Humboldt State University ® Department of Chemistry

Richard A. Paselk


General Chemistry

Spring 2011

Exercise: Colligative Properties of Solutions

© R. Paselk 2008

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Colligative Properties Problems

Remember that Colligative properties depend only on the number or concentration of particles in a solution. The properties are, for ideal solutions, independent of the kind or size of the particles, whether ionic, large or small etc. As a consequence:

In these exercises we will be using the equations for the various colligative properties we have studied:

Recall also the definitions of the three concentration measures fundamental to solution colligative behaviors:

1. Find the vapor pressure of acetone (CH3COCH3) in a solution made by dissolving 1.00g of a non-volatile compound (sulfanilamide, MW = 172.1) in 10.0g of acetone. The vapor pressure of pure acetone under these conditions is 4.00 x 103mmHg.

2. Determine the total vapor pressure over a solution of 0.600 moles of toluene and 0.400 moles of benzene @ 60 °C assuming ideal behavior. The vapor pressures of pure toluene and benzene are, respectively, 139 mmHg and 392 mmHg @ 60°C.

3. Find the boiling point of a solution of 0.300 g urea (NH2CONH2, MW = 60.1) dissolved in 10.0 g of pure water at 1.00 atm. (Kb = 0.512 °C m-1)

4. Calculate the molecular weight of an unknown substance if dissolving 7.39 g in 85.0 g of benzene (a non-polar solvent) raises the boiling point from 80.2 °C to 82.6 °C. (Kb = 2.52 °C m-1)

5. Calculate the boiling point of a 0.250 m aqueous solution of iron(III) chloride at 1.00 atm. (Kb = 0.512 °C m-1)

6. Calculate the molecular weight of an unknown substance if dissolving 1.42 g in 25.0 g of pure benzene lowers the freezing point by 1.96°C. (Kf = 5.12 °C m-1)

7. What is the freezing point of a 0.100 m aqueous solution of aluminum chloride? (Kf = 1.855 °C m-1)

8. A 0.150 m solution of acetic acid in water is found to have a freezing point of - 0.28 °C. What is the concentration of hydrogen ions in this weak acid solution? (Kf = 1.855 °C m-1)

9. Find the osmotic pressure of a 0.500 M solution of sodium chloride at 0 °C.

10. In order to find the molecular weight of hemoglobin 0.500 g was dissolved in enough water in a volumetric flask to give 100.0 mL of solution. The osmotic pressure of this solution was then measured at 25°C and found to be 1.35 mmHg. Calculate the moleculaar weight.

Answers 1-10

In addition to these exercises you should familiarize yourself with the text materials.

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© R A Paselk

Last modified 17 April 2009