1.000 mol / (110.44 g / 1000 g/kg) = 9.06 mol/kg

The molal boiling point elevation constant for chloroform is 3.63 °C/m, so:

ΔTb = 3.63 °C/m x 9.06 mol/kg = 32.91 °C

The boiling point of the solution will be the sum of the boiling point of chloroform and the boiling point elevation:

Boiling point of solution = 61.1°C + 32.91°C = 94.01°C

Therefore, the solution will have a boiling point between 40.0°C and 61.1°C, but closer to 61.1°C. Choice A is the correct answer.

Chloroform Has a Boiling Point of 61