Advance Study Assignment: Determination of Molar Mass by Depression of the Freez

Question

Advance Study Assignment: Determination of Molar Mass by Depression of the Freezing Point

1. A student determines the molar mass of methanol, CH3OH, by the method used in this experiment. She found that the equilibrium temperature of a mixture of ice and pure water was 0.4°C on her thermometer. When she added 10.0 g of her sample to the mixture, the temperature, after thorough stirring, fell to 5.4°C. She then poured off the solution through a screen into a beaker. The mass of the solution was 101.8 g.

Whatwasthefreezingpointdepression?

What was the molality of the methanol?

Howmuchmethanolwasinthedecantedsolution?

How much water was in the decanted solution?

______________°C

______________ m

______________g

______________ g

Howmuchmethanolwouldtherebeinasolutioncontaining1kgofwater,withmethanolatthesame concentration as she had in her experiment?

Explanation / Answer

Mass of methanol in the solution = 10 g

Let the molar mass of methanol = 'M' g/mole

Thus, moles of methanol in the solution = mass/molar mass = 10/M

Mass of solvent = 101.8 - 10 = 91.8 g = 0.0918 kg

Thus, molality of solution = moles of methanol/mass of solvent in kg = (10/M)/0.0918 = 108.93/M

Now, depression in F.P = -5.4 - 0.4 = -5.8

Thus, depression in F.P = molality*Kf of water

or, 5.8 = (108.93/M)*5.12

or, molar mass of methanol,M = 96.16 g/mole

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