Chapter 14 Solutions & Colligative Properties

Solution

A homogeneous mixture where one substance is dissolved in another.

Solute

The substance being dissolved; usually present in a smaller amount.

Solvent

The substance doing the dissolving; usually present in a larger amount.

Molarity (M)

Moles of solute per liter of solution (mol/L); changes with temperature.

Molality (m)

Moles of solute per kilogram of solvent; does NOT change with temperature.

Mole fraction (X)

Moles of one component divided by total moles of all components.

Percent by mass

mass of solute / mass of solution × 100.

Dilution equation

M1V1 = M2V2; moles stay the same before and after dilution.

Strong electrolyte

A substance that fully dissociates into ions in solution (like NaCl).

Weak electrolyte

A substance that partially dissociates in solution (like weak acids).

Nonelectrolyte

A substance that does NOT dissociate into ions (like sugar).

Hydration

The process of water molecules surrounding ions in solution.

Enthalpy of solution

Heat absorbed or released when a solute dissolves in a solvent.

Saturated solution

A solution containing the maximum amount of dissolved solute.

Unsaturated solution

A solution that can dissolve more solute.

Supersaturated solution

A solution containing more solute than normal due to special conditions.

Henry’s Law

The solubility of a gas is directly proportional to its partial pressure: S = kP.

Factors affecting solubility of solids

Solubility increases with temperature for most solids.

Factors affecting solubility of gases

Solubility decreases with temperature; increases with pressure.

Raoult’s law (vapor pressure lowering)

Psolution = Xsolvent × P°solvent.

Vapor pressure lowering

The vapor pressure of a solution is lower than the pure solvent because solute particles block evaporation.

Boiling point elevation

ΔTb = i × Kb × m; solution boils at a higher temperature.

Freezing point depression

ΔTf = i × Kf × m; solution freezes at a lower temperature.

Osmotic pressure

π = iMRT; pressure needed to stop solvent flow across a membrane.

van’t Hoff factor (i)

Number of particles a solute forms when dissolved; ionic solutes > 1, covalent solutes = 1.

i value for nonelectrolytes

i = 1 because they do not dissociate.

i value for NaCl

i = 2 because NaCl → Na⁺ + Cl⁻.

i value for CaCl2

i = 3 because CaCl₂ → Ca²⁺ + 2Cl⁻.

Why colligative properties depend on number of particles

They depend on the number of dissolved particles, not the type.

Osmosis

The movement of solvent from low solute concentration → high solute concentration.

Hypertonic solution

Solution with higher solute concentration; water moves OUT of the cell.

Hypotonic solution

Solution with lower solute concentration; water moves INTO the cell.

Isotonic solution

Solution with equal solute concentration inside/outside the cell.

Reverse osmosis

Using pressure to force water from a concentrated solution to a less concentrated one.

Strong electrolyte effect on boiling point elevation

Increases ΔTb because more particles → larger i.

Strong electrolyte effect on freezing point depression

Increases ΔTf for the same reason.

Colligative property definition

Physical properties that depend only on number of solute particles.

Solubility curve

A graph showing solubility vs. temperature.

When vapor pressure decreases

Boiling point increases because more heat is needed to reach atmospheric pressure.

Why adding salt melts ice

Salt lowers the freezing point → ice melts at lower temperatures.