Chapter 12 Equations
c = kP (Henry’s Law)
c is the molar solubility of the gas
P is the partial pressure of the gas over the solution (in atm)
k is the Henry’s law constant (unit is M/atm). The value of k depends only on
temperature and the identity of the gas. (It is the equilibrium constant for the
dissolving of a gas in water.)
Note: This is how Chang writes the equation. I prefer to use symbols that make it
easier to remember what the variables represent: Sgas = KHPgas (The Henry’s law
constant is an equilibrium constant. Equilibrium constants are usually written as an
upper case K.)
Colligative Properties
Vapor pressure reduction: P1 = X1P1o (Raoult’s law for ideal solutions)
P1 is the vapor pressure of the solvent (usually water)
X1 is the mole fraction of the solvent
P1o is the vapor pressure of pure solvent (when no solute is present)
(note: If the solute is volatile it also obey’s Raoult’s law)
Freezing point reduction:
Tf = −Kf mi Kf for water = 1.86oC/m
Tf = Tf (solution) – Tf (pure solvent)
Kf is the freezing point depression constant
m is the molality of the solute
i is the number of particles produced in solution for each particle that dissolves
(note: Tf is a negative number.)
Boiling point elevation:
Tb = Kb mi Kb for water = 0.51oC/m
Tfb = Tb (solution) – Tb (pure solvent)
Kb is the boiling point elevation constant
m is the molality of the solute
i is the number of particles produced in solution for each particle that dissolves
(note: Tf is a negative number.)
Note: i (the Van’t Hoff factor) is only important for compounds that ionize when
dissolved in water (ionic compounds and acids). For molecular compounds, i=1.
Osmotic pressure elevation: V = nRT or  = MRT
is the osmotic pressure (in atm)
V is the volume of the solution (in liters)
n is the number of moles of solute particles
R is the ideal gas constant (L-atm/mol-K)
T is the Kelvin temperatrue
M is the molarity (n/V) of solute particles