Ch 15.2 & 15.3 molality and Colligative Properties Tutorial Std 6e


You have used M (molarity) before.


mole (solute)

M = --------

L (solution)


In this chapter you have m (molality). Note the difference, replaces r in molarity with l to make it molality. Instead of big M it is a lower case m. Also notice in M you use L of solution, where in m you use Kg of solvent.


mole (solute)

m = --------

Kg (solvent)



Colligative Properties

Physical properties of a solution that are affected by the number of particles of solute in solution are called colligative properties.


We use m (molality) predict the exact change in freezing & boiling points when we add solutes to solvents. Particles (solute) in a solution decrease freezing points and increase boiling points. See phase diagram p. 473.


Boiling & freezing point changes are determined by # of particles of solute.


ionic solutes like NaCl, give 2 moles of particles (ions) for every mole of molecules


NaCl -----> Na1+ + Cl1- (2 for 1)


non-ionic solutes like sugar dissolve in water but dont form ions. For non-ionic solutes 1 mol of solute gives one mole of particles. (1 for 1)


(see page 2)




Boiling Point Elevation


ΔTb = Kbm = (+) change in boiling point Remember m = molality

Kb can be found in Table 15-4 on p. 472 for 5 different solvents.

Unit of measure for Kb = C/m


Freezing Point Depression


ΔTf = Kfm = (-) change in freezing point Remember m = molality

Kf can be found in Table 15-5 on p. 474 for the same 5 solvents.

Unit of measure for Kf = C/m



1. What is the unit of measure for Kb and Kf ?

2. Compare Tables 15-4 (p. 472) and 15-5 (p. 474). Does a 1m solution have the same value for both Kb and Kf ?

3. Which is larger for a 1 m solution of water, Kb and Kf ?

4. Is Kf larger than Kb for all 5 of the 1m solutions given in Tables 15-4 & 15-5?

5. NaCl is an electrolyte. How many times larger will the value of ΔTb and ΔTf be for NaCl vs sugar?

6. How many times do ΔTb and ΔTf increase for a 1m solution of AlCl3 vs a 1m solution of sucrose, if the AlCl3 dissolves completely?