Solutions & Colligative Properties Lecture

Vocabulary flashcards covering definitions, laws, examples, and calculations related to solutions and colligative properties.

Molarity (M)

The number of moles of solute present in 1 litre of solution; M = n/V (mol L⁻¹).

Molarity of 0.4 g NaOH in 250 mL

Moles NaOH = 0.4 g / 40 g mol⁻¹ = 0.01 mol; Volume = 0.250 L; M = 0.01 / 0.250 = 0.04 M.

Molality (m)

The number of moles of solute present in 1 kg of solvent; m = n / mass of solvent (mol kg⁻¹).

Molality of 1.2 g CH₃COOH in 200 g H₂O

Moles CH₃COOH = 1.2 g / 60 g mol⁻¹ = 0.02 mol; Solvent = 0.200 kg; m = 0.02 / 0.200 = 0.10 m.

Henry's Law

At a constant temperature, the partial pressure of a gas (p) in a liquid is directly proportional to its mole fraction (x): p = k_H x.

Applications of Henry's Law

1. Carbonation of soft drinks (CO₂ dissolved under pressure).
2. Scuba diving tanks use O₂–He mixtures to reduce nitrogen bends.

Raoult's Law

For an ideal solution, the partial vapour pressure of each component equals the product of its mole fraction and the vapour pressure of the pure component: pi = xi P_i⁰.

Ideal Solution

A mixture that obeys Raoult’s law over the entire composition range and shows ΔHmix = 0 and ΔVmix = 0; e.g., benzene–toluene, n-hexane–n-heptane.

Non-ideal Solution (Positive Deviation)

Mixture whose total vapour pressure is higher than predicted by Raoult’s law due to weaker A–B interactions (ΔHmix > 0, ΔVmix > 0).

Non-ideal Solution (Negative Deviation)

Mixture whose total vapour pressure is lower than predicted by Raoult’s law owing to stronger A–B interactions (ΔHmix < 0, ΔVmix < 0).

Examples of Positive Deviation

Ethanol–acetone, cyclohexane–ethanol.

Examples of Negative Deviation

Chloroform–acetone, HCl–water.

Azeotrope

A constant-boiling mixture that distils without change in composition because liquid and vapour phases have the same composition.

Minimum-boiling Azeotrope

Ethanol (≈95 %)–water forms an azeotrope that boils at a lower temperature than either component.

Maximum-boiling Azeotrope

Hydrochloric acid (≈20.2 %)–water forms an azeotrope that boils at a higher temperature than either component.

Colligative Properties

Solution properties that depend only on the number of solute particles, not their nature.

Four Colligative Properties

Relative lowering of vapour pressure, elevation of boiling point, depression of freezing point, and osmotic pressure.

Osmosis

The spontaneous flow of solvent molecules through a semipermeable membrane from a region of lower solute concentration to higher concentration.

Osmotic Pressure (π)

The external pressure that must be applied to a solution to prevent osmosis; π = CRT for dilute solutions.

Reverse Osmosis

Process in which a pressure greater than π is applied to a solution to force solvent to flow in the reverse direction, used for desalination.

Isotonic Solution

Two solutions having the same osmotic pressure; no net water flow occurs across a semipermeable membrane between them.

Hypotonic Solution

A solution with lower osmotic pressure than another; solvent flows into the more concentrated (hypertonic) solution.

Hypertonic Solution

A solution with higher osmotic pressure than another; draws solvent from the hypotonic solution.

Example of Isotonic Solution

0.9 % (w/v) NaCl solution is isotonic with human blood plasma.

Van’t Hoff Factor (i)

The ratio of the observed colligative property to the value calculated for an ideal nonelectrolyte: i = (observed ΔT, π etc.) / (calculated).

Calculation of Van’t Hoff Factor

i = (measured molar mass of solute) / (theoretical molar mass); for dissociation, i > 1; for association, i < 1.

Abnormal Molar Mass

Apparent molar mass different from the true molar mass due to solute association or dissociation altering particle count in solution.

KCl & Abnormal Molar Mass

KCl dissociates into K⁺ and Cl⁻ (i ≈ 2), doubling the number of particles and yielding a lower observed molar mass.

Acetic Acid & Abnormal Molar Mass

In non-polar solvents, CH₃COOH dimerises (i < 1), halving particle number and giving an observed molar mass higher than the true 60 g mol⁻¹.