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