Question

If a 2.20g of a molecular substance is dissolved into 100g of water and the freezing point of the water lowers to -1.2 degrees C, what is the molecular weight of the substance?

Answer 1

we have below equation to be used:

delta Tf = Kf*mb

1.2 = 1.86 *mb

mb= 0.6452 molal

mass of solvent = 100 g

= 0.1 kg [using conversion 1 Kg = 1000 g]

we have below equation to be used:

number of mol,

n = Molality * mass of solvent in Kg

= (0.6452 mol/Kg)*(0.1 Kg)

= 6.452*10^-2 mol

mass of solute = 2.20 g

we have below equation to be used:

number of mol = mass / molar mass

6.452*10^-2 mol = (2.2 g)/molar mass

molar mass = 34.1 g/mol

Answer: 34.1 g/mol