1. CALCULATIONS INVOLVING COLLIGATIVE PROPERTIES
Section 18.4

2. AFTER READING SECTION 18.4, YOU SHOULD KNOW:
How to calculate molality (m)
How to calculate the molar mass of a molecular compound from the freezing point depression or boiling point elevation

3. MOLALITY
Molality (m) – the number of moles of solvent dissolved per kilogram of solvent
m = mol solute / kg solvent
Mole fraction
nA = moles of solute A nB moles of solvent B
XA = nA XB = nB
nA + nB nA + nB

4. BOILING POINT ELEVATION
Change in boiling point temperature
ΔTb = Kb * m
ΔTb = change in boiling pt temp
Kb = molal boiling-point elevation constant
Depends on the solvent
Units (oC / m)
m = molality (mole / kg)

5. FREEZING POINT DEPRESSION
Change in freezing point temperature
ΔTf = Kf * m
ΔTf = change in freezing pt temp
Kf = molal freezing-point depression constant
Depends on the solvent
Units (oC / m)
m = molality (mole / kg)

6. AFTER READING SECTION 18.4, YOU SHOULD KNOW:
How to calculate molality (m)
How to calculate the molar mass of a molecular compound from the freezing point depression or boiling point elevation