Homework Help: Chemistry II

Posted by Lindsay on Tuesday, September 8, 2009 at 8:43pm.

An unknown solute (a nonelectrolyte) was obtained and 5.37 g was weighed out. After dissolving the solute in water, the mass of the solution was 26.58 g. From the experimental results, the freezing point depression was found to be 3.6oC. If the freezing point depression constant, Kf, for water is 1.86oC.kg/mol, what is the molar mass of the unknown solute?

Chemistry II - DrBob222, Tuesday, September 8, 2009 at 11:40pm
delta T = i*Kf*molality
First, I don't know about the value you show for Kf for water. It is 1.86 degrees C/molal and not kg/mol.
You know delta T, i = 1, Kf = 1.86, calculate molality.
Then molality = moles/kg solvent. You now know molality and kg solvent (26.58 g - 5.37 g = ?? changed to kg). Calculate moles.
Then moles = grams/molar mass. You know moles and grams, calculate molar mass.

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