Data Sheet 3 IlL. Determining the Molar Mass of an Unknown unknown identificatio

Question

Data Sheet 3 IlL. Determining the Molar Mass of an Unknown unknown identification code mass of test tube, g mass of test tube plus naphthalene. g mass of test tube plus naphthalene plus unknown, g mass of naphthalene,9 mass of unknown, g sal 30.009 oog time, min temperature, time, min temperature, 4ac_ 13.0 uC14.0 4.0 33 C 3.3 a.o temperature at which crystals first appeared, °C maximum temperature attained after crystals appeared, °C freezing point of naphthalene/unknown solution, "C (olution freezing point depression, °C (At) molality of naphthalene/unknown solution, movkg (m) experimental molar mass of unknown, g/mol actual molar mass of unknown, g/mol percent error in the determination, % 3O

Explanation / Answer

Determining molar mass of unknown by freezing point depression

mass of unknown = 1.012 g

mass of naphthalene = 5.136 g = 0.005136 kg

molality of solution (m) = 1.012/molar mass of unknown x 0.005136 kg

depression in freezing point (dTf) = 9.5 oC

So using,

dTf = Kf.m

Kf for naphthalene = 6.91 oC/m

or,

9.5 = (1.012/molar mass of unknown x 0.005136)(6.91)

molar mass of known = 143.32 g/mol

literature molar mass of unknown is not given and thus percent error we cannot calculate in such a case.

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