AIEM — ICSE Exam Prep

Solutions

A solution is a homogeneous mixture of two or more components; its composition and properties are uniform throughout. The component present in the largest amount is the solvent (it fixes the physical state); the rest are solutes. Solutions are classified as gaseous, liquid or solid depending on the state of the solvent.

Expressing concentration

Mass % (w/w) = mass of component / total mass x 100
Volume % (v/v) and mass by volume % (w/V)
ppm = parts of component / total parts x 10^6 (for trace amounts)
Mole fraction (x) = moles of component / total moles; sum of all mole fractions = 1
Molarity (M) = moles of solute / litre of solution
Molality (m) = moles of solute / kg of solvent

Molality, mole fraction and mass % are temperature independent; molarity depends on temperature (volume changes with T).

Solubility and Henry's law

Like dissolves like. Solid-in-liquid solubility rises with temperature if dissolution is endothermic. Henry's law: at constant T the solubility of a gas is proportional to its partial pressure - p = KH x. Higher KH means lower solubility. Gas solubility decreases as temperature rises (dissolution is exothermic).

Raoult's law and deviations

For volatile components, p_i = x_i p_i(pure); total pressure varies linearly with composition. Ideal solutions obey Raoult's law throughout, with Dmix H = 0 and Dmix V = 0. Non-ideal solutions show positive deviation (A-B interactions weaker; e.g. ethanol-acetone) or negative deviation (A-B stronger; e.g. chloroform-acetone). Large deviations give azeotropes (constant-boiling mixtures, same liquid and vapour composition).

Colligative properties

These depend only on the number of solute particles: relative lowering of vapour pressure (= x_solute), elevation of boiling point (DTb = Kb m), depression of freezing point (DTf = Kf m) and osmotic pressure (P = CRT). All are used to find molar masses of solutes.

Abnormal molar mass and van't Hoff factor

Dissociation (more particles) lowers the observed molar mass; association (fewer particles) raises it. The van't Hoff factor i = normal molar mass / observed molar mass = observed property / calculated property. i > 1 for dissociation, i < 1 for association. Modified relations: DTb = i Kb m, DTf = i Kf m, P = i n2 RT / V.