Is it correct to say: Using molarity instead of molality in ΔTf or ΔTb?

K_f and K_b are defined per unit molality. Using molarity gives wrong answer.

Is it correct to say: Forgetting van't Hoff factor for electrolyte solutions?

For NaCl, i ≈ 2 (approx). For CaCl₂, i ≈ 3. Multiply into colligative expression.

Is it correct to say: Assuming Raoult's law applies to non-ideal (positive/negative deviation) solutions strictly?

Raoult's law is exact only for ideal solutions. State the ideality assumption in your answer.

Is it correct to say: Confusing Kf (freezing) with Kb (boiling)?

For water Kf = 1.86 K·kg/mol; Kb = 0.52 K·kg/mol. Very different values.

Is it correct to say: Applying π = CRT with concentration in mol/L for R in cal — unit mismatch?

Use R = 0.0821 L·atm·K⁻¹·mol⁻¹ if you want π in atm; or SI throughout.

Is it correct to say: Not distinguishing between dissociation (i > 1) and association (i < 1)?

Dissociation of NaCl: i > 1. Association of acetic acid to dimer in benzene: i < 1.

Solutions

Q: Is it correct to say: Not distinguishing between dissociation (i > 1) and association (i < 1)?

Dissociation of NaCl: i > 1. Association of acetic acid to dimer in benzene: i < 1.

Types of solutions, concentration terms, Raoult's law, colligative properties, abnormal molar mass — Maharashtra HSC Chemistry Ch 2

📐 12 formulas✏️ 3 examples🎯 5 practice⚖️ 5 marks🏫 Maharashtra State Board📚 Class 12✓ 2025–26 syllabus

💡

Board Exam Tips

→Derivation of the relation between molality and depression in freezing point (ΔTf = Kf·m) or elevation in boiling point (ΔTb = Kb·m) is asked for 2 marks.
→Numerical on determination of molar mass by ΔTf or π (osmotic pressure) is a very common 3-marker.
→State Raoult's law for both volatile and non-volatile solutes — HSC often asks the distinction.
→van't Hoff factor i and its relation to degree of dissociation α is a Maharashtra Board favourite HOTS topic.
→Use molality (mol/kg) — NOT molarity — in freezing/boiling point numericals. Molarity depends on T; molality does not.

📐 Formulas(12)

Molarity

M = \frac{n _{so l u t e}}{V _{so l u t i o n} ( L )}

Molality★ Board fav

m = \frac{n _{so l u t e}}{w _{so l v e n t} ( kg )}

Mole Fraction

x_{A} = \frac{n _{A}}{n _{A} + n _{B}}, x_{A} + x_{B} = 1

Raoult's Law (volatile solute)★ Board fav

p_{A} = p_{A}^{\circ} x_{A}, p_{B} = p_{B}^{\circ} x_{B}

Raoult's Law (non-volatile solute)★ Board fav

\frac{p ^{\circ} - p}{p ^{\circ}} = x_{B}

Elevation in Boiling Point

Δ T_{b} = K_{b} m

Depression in Freezing Point★ Board fav

Δ T_{f} = K_{f} m

Osmotic Pressure★ Board fav

π = C R T = \frac{n}{V} R T

van't Hoff Factor

i = \frac{observed colligative property}{calculated colligative property} = \frac{normal molar mass}{abnormal molar mass}

Modified Colligative Property (with i)★ Board fav

Δ T_{f} = i K_{f} m, π = i C R T

Degree of Dissociation from i

α = \frac{i - 1}{n - 1}

Henry's Law

p = K_{H} x

✏️ Solved Examples

1Solved Exampleeasy3 steps

Calculate the molality of a solution containing 4.0 g of NaOH in 250 g of water.

Moles of NaOH (molar mass 40 g/mol).

n = \frac{4.0}{40} = 0.10 mol

2Solved Exampleboard3 steps

18.0 g of glucose (M = 180 g/mol) is dissolved in 1 kg of water. Calculate (i) the freezing point depression, (ii) the freezing point of the solution. (K_f for water = 1.86 K·kg/mol)

Moles of glucose and molality.

n = \frac{18.0}{180} = 0.10 mol; m = 0.10/1 = 0.10 mol/kg

3Solved ExampleHOTS4 steps

0.5 g of a salt CaCl₂ (M = 111 g/mol) dissolved in 100 g of water lowers the freezing point by 0.223 K. Given K_f = 1.86 K·kg/mol, find the van't Hoff factor and the degree of dissociation.

Molality.

m = \frac{0.5/111}{0.1} = 0.045 mol/kg

⚠️ Traps & Common Mistakes

⚠️Common Mistakes6

1
Using molarity instead of molality in ΔTf or ΔTb
✓K_f and K_b are defined per unit molality. Using molarity gives wrong answer.
2
Forgetting van't Hoff factor for electrolyte solutions
✓For NaCl, i ≈ 2 (approx). For CaCl₂, i ≈ 3. Multiply into colligative expression.
3
Assuming Raoult's law applies to non-ideal (positive/negative deviation) solutions strictly
✓Raoult's law is exact only for ideal solutions. State the ideality assumption in your answer.
4
Confusing Kf (freezing) with Kb (boiling)
✓For water Kf = 1.86 K·kg/mol; Kb = 0.52 K·kg/mol. Very different values.
5
Applying π = CRT with concentration in mol/L for R in cal — unit mismatch
✓Use R = 0.0821 L·atm·K⁻¹·mol⁻¹ if you want π in atm; or SI throughout.
6
Not distinguishing between dissociation (i > 1) and association (i < 1)
✓Dissociation of NaCl: i > 1. Association of acetic acid to dimer in benzene: i < 1.

🎯 Practice Yourself

🎯Practice Yourself5 questions

Q1
Calculate the molarity of a solution containing 5.85 g of NaCl in 500 mL of solution.
Q2
Vapour pressure of a solvent decreases from 100 mmHg to 90 mmHg on adding a non-volatile solute. Find mole fraction of solute.
Q3
0.6 g of urea is dissolved in 100 g water; observed ΔTf is 0.186 K. Molar mass? (Kf = 1.86)
Q4
State Raoult's law and derive the expression for relative lowering of vapour pressure for a non-volatile solute.
Q5
Calculate osmotic pressure at 27 °C of a 0.01 M glucose solution.

📝 Notes

Solutions — Maharashtra HSC Overview

Chapter 2 of the Maharashtra Class 12 Chemistry textbook covers types of solutions, concentration terms, Raoult's law, colligative properties, and the van't Hoff factor.

Maharashtra syllabus specifics

The Maharashtra syllabus places explicit numericals on van't Hoff factor and degree of dissociation — the derivation α = (i−1)/(n−1) is a standard 2-mark board question.
Osmotic pressure and its uses in molar mass determination of biomolecules is emphasised in Balbharati. CBSE mentions it too but Maharashtra board expects a full derivation.
Non-ideal solutions and positive/negative deviations from Raoult's law — with examples (ethanol + acetone; chloroform + acetone) — is a Balbharati-specific short-answer favourite.
Ideal vs non-ideal solutions comparison in tabular form is a common 2-marker.

Working tips

Molality is preferred for all colligative property numericals: temperature-independent, and K_f/K_b are defined per molality.
For electrolytes always calculate i first (using van't Hoff factor definition) before deriving abnormal molar mass.
π = CRT is analogous to PV = nRT — very handy shortcut; remember R = 0.0821 L·atm·K⁻¹·mol⁻¹ or 8.314 J·K⁻¹·mol⁻¹.