Question

Find the molecular mass of a solute by freezing point depression. Solvent: para-dichlorobenzene Freezing point of pure solvent: 53.02C Mass of unknown substance: 2.04g Freezingpointdepressin constant:7.1c/m Mass of para-dichlo...: 24.80g Freexing point solution: 50.78 C Can you explain how you got it, Please and thank you

Answer 1

delta T = Kf*molality.
Calculate molality.

molality = moles/kg solvent.
Calculate moles.

moles = grams/molar mass.
Calculate molar mass.

Answer 2

260.73 g/mol

Temp of solvent - Temp of solution = constant x molality

solve for molality.
molality = mol of solute/kg of solvent

molality = change in Temp/constant

molality = (53.02-50.78)/7.1 = 0.3155 mol/kg

Convert to mol/g, so divide by 1000g.

molality = 3.155e-4 mol/g

multiply molality by grams of solvent to get moles of solute.

3.155e-4 mol/g x 24.8 g = 0.007824 mol

Now you have moles of solute. Solve for molecular mass by diving the grams of the solute by the calculated moles of solute.

Molecular mass of solute = 2.04 g/0.007824 mol = 260.73 mol/g of solute.