1. Calculations Involving Colligative Properties
Section 18.4
Calculations Involving Colligative Properties

2. Colligative properties depend on solute concentration
There are two ways to express this:
Molality
Mole Fraction

3. Molality = moles of solute
kilograms of solvent
Pronounced “molal”

4. Example
How many grams of potassium iodide must be dissolved in 500.0g of water to produce a molal KI solution?

5. Mole Fraction
Ratio of # of moles of solute to total # of moles of solute and solvent
XA = nA
nA + nB
XB = nB

6. Example
Compute the mole fraction of each component in a solution of 1.25 mol of ethylene glycol (EG) and 4.00 mol of water.

7. Boiling Point Elevation and Freezing Point Depression
Magnitude of boiling point elevation can be calculated:
ΔTb = Kb x m
Kb = molal boiling point elevation constant
see page 523 for values

8. Example
What is the boiling point of a 1.50m NaCl solution? p523

9. Also freezing point depression
ΔTf = Kf x m
Kf = molal freezing-point depression
See page 524