Solutions – Colligative Properties
Properties that depend only on the concentration
of particles, not their identities.
Solutions – Colligative Properties
We use ‘molality (m)’ instead of ‘molarity’ for
colligative properties.
(molality is used only for colligative effects)
Boiling Point Elevation:
DTb = i Kb m
DTb = change in boiling point temperature
i
= Van’t Hoff Factor (= number of particles)
Kb
= molal boiling point constant (q.v. table)
m
= molality (moles solute / kg solvent)
DTb = i Kb m
Boiling Point Elevation:
i
= Van’t Hoff Factor (= number of particles)
Substance
i
sucrose (C12H22O11)
1
NaCl
2
MgCl2
3
Ca3(PO4)2
5
CH4
1
CaCl2
3
Solvent
water
Molal Freezing-Point and Boiling-Point Constant
Normal f.p. (oC)
Molal f.p.
Normal b.p.
constant, Kf
(oC)
(oC/m)
0.00
–1.86
100.0
Molal b.p.
constant, Kb
(oC/m)
0.51