Section 1: Concentration Terms and Henry's Law

Concentration of a solution is expressed as molality (mol/kg solvent — temperature independent), molarity (mol/L solution — temperature dependent), or mole fraction. Henry's law (p = K_H × x) describes gas dissolution: at constant temperature, the amount of gas dissolved in a liquid is proportional to the partial pressure of that gas above the liquid. K_H increases with temperature, so gas solubility decreases with rising temperature. This underpins the physics of carbonated drinks and the physiology of decompression sickness.

Section 2: Raoult's Law and Types of Solutions

Raoult's law gives the vapour pressure of each component in a binary liquid mixture. Ideal solutions (benzene + toluene) obey it perfectly with $\Delta H$_mix = 0 and $\Delta V$_mix = 0. Non-ideal solutions deviate: positive deviation (ethanol + water — weaker A-B forces, $\Delta H$_mix > 0, $\Delta V$_mix > 0, minimum boiling azeotrope) and negative deviation ($CHCl_{3}$ + acetone — stronger A-B forces, $\Delta H$_mix < 0, $\Delta V$_mix < 0, maximum boiling azeotrope). Azeotropes cannot be separated by simple distillation because the vapour and liquid phases have the same composition at the azeotrope.

Section 3: Colligative Properties

All four colligative properties depend exclusively on the number of solute particles per unit of solvent:

• Relative lowering of VP: $\Delta P$/P° = x_solute (measured by manometer)
• Boiling point elevation: $\Delta Tb$ = Kb·m (measured by Beckmann thermometer; Kb water = 0.52 K·kg/mol)
• Freezing point depression: $\Delta Tf$ = Kf·m (Kf water = 1.86 K·kg/mol; Kf > Kb because $\Delta Hf$us < $\Delta Hv$ap)
• Osmotic pressure: π = CRT (measured by osmometer; most sensitive — preferred for macromolecules)

Molar mass is calculated from each: most commonly $M_{2}$ = Kb × w_{2} × 1000 / ($\Delta Tb$ × w_{1}). Camphor (Kf = 40 K·kg/mol) gives the largest $\Delta Tf$ per mole — ideal for Rast's camphor method.

Section 4: Electrolytes and Van't Hoff Factor

The van't Hoff factor (i) corrects colligative property formulas for electrolytes: $\Delta Tb$ = iKbm, $\Delta Tf$ = iKfm, π = iCRT. For dissociating electrolytes: i = 1 + (n−1)α. For associating solutes: i = 1 − (n−1)α/n (for dimerization: i = 1 − α/2). Complete dissociation of NaCl gives i = 2, $CaCl_{2}$ gives i = 3, $AlCl_{3}$ gives i = 4. Acetic acid in benzene completely dimerizes (α = 1) giving i = 0.5. Association causes the apparent molar mass to exceed the true value; dissociation makes it appear lower.

Section 5: Osmosis and Applications

Osmosis is the spontaneous passage of solvent through a semipermeable membrane from lower to higher solute concentration (lower to higher osmotic pressure). Osmotic pressure π = iCRT. Applying pressure exceeding π reverses the flow (reverse osmosis) — the basis of RO water purification and desalination. Isotonic solutions (equal π) do not cause osmotic stress on cells. Blood cells require isotonic environment (0.9% NaCl) — hypotonic solutions cause haemolysis; hypertonic cause crenation.