Honors Chemistry                                                                                Name____________________________

Colligative Properties PMP

Mr. McAfoos                                                                                       Period_____ Date__________________

 

Solutions are more stable than pure solvents, that is to say that the liquid phase of a solution freezes at a lower temperature and boils at a higher temperature than the pure solid does. These changes are called freezing point depression and boiling point elevation. The change in the freezing point or boiling point can be calculated with the following formula:

 

                                ΔT = kf m #            or            ΔT = kb m #

 

Where D means change in, kf is the freezing point depression constant and kb is the boiling point elevation constant (both known for a given solvent), m is the molality (see below) of the solution and # is the number of particles that the solute breaks into when it dissolves. For instance NaCl breaks into Na+1 and Cl-1, so # = 2 but MgCl2 breaks into Mg+2 and 2 Cl-1's so # = 3.

 

Molality (m) is a different unit of concentration. Remember that molarity (M) is mols solute/ liter of solution, molality is mols solute/ kilogram of solvent.

 

Given this formula and the appropriate information you can solve many different problems, such as the ones below.

 

1.        What is the freezing point of the solution of 92.1 g of I2 in 800.0 g of CHCl3? (Kf = 4.68oC/m, normal freezing point of CHCl3 = 4.5oC)

 

 

 

 

 

2.        What is the boiling point of the solution created when 1.00 g of glycerin (C3H8O3) is dissolved in 47.8g of water? (Kb = 0.512oC/m)

 

 

 

 

 

3.        What is the freezing point of 0.724 g of calcium chloride in 175 g of water? (Kf=1.86oC/m)

 

 

 

 

 

 

4.        A solution of 35.7 g of a non-electrolyte (i.e. it's not ionic) in 220.0g of chloroform has a boiling point of 64.5oC What is the GMM of the compound? (the normal bp of chloroform = 61.26oC, kb=5.02oC/m)

 

 

 

 

 

 

5.        When 4.00 g of a certain non-electrolyte is dissolved in 55.0 g of benzene, the resulting solution freezes are 2.32oC. What is the GMM of the compound? (Normal freezing point of benzene = 5.12oC, kf=5.12oC/m)