JEE Mains 2026 Chemistry Revision: Mole Concept Notes, Formulas & PYQ Patterns

 

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JEE Mains 2026 — Revision Capsule #1: Mole Concept

30-Second Refresher

The mole connects particles ↔ mass ↔ volume ↔ concentration. Most Physical Chemistry problems reduce to converting between these.

Essential Ideas (Ultra-Compressed)

• One mole = Avogadro number \(N_A = 6.022\times10^{23}\) particles.
• Moles ↔ mass: \(n=\dfrac{W}{M}\) (mass ÷ molar mass).
• Gases at STP: \(1\ \text{mol} = 22.4\ \text{L}\).
• Ideal gas: \(PV=nRT\). Watch units.
• Concentration: Molarity \(M=\dfrac{n}{V_{\text{soln}}}\), Molality \(m=\dfrac{n_{\text{solute}}}{\text{kg solvent}}\).
• Equivalents useful for redox/acid-base: \(n_{\text{eq}}=\dfrac{W}{\text{equivalent mass}}\).

High-Yield Formulas (Memorize)

n = W / M
moles = mass ÷ molar mass

n = V / 22.4
(gases at STP)

PV = nRT
ideal gas equation

n = N / N_A
particles → moles

ρ = PM / RT
gas density relation

M_1V_1 = M_2V_2
dilution formula

Classic JEE Traps

1. Using \(22.4\ \text{L/mol}\) for non-STP conditions.
2. Mixing up molarity (depends on solution volume) and molality (depends on mass of solvent).
3. Using volume of solute instead of solution in molarity calculations.
4. Forgetting to count atoms inside molecules: 1 mol CO₂ = 3 mol atoms.

Question Patterns (Repeat Every Year)

Pattern 1 — Mass ↔ Mole ↔ Number

Q: How many molecules in 4.4 g CO₂?

Solution

Moles \( = \dfrac{4.4}{44} = 0.1\).
Molecules \(= 0.1 \times 6.022\times10^{23} = 6.022\times10^{22}\).

Pattern 2 — Gas density → Molar mass

Q: Density \(=1.25\ \text{g·L}^{-1}\) at 1 atm, 273 K. Find molar mass \(M\).

Solution

Use \( \rho=\dfrac{PM}{RT}\Rightarrow M=\dfrac{\rho RT}{P}\). Substitute \(R=0.0821\ \text{L·atm·K}^{-1}\text{mol}^{-1}\).

Pattern 3 — Mole fraction in gas mixture

Q: Mixture has 2 g H₂ and 32 g O₂. Mole fraction of H₂?

Solution

Moles H₂ = 2/2 = 1; Moles O₂ = 32/32 = 1 → total 2. Mole fraction H₂ = 1/2 = 0.5.

Mini Quiz (Try mentally — answers below)

1. How many moles in 9 g of H₂O?
2. Volume of 0.5 mol gas at STP?
3. Number of atoms in 0.5 mol of O₃?

Show Answers

1. 9 g H₂O: Molar mass = 18 → moles = 9/18 = 0.5 mol.
2. 0.5 mol at STP: Volume = 0.5 × 22.4 = 11.2 L.
3. 0.5 mol O₃: 1 mol O₃ has 3 mol atoms → 0.5 × 3 = 1.5 mol atoms = \(1.5\times 6.022\times10^{23}\) atoms.

One-Line Summary

Mole concept is the conversion engine of chemistry — master mass ⇄ mole ⇄ volume ⇄ particles and the rest becomes routine.

Next: Stoichiometry (Capsule 2) Download PDF