NEET Chemistry — Solutions Complete Chapter Guide

Chapter Overview & Weightage

Solutions is a high-scoring chapter in NEET Physical Chemistry. Colligative properties dominate, and most questions involve direct formula application. If you master Raoult’s law and the four colligative properties, this chapter is nearly guaranteed marks.

Solutions carries 4-5% weightage in NEET with 2-3 questions. Colligative properties and Raoult’s law are tested every year without exception.

Year	NEET Q Count	Key Topics Tested
2025	2	Elevation in boiling point, osmotic pressure
2024	3	Raoult’s law, van’t Hoff factor
2023	2	Depression in freezing point, abnormal molar mass
2022	2	Colligative properties, Henry’s law
2021	2	Osmotic pressure, molality

graph TD
    A[Solutions] --> B[Concentration Terms]
    A --> C[Raoult's Law]
    A --> D[Colligative Properties]
    A --> E[Abnormal Molar Mass]
    B --> F[Molarity, Molality, Mole Fraction]
    C --> G[Ideal Solutions]
    C --> H[Non-ideal: +ve and -ve deviation]
    D --> I[Relative Lowering of VP]
    D --> J[Elevation in BP]
    D --> K[Depression in FP]
    D --> L[Osmotic Pressure]
    E --> M[van't Hoff Factor i]

Key Concepts You Must Know

Tier 1 (Always asked)

Raoult’s law: $P = P^0 \cdot x_{solvent}$
Colligative properties depend on number of particles, not their nature
$\Delta T_b = K_b \cdot m$ and $\Delta T_f = K_f \cdot m$
Osmotic pressure: $\pi = iCRT$

Tier 2 (Frequently asked)

van’t Hoff factor $i$ and its effect on colligative properties
Positive and negative deviations from Raoult’s law
Henry’s law: $P = K_H \cdot x_{solute}$
Inter-conversion of concentration terms

Tier 3 (Occasional)

Isotonic solutions
Reverse osmosis
Azeotropes

Important Formulas

Relative lowering of vapour pressure: $\dfrac{P^0 - P}{P^0} = x_{solute}$

Elevation in boiling point: $\Delta T_b = i \cdot K_b \cdot m$

Depression in freezing point: $\Delta T_f = i \cdot K_f \cdot m$

Osmotic pressure: $\pi = iCRT = i \cdot \dfrac{n_{solute}}{V} \cdot RT$

where $m$ = molality, $C$ = molarity, $i$ = van’t Hoff factor.

i = \frac{\text{observed colligative property}}{\text{calculated colligative property}}

For dissociation: $i = 1 + (n-1)\alpha$ where $n$ = number of ions, $\alpha$ = degree of dissociation.

For association: $i = 1 - (1 - 1/n)\alpha$ where $n$ = number of molecules associating.

NaCl: $i \approx 2$ , CaCl $_2$ : $i \approx 3$ , glucose: $i = 1$ .

Colligative properties depend on the NUMBER of solute particles. NaCl gives 2 particles per formula unit, so its effect is roughly double that of glucose at the same molality. This is the entire point of the van’t Hoff factor.

Solved Previous Year Questions

PYQ 1 — NEET 2024

Problem: The boiling point elevation of a 0.1 m aqueous NaCl solution is ( $K_b = 0.52$ K kg/mol, assume complete dissociation):

Solution:

NaCl dissociates completely: $i = 2$

\Delta T_b = i \cdot K_b \cdot m = 2 \times 0.52 \times 0.1 = \mathbf{0.104 \text{ K}}

PYQ 2 — NEET 2023

Problem: Two solutions are isotonic. Solution A has 6 g of urea (M = 60) in 1 L. What mass of glucose (M = 180) in 1 L gives the same osmotic pressure?

Solution:

Isotonic means same $\pi$ , so same $C$ (both non-electrolytes, $i = 1$ ):

Moles of urea = 6/60 = 0.1 mol in 1 L, so C = 0.1 M

For glucose: $0.1 = m/180 \implies m = \mathbf{18 \text{ g}}$

Difficulty Distribution

Difficulty	% of Questions	What to Expect
Easy	40%	Direct colligative property formula
Medium	45%	van’t Hoff factor, abnormal molar mass
Hard	15%	Non-ideal solutions, azeotropes

Expert Strategy

Week 1: Master all four colligative property formulas with the van’t Hoff factor. Solve 20 problems on boiling point elevation and freezing point depression.

Week 2: Raoult’s law and deviations. Understand why ethanol + water shows positive deviation (weaker interactions) and chloroform + acetone shows negative deviation (stronger interactions).

Week 3: PYQs and mixed problems. Focus on questions that combine colligative properties with dissociation/association.

Common Traps

Trap 1 — Forgetting the van’t Hoff factor for electrolytes. NaCl, KCl, CaCl $_2$ are electrolytes — they dissociate. Using $i = 1$ for them gives answers that are half (or one-third) of the correct value.

Trap 2 — Molality vs molarity. $\Delta T_b$ and $\Delta T_f$ use molality (moles per kg of solvent). Osmotic pressure uses molarity (moles per litre of solution). Mixing these up is a frequent error.

Trap 3 — Positive deviation means HIGHER vapour pressure. Solutions with positive deviation from Raoult’s law have vapour pressure higher than expected. They form minimum boiling azeotropes (like ethanol-water at 95.5%).