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 don’t 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

Questions

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?