Class 11 Chemistry – Some Basic Concepts of Chemistry

CBSE 2026–27 | NCERT-aligned free study resource

Chapter Overview

This chapter develops the quantitative language of chemistry. It covers matter and its measurement, laws of chemical combination, Dalton’s atomic theory, atomic and molecular masses, the mole concept, molar mass, percentage composition, stoichiometry and stoichiometric calculations.

1. Nature of Matter

Matter has mass and occupies space. It can be classified broadly as pure substances and mixtures. Elements contain one kind of atom, compounds contain elements chemically combined in fixed proportions, and mixtures contain substances physically combined in variable proportions. Properties may be physical or chemical, and extensive or intensive.

2. Measurement and Uncertainty

Important measurements include mass, volume, temperature and amount of substance. Significant figures communicate the precision of a measured value. In calculations, the result should not imply greater precision than the measurements allow.

3. Laws of Chemical Combination

The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. The law of definite proportions states that a pure compound contains the same elements in a fixed ratio by mass. The law of multiple proportions applies when two elements form more than one compound: the masses of one element that combine with a fixed mass of the other are in small whole-number ratios.

4. Dalton’s Atomic Theory

Dalton proposed that matter consists of atoms, atoms of an element are similar in their chemical properties, compounds form by combination of atoms in simple whole-number ratios, and chemical reactions involve rearrangement of atoms. Modern discoveries refined several of these statements.

5. Atomic and Molecular Masses

Relative atomic mass is compared with 1/12 of the mass of a carbon-12 atom. Molecular mass is the sum of the relative atomic masses of atoms present in a molecule. Molar mass is the mass of one mole of a substance, expressed in g mol⁻¹.

6. Mole Concept

One mole contains Avogadro’s number, 6.022 × 10²³, of specified entities. Use n = m/M to convert mass to moles, and N = nN_A to convert moles to number of entities.

7. Percentage Composition

Mass percentage of an element = (mass of that element in one mole of compound / molar mass of compound) × 100.

8. Stoichiometry

A balanced chemical equation gives mole relationships among reactants and products. Always balance the equation before using its coefficients for calculations. Identify the limiting reagent when reactants are supplied in unequal amounts. The limiting reagent determines the maximum theoretical amount of product.

Worked Example

For 2H₂ + O₂ → 2H₂O, 2 mol H₂ react with 1 mol O₂ to produce 2 mol H₂O. If 3 mol H₂ and 1 mol O₂ are supplied, O₂ is limiting and 2 mol H₂O can form.

Important Formulae

• n = m/M
• N = nN_A
• Molarity = moles of solute / volume of solution in L
• Mass percentage = (mass of component / total mass) × 100

MCQs

1. One mole contains: (A) 6.022×10²³ entities (B) 6.022×10²² (C) 10²³ kg (D) 1 entity. Answer: A.
2. The limiting reagent is the reactant that: (A) has greatest mass (B) is consumed first (C) has greatest molar mass (D) remains after reaction. Answer: B.
3. Which law states that elements in a compound occur in fixed mass ratios? (A) conservation of mass (B) definite proportions (C) reciprocal proportions (D) gaseous volumes. Answer: B.
4. Moles in 18 g water are: (A) 0.5 (B) 1 (C) 2 (D) 18. Answer: B.
5. The coefficients of a balanced equation represent: (A) masses only (B) mole ratios (C) atomic numbers (D) densities. Answer: B.

Short-Answer Practice

1. Differentiate between molar mass and molecular mass.
2. Explain significant figures with one example.
3. State the law of definite proportions.
4. What is a limiting reagent?

Long-Answer / Numerical Practice

1. Calculate the number of molecules in a given mass of water and explain each conversion step.
2. For a balanced reaction with specified masses of two reactants, identify the limiting reagent and calculate theoretical product.

HOTS / Competency

A reaction requires reactants in a 2:1 mole ratio. A student uses 5 mol and 2 mol respectively. Determine which reactant limits the reaction and justify using the balanced equation.

Quick Revision

Balance → convert quantities to moles → use coefficient ratio → identify limiting reagent → calculate product → convert to required unit.