Some Basic Concepts of Chemistry

Chapter focus: Mole concept, stoichiometry, concentration terms, laws of chemical combination (JEE Main).

1. Substance (Matter)

Anything having mass and volume is called a substance.

• Physical classification: Solid, Liquid, Gas
• Chemical classification:
  • Pure substances → Elements (metal, non-metal, metalloid), Compounds
  • Impure substances → Mixtures (homogeneous, heterogeneous)

2. Mole Concept

Avogadro’s Number (N_A):
N_A = 6.022 × 10²³

Mole:
One mole is the amount of substance containing Avogadro’s number of particles.

Atomic Mass:
= (Mass of one atom) / (1/12 × mass of one C-12 atom)

Molecular Mass:
Sum of atomic masses of all atoms in a molecule

Example:
Molecular mass of H₂SO₄ = (2 × 1) + 32 + (4 × 16) = 98

3. Number of Moles

Moles of atoms:
Moles = Given mass / Atomic mass

Moles of molecules:
Moles = Given mass / Molecular mass

Number of particles:
Number = Moles × N_A

4. Gaseous State (STP)

Molar volume at STP:
1 mole of any gas occupies 22.4 L

Moles of gas:
Moles = Volume (in L at STP) / 22.4

Loschmidt Number:
Number of molecules in 1 mL gas at STP = 2.687 × 10¹⁹

5. Equivalent Weight & n-Factor

Equivalent Weight:
= Molecular weight / n-factor

• Acid: n-factor = number of H⁺ ions replaced
• Base: n-factor = number of OH⁻ ions replaced
• Salt: n-factor = total ionic charge
• Oxidising agent: electrons lost
• Reducing agent: electrons gained

Gram equivalent:
= Weight (g) / Equivalent weight

6. Concentration Terms

Molarity (M):
M = Moles of solute / Volume of solution (in L)

Normality (N):
N = Molarity × n-factor

Molality (m):
m = Moles of solute / Mass of solvent (in kg)

Strength (g/L):
= Molarity × Molar mass

7. Percentage Composition

• w/w %: (Mass of solute / Mass of solution) × 100
• v/v %: (Volume of solute / Volume of solution) × 100
• w/v %: (Mass of solute / Volume of solution) × 100

Relation:
If % (w/w) = x, density = d, molar mass = M
Molarity = (10 × x × d) / M

8. Dilution Formula

M₁V₁ = M₂V₂

During dilution, moles of solute remain constant.

9. Empirical & Molecular Formula

Empirical formula: Simplest whole-number ratio of atoms

Molecular formula:
= Empirical formula × n

10. Laws of Chemical Combination

1. Law of Conservation of Mass: Mass is neither created nor destroyed
2. Law of Definite Proportions: Elements combine in fixed mass ratios
3. Law of Multiple Proportions: Masses combine in simple whole-number ratios
4. Law of Reciprocal Proportions
5. Law of Gaseous Volumes
6. Avogadro’s Law: Equal volumes of gases contain equal number of molecules