Question

A solution containing 0.200g of an unknown substance in 2.50g of cyclohexane was found to freeze at 5.1^oC. What is the molar mass of the unknown substance? Show your work.

Answer 1

Freezing point depresion = kf*m*i, m =molality, i= van't Hoff factor for cyclohexane= 1

Meltinng point of cyclhexane =Freezing point = 6.55 deg.c

Freezing point depression= -6.55+5.10=-1.45 deg.c

1.45= 20*m*1

m= 1.45/20 =0.0725 moles/kg solvent

1 kg =1000 kg solvent contains 0.0725 kg moles of solute (unknown compound)

2.5 gms cyclohexane contains 0.0725*2.5/1000 =0.000181 moles

Moles = mass of unknown compounds/ Molecular weight= 0.000181

0.2/Molecular weight =0.000181

Molecular weight =0.2/0.000181=1103.448 =Molar mass