Here are perfect notes for the Chapter: Solutions from Class 12 Chemistry (NCERT & CBSE syllabus).
Chapter: Solutions
1. Introduction
A solution is a homogeneous mixture of two or more substances. The substance present in a larger amount is the solvent, while the substance present in a smaller amount is the solute.
Types of Solutions
- Gaseous Solutions (Gas in gas, liquid in gas, solid in gas)
- Liquid Solutions (Gas in liquid, liquid in liquid, solid in liquid)
- Solid Solutions (Gas in solid, liquid in solid, solid in solid)
2. Expressing Concentration of Solutions
The amount of solute dissolved in a given amount of solvent is expressed as:
- Mass Percentage = (Mass of solute / Mass of solution) × 100
- Volume Percentage = (Volume of solute / Volume of solution) × 100
- Mass by Volume Percentage = (Mass of solute / Volume of solution) × 100
- Molarity (M) = Moles of solute / Volume of solution (in L)
- Molality (m) = Moles of solute / Mass of solvent (in kg)
- Mole Fraction (χ) = Moles of component / Total moles of solution
- Normality (N) = Moles of equivalents / Volume of solution (in L)
3. Solubility
The maximum amount of solute that can dissolve in a given solvent at a specific temperature is called solubility.
- Effect of Temperature: Solubility of solids in liquids increases with temperature, but solubility of gases decreases.
- Effect of Pressure: Solubility of gases increases with pressure (Henry’s law).
Henry’s Law
It states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid.
Mathematical Expression:
where,
- C = concentration of the gas,
- k_H = Henry’s constant,
- P = pressure of the gas.
4. Vapour Pressure of Liquid Solutions
The vapour pressure of a liquid solution depends on the nature of solute and solvent.
Raoult’s Law
It states that the partial vapour pressure of a volatile component is directly proportional to its mole fraction in the solution.
Mathematical Expression:
where,
- = vapour pressure of component A,
- = vapour pressure of pure A,
- = mole fraction of A.
For a binary solution:
- Ideal Solutions follow Raoult’s Law and show no change in enthalpy and volume during mixing.
- Non-Ideal Solutions deviate from Raoult’s Law and show either positive or negative deviation.
5. Colligative Properties
These properties depend only on the number of solute particles, not their identity.
(i) Relative Lowering of Vapour Pressure
where,
- P⁰ = vapour pressure of pure solvent,
- P = vapour pressure of solution,
- χ_B = mole fraction of solute.
(ii) Boiling Point Elevation
When a solute is added, the boiling point increases.
Formula:
where,
- ΔT_b = boiling point elevation,
- K_b = molal boiling point constant,
- m = molality.
(iii) Freezing Point Depression
When a solute is added, the freezing point decreases.
Formula:
where,
- ΔT_f = freezing point depression,
- K_f = molal freezing point constant,
- m = molality.
(iv) Osmotic Pressure ()
It is the pressure applied to stop osmosis.
Formula:
where,
- C = molarity,
- R = gas constant,
- T = temperature (in K).
Van’t Hoff Factor (): Accounts for ionization or association of solutes.
Modified formulas:
6. Abnormal Molar Mass
Due to dissociation or association, the experimentally determined molar mass differs from the theoretical value. This is corrected using the Van’t Hoff Factor (i):
- i > 1 for dissociation (e.g., NaCl → Na⁺ + Cl⁻).
- i < 1 for association (e.g., acetic acid dimerizes).
Key Points to Remember
- Colligative properties depend only on the number of solute particles.
- Raoult’s Law applies to ideal solutions, while non-ideal solutions show deviations.
- Henry’s Law explains gas solubility in liquids.
- Osmotic pressure is crucial for biological processes.
- Van’t Hoff Factor corrects molar mass for dissociation/association.
Important Questions for Practice
- State Raoult’s Law and derive its mathematical expression.
- Explain Henry’s Law with an example.
- Differentiate between ideal and non-ideal solutions.
- Derive the boiling point elevation formula.
- Explain the concept of Van’t Hoff Factor with an example.
- Calculate the osmotic pressure of a 0.5 M urea solution at 27°C.
- Why does NaCl show an abnormal molar mass in aqueous solution?
- Explain the significance of colligative properties.
These notes cover all key concepts, formulas, and important questions in a structured manner for quick revision and effective learning. Let me know if you need additional explanations or numericals!
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