Solutions Class 12 Chemistry Notes

Types of Solutions and Concentration Terms

• $\text{Molarity (M)}:$
• moles of solute per litre of solution.
• $\text{Molality (m)}:$
• moles of solute per kg of solvent (temperature independent).
• $\text{Mole fraction (x)}:$
• ratio of moles of one component to total moles.
• $\text{Mass percentage}:$
• mass of solute per 100 units of solution mass.
• $\text{Parts per million (ppm)}:$
• used for very dilute solutions.

Henry's Law and Solubility

• Applications: carbonated drinks, oxygen in blood foams, anaesthesia using gases.
• Effervescence on opening a soda bottle — pressure reduces, gas escapes.
• Solubility of gases in liquids generally decreases with rising temperature.

Vapour Pressure and Raoult's Law

• $\text{Ideal solution}:$
• obeys Raoult's law at all compositions with
• $\Delta H_{\text{mix}}=0$
• and
• $\Delta V_{\text{mix}}=0$
• .
• $\text{Non-ideal (+ve deviation)}:$
• weaker A-B interaction — vapour pressure higher than ideal.
• $\text{Non-ideal (-ve deviation)}:$
• stronger A-B interaction — vapour pressure lower than ideal.

Colligative Properties

Colligative properties depend only on the number of solute particles, not their nature. The four key ones are lowering of vapour pressure, boiling point elevation, freezing point depression and osmotic pressure.

• $\text{Relative lowering of v.p.}:$
• $\frac{p^{\circ}-p}{p^{\circ}} = x_{\text{solute}}$
• .
• $\text{Elevation of boiling point}:$
• $\Delta T_b = K_b \cdot m$
• .
• $\text{Depression of freezing point}:$
• $\Delta T_f = K_f \cdot m$
• .
• $\text{Osmotic pressure}:$
• $\pi = CRT$
• .

Relative Molecular Mass and Abnormal Molar Mass

Colloidal State

A colloid is a heterogeneous system of two phases — a dispersed phase (particles 1–1000 nm) and a dispersion medium. Colloids show the Tyndall effect and Brownian motion and carry charges on their particles.

• $\text{Lyophilic}:$
• solvent-loving colloids that are reversible (e.g. gum, starch).
• $\text{Lyophobic}:$
• solvent-hating colloids that are irreversible (e.g. metal sols).
• Coagulation of a colloid occurs by adding an electrolyte that neutralises the particle charge.
• Gold number and Hardy-Schulze rule describe coagulation effectiveness.

Solved Examples

Example: The freezing point of a 0.5 m aqueous solution of a non-electrolyte is depressed by 0.93 °C. Given $K_f = 1.86\,\text{K kg mol}^{-1}$, find the van't Hoff factor.

Solution: $\Delta T_f = i\,K_f\,m \Rightarrow 0.93 = i \times 1.86 \times 0.5 \Rightarrow i = 1$. The observed depression matches the theoretical value, so the solute neither dissociates nor associates.