Solutions

This chapter explores liquid solutions, vapour pressure, and Colligative Properties. The math is simple, but the concepts are subtle. Pay attention to ideal vs. non-ideal behaviour and the crucial Van't Hoff Factor.

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⚠️ Prerequisites

Mole Concept: Calculations of Molarity, Molality, and Mole Fraction.
Equilibrium: Degree of dissociation ($\alpha$) for Van't Hoff factor.
States of Matter: Concept of Vapour Pressure.

🧠 Study Approach

Identify the Solute: Is it an electrolyte (like NaCl) or non-electrolyte (like Urea)? This single question determines if you need the Van't Hoff factor ($i$) in your calculations.

Study Sequence

1. Concentration Terms (Recap)

Mass %, Volume %, ppm. Mole Fraction ($x$). Molarity ($M$, Temp dependent) vs Molality ($m$, Temp independent).

2. Solubility & Henry's Law

Solubility of solids in liquids. Solubility of gases in liquids (Pressure effect).
Henry's Law: $p = K_H \cdot x$. Applications (Scuba diving, Soda bottles).

3. Vapour Pressure & Raoult's Law

For volatile solutes: $p_{total} = p_A + p_B$.
Ideal vs Non-Ideal: Positive Deviation (A-B interaction weaker) vs Negative Deviation (A-B interaction stronger). Azeotropes.

4. Colligative Prop 1: RLVP

Relative Lowering of Vapour Pressure.
Formula: $\frac{p^\circ - p}{p^\circ} = x_{solute} \approx \frac{n}{N}$ (for dilute solutions).

5. Elevation in BP & Depression in FP

Boiling Point: $\Delta T_b = K_b \cdot m$. (Ebullioscopic constant).
Freezing Point: $\Delta T_f = K_f \cdot m$. (Cryoscopic constant).
Understanding the Vapour Pressure vs Temperature graphs is crucial.

6. Osmotic Pressure ($\pi$)

$\pi = CRT$. Reverse Osmosis (Desalination).
Why is this preferred for determining molar mass of proteins? (Magnitude is large even for dilute solutions).

7. Abnormal Molar Mass & Van't Hoff Factor

Factor $i$. Dissociation ($i > 1$) vs Association ($i < 1$).
Corrected formulas: $\Delta T_b = i K_b m$, $\pi = i CRT$, etc.

🎯 How to Practice

The "i" Check: Before solving ANY problem, write down the solute and ask "Does it break?"
Glucose/Urea: $i=1$.
NaCl: $i=2$.
$MgCl_2$: $i=3$.

Deviation Logic: Positive Deviation = Weaker interactions = Lower BP = Higher Vapour Pressure. Negative Deviation = Stronger interactions (H-bond) = Higher BP = Lower Vapour Pressure.

Unit Conversion: For Osmotic Pressure ($\pi = iCRT$), pressure is usually in atm/bar, so use $R = 0.0821$ or $0.083$. Do not use $8.314$ (Joules).

📝 Quick Revision Formulas

Laws $$p = K_H \chi \quad (\text{Henry})$$ $$p_A = x_A p_A^\circ \quad (\text{Raoult})$$

Colligative Properties $$\frac{p^\circ - p}{p^\circ} = i \cdot x_{solute}$$ $$\Delta T_b = i K_b m$$ $$\Delta T_f = i K_f m$$ $$\pi = i C R T$$

Van't Hoff Factor $$i = 1 + (n-1)\alpha \quad (\text{Dissociation})$$ $$i = 1 + (\frac{1}{n}-1)\alpha \quad (\text{Association})$$

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Lesson Plan: Solutions (Class 12)