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Study Problems

Experiment 7:

Colligative Properties

> 1. You dissolve 1.2g of sulfur in 15 g of naphthalene (C10H8). If the freezing-point depression constant, Kf, is 6.9 deg C / m and the normal freezing point is 80.6 deg C, what is the new freezing point of the naphthalene solution?

> Answer: Tf is 63.4 deg C

> 2. Suppose you dissolve 2 g of a subtance, CaCl2, in 50 mL of water. Given that the Kf of H2O is 1.86 deg C / m and that the freezing point drops down to -3.0 deg C, into how many particles does CaCl2 dissociate? Assume that your thermometer reads the freezing point as -2 deg C. (The density of water .99823 g / mL at 20 deg C.)

> Answer: i is approximately 3

> (Note, the depression in freezing point is smaller than in the manual.)

> 3. You add .5 g of p-dichlorobenzene to 10 mL of cyclohexane (C6H12) which has a Kb equal to 2.79 deg C / m and a boiling point of 80.7 deg C. Given that the density of cyclohexane is .7785 g / mL and that the new boiling point is 81.9 deg C, what is the molecular weight of p-dichlorobenzene?

> Answer: MW is 149 g/mol

> 4. On what basis would you criticize an experiment studying the colligative properties of water with cyclohexane as the solute?

> Answer: Cyclohexane and water are immiscible.

> 5. In question 2, knowing the formula weight of a solute, the freezing-point depression constant of a solvent, and the resulting depression in freezing point, we were able to calculate the number of particles into which a solute dissociates in solution. We obtained an integer as our answer, called the van't Hoff , but it is not always an integer. Some materials only dissociate partially in solution like salicylic acid. Additionally, as concentration increases, ion-pairing increases. Ion-pairing occurs when ions in solution join together as a result of electrostatic attraction. This further reduces i. Suppose you dissolve a small amount of KHP (potassium hydrogen phthalate) in distilled water. KHP is a weak, monoprotic, organic acid. Give a rough estimate of the value of i.

> Answer: 2 < i < 1