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chem 20.docx - Post Lab Determination of Molecular Weight...

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Post Lab: Determination of Molecular Weight by Freezing Point Depression Calculations 1. T f Pure Cyclohexane: 6.3 C T f Cyclohexane + Unknown: 4.5 C 2. ΔT = T f − T f = (1000 g/kg) (K f ) (g of solute) / (molar mass solute ,g/mol) (g of solvent) ΔT = 6.3 – 4.5 = (1000)(20.2)(0.140) / (Molar Mass)(9.358) 1.8 = (2828) / (Molar Mass)(9.358) Molar Mass = 167.89 g/mol 3. Code 321I Atomic Weight: 167.89 ID: Biphenyl (C 6 H 5 ) 2 ; Atomic Weight 154.21 % Error = (154.21 – 167.89) / 154.21) X 100 % Error = -8.87 % 4. Molality = moles solute / kg solvent a. Urea Molality = 1.04g / 0.0337 kg = 30.86 Sodium Chloride Molality = 1.051 / 0.03357 kg = 31.31 b. ΔT Urea = 1.25 – 0.20 = 1.05 C ΔT Sodium Chloride = 1.25 - (-1.05 ) = 3.30 C Questions 1. A student’s unknown compound contained an impurity that is insoluble in cyclohexane. What is the effect on the molecular weight determination? Explain. Such an insoluble impurity would cause the determined molecular weight to be slightly larger than the actual molecular weight, for the mass of the solute used in the calculation would be greater than the actual mass.
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