I. A solution was prepared by mixing together a nonelectrolyte solute with 25.5

Question

I. A solution was prepared by mixing together a nonelectrolyte solute with 25.5 g of solvent. Kf = 3.25 oC/m for the pure solvent. The freezing point depression, ?T is 5.40 oC. What is the molality of the solution? Enter your answer as a decimal number.

II. You have four related molecules, XA (mw =140), XB (mw = 100), XC (mw = 80) and XD (mw = 120). None of the four can hydrogen bond. During the evaporation of XB, |?T| = 7.2oC. During the evaporation of XD, |?T| = 4.7oC. What would be the expected |?T| during the evaporation of XA?

III. A solution was prepared by mixing together 6.70 g of a nonelectrolyte solute with 25.0 g of solvent. The cooling curves for both the pure solvent and the solution are shown above. Kf= 3.91 oC/m for the pure solvent. What is the molecular weight (molar mass) of the solute?

Explanation / Answer

Question 1.

The data

m = 25.5 g of solvent = 0.0255 kg

Kf = 3.25

dTf = 5.4

Apply

dTf = -Kf*m

m = mol of solute / kg solvent

substitute

-5.4 = -3.25 * mol of solute / 0.0255

mol of solute = 5.4*0.0255/3.25 = 0.0423692 moles

molality = mol of S / kg solvent = 0.0423692 /0.0255 = 1.6615 molal

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