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Mole Concept - Revision capsule JEE Mains

 

Mole Concept – Complete Quantitative Chemistry Revision

The Mole Concept is the backbone of Physical Chemistry. Almost every numerical problem in chemistry starts or ends here. JEE tests this chapter through unit conversion, limiting reagent logic, stoichiometry, and concentration traps.

1. Fundamental Definitions

Mole: Amount of substance containing $6.022 \times 10^{23}$ entities

Avogadro Number:

$N_A = 6.022 \times 10^{23} \, mol^{-1}$

Molar Mass:

Mass of 1 mole of a substance (in g/mol)

Golden Relation:
$\text{Moles} = \dfrac{\text{Given quantity}}{\text{Molar mass}}$

2. Mole – Particle – Mass Relations

$\text{Number of particles} = n \times N_A$

$n = \dfrac{w}{M}$

JEE Trap: Atoms vs molecules vs ions — always check entity asked

3. Mole Concept in Gases

Molar Volume (STP):

$1 \text{ mole gas} = 22.4 \, L \; (STP)$

$n = \dfrac{V}{22.4} \quad (STP)$

Using Ideal Gas Equation:

$PV = nRT$

Trap: STP vs RTP confusion

4. Stoichiometry & Balanced Equations

All mole-based problems depend on balanced chemical equations.

$aA + bB \rightarrow cC + dD$

Mole ratio = coefficient ratio

Rule: Never compare masses directly — convert to moles first

5. Limiting Reagent (Most Tested)

The reactant which gets completely consumed first is the limiting reagent.

Steps:

  • Convert all reactants to moles
  • Divide by stoichiometric coefficient
  • Smallest value → limiting reagent

Advanced Trap: Excess reagent calculation after reaction

6. Percentage Composition

$\%\text{ of element} = \dfrac{\text{Mass of element}}{\text{Molar mass of compound}} \times 100$

7. Empirical & Molecular Formula

Empirical Formula: Simplest whole number ratio

Molecular Formula: Actual number of atoms

$\text{Molecular formula} = (\text{Empirical formula})^n$

$n = \dfrac{\text{Molecular mass}}{\text{Empirical formula mass}}$

8. Concentration Terms (JEE Main Favourite)

Molarity (M):

$M = \dfrac{n}{V(L)}$

Molality (m):

$m = \dfrac{n}{\text{mass of solvent (kg)}}$

Mole Fraction:

$\chi_A = \dfrac{n_A}{n_A + n_B}$

Trap: Temperature affects molarity, not molality

9. Dilution Formula

$M_1V_1 = M_2V_2$

Common Mistake: Forgetting to convert mL to L

10. How JEE Asks Mole Concept

  • Multi-step stoichiometry
  • Limiting reagent + yield
  • Gas volume + mole conversion
  • Concentration mixing problems

✔ NCERT aligned
✔ JEE Main 2026 ready
✔ PYQ linked for active learning

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