
Fall 2009 

Exercise: Colligative Properties of Solutions 


Discussion Modules 


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 (CH_{3}COCH_{3}) in a solution made by dissolving 1.00g of a nonvolatile compound (sulfanilamide, MW = 172.1) in 10.0g of acetone. The vapor pressure of pure acetone under these conditions is 4.00 x 10^{3}mmHg.
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 (NH_{2}CONH_{2}, MW = 60.1) dissolved in 10.0 g of pure water at 1.00 atm. (K_{b} = 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 nonpolar solvent) raises the boiling point from 80.2 °C to 82.6 °C. (K_{b} = 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. (K_{b} = 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. (K_{f} = 5.12 °C m^{1})
7. What is the freezing point of a 0.100 m aqueous solution of aluminum chloride? (K_{f} = 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? (K_{f} = 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 110
In addition to these exercises you should familiarize yourself with the text materials.
© R A Paselk
Last modified 7 November 2009