Colligative Properties of Solutions | General Chemistry 2
Solution Concentrations
Mass Percent:
% w/w = x 100
% w/v = x 100
% v/v = x 100
msolution = msolute + msolvent
Mole Fraction:
xsolute =
n = number of moles (in mol)
Molarity M (in mol.L-1):
M =
Molality m (in mol.kg-1):
m =
Solutions and Solubility
A solute dissolves in a solvent to make a solution
Solubility: maximum amount that dissolves in a given amount of solvent at a particular temperature
Substances with similar types of intermolecular forces dissolve in each other
H2O and CH3OH exhibit hydrogen bonding ⇒ they are miscible
Solids are more soluble at higher temperatures
Gas Solubility and Henry’s Law
Gases become:
- less soluble as temperature increases
- more soluble at higher pressure
Henry’s Law:
Solubility of a gas is proportional to its pressure:
Pgas = k x Mgas
Pgas = partial pressure of the gas (in atm)
Mgas = molarity of the dissolved gas (in mol.L-1)
k = proportionality constant (Henry’s law constant) ⇒ depends upon the gas, the solvent and T
Raoult’s Law
Psolvent = xsolvent P0solvent
Psolvent = vapor pressure of the solvent
P0solvent = vapor pressure of the pure solvent
xsolvent = mole fraction of solvent in the solution
(0 ≤ xsolvent < 1 ⇒Psolvent < P0solvent)
Boiling Point Elevation
Vapor pressure of the solution < Vapor pressure of the pure solvent
⇒ more energy is required to convert solution from liquid to gas
⇒ boiling point elevation
The magnitude of the boiling point elevation ΔTb (in K) is:
ΔTb = i x Kb x m
ΔTb = Tb solution – Tb solvent
i = van’t Hoff i-factor = number of ions produced per formula unit
m = solution molality (in mol.kg-1)
Kb = proportionality constant (boiling point elevation constant) ⇒ depends upon the solvent
Freezing Point Depression
At the freezing temperature:
vapor pressure of the solid (pure solvent) = vapor pressure of the solution
BUT the vapor pressure of the solution has been lowered by the solute
⇒ freezing point is also lower
⇒ solid solvent has a smaller vapor pressure
⇒ freezing point depression
The magnitude of the freezing point depression ΔTf (in K) is:
ΔTf = - i x Kf x m
ΔTf = Tf solution – Tf solvent
i = van’t Hoff i-factor = number of ions produced per formula unit
m = solution molality (in mol.kg-1)
Kf = proportionality constant (freezing point depression constant) ⇒ depends upon the solvent
Osmotic Pressure
Osmosis: movement of solvent particle through a semipermeable membrane while the solute particles stay within their respective solutions
Solvent particles move from the solution of lower concentration to the solution of higher concentration
Osmotic pressure Π: amount of pressure that must be applied to the higher concentration solution to prevent the osmosis
Π = i x M x R x T
i = van’t Hoff i-factor = number of ions produced per formula unit
M = molarity (in mol.L-1)
R = ideal gas constant = 8.314 (in J.K-1.mol-1)
T = temperature (in K)