Overview
Solutions deals with the homogeneous mixture of two or more components and focuses on how concentration and intermolecular forces dictate physical properties. It is a high-weightage topic in chemistry, requiring a strong conceptual grasp of thermodynamics and colligative behaviors to solve numerical problems effectively.
Concentration Terms
Understanding concentration units is the foundation for all numerical problems in this chapter. Aspirants must master the interconversion between molarity, molality, and mole fraction, as exams often provide one and ask for another.
- Molarity (M) = mol solute / L solution
- Molality (m) = mol solute / kg solvent
- Mole Fraction (x) = n_A / (n_A + n_B)
- Mass Percentage = (mass solute / total mass) * 100
- Molality is independent of temperature while Molarity is temperature-dependent
Raoult's Law and Vapour Pressure
Raoult's Law describes the vapour pressure of volatile components in an ideal solution. Deviations from this law define non-ideal solutions, which are frequently tested through conceptual questions regarding intermolecular forces.
- P_total = p_A + p_B = pA°x_A + pB°x_B
- Positive deviation: P_obs > P_calc, ΔH_mix > 0, ΔV_mix > 0
- Negative deviation: P_obs < P_calc, ΔH_mix < 0, ΔV_mix < 0
- Azeotropes: Constant boiling mixtures that cannot be separated by fractional distillation
- Ideal solution: ΔH_mix = 0 and ΔV_mix = 0
Colligative Properties
Colligative properties depend solely on the number of solute particles rather than their identity. These properties are the most common source of multi-step numerical problems in board exams.
- Relative Lowering of Vapour Pressure: (pA° - pA) / pA° = x_B
- Elevation in Boiling Point: ΔTb = Kb * m
- Depression in Freezing Point: ΔTf = Kf * m
- Osmotic Pressure: Π = iCRT
- Van't Hoff factor (i) = normal molar mass / abnormal molar mass
Formula Sheet
ΔTb = i * Kb * m
ΔTf = i * Kf * m
Π = i * C * R * T
(pA° - pA) / pA° = i * xB
Exam Tip
Always verify if the solute is an electrolyte or non-electrolyte first, as missing the Van't Hoff factor (i) is the quickest way to lose marks on numerical problems.
Common Mistakes
- Confusing Molality (m) with Molarity (M) when using boiling point elevation or freezing point depression formulas.
- Forgetting to apply the Van't Hoff factor (i) for electrolytes which dissociate in solution.
- Incorrectly assigning units (e.g., using liters instead of kilograms in molality calculations).
More Revision Notes
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