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Some Basic Concepts of Chemistry

NCERT Class 11 Chemistry Chapter 1

Some Basic Concepts of Chemistry for Class 11, JEE and NEET: laws of chemical combination, atomic and molecular masses, the mole concept and stoichiometry.

10 topics Updated 8 Nov 2026

For: Class 11, JEE and NEET, unless a topic says otherwise

In the syllabus: JEE Class 11 syllabus, NEET Class 11 syllabus

With your textbook

NCERT Chemistry Part I (Class 11), Chapter 1 “Some Basic Concepts of Chemistry”, reprint 2026-27.

Open the official NCERT chapter (PDF, 4.0 MB, ncert.nic.in)

Checked on 19 Oct 2026.

Learn in this order

  1. Importance of chemistry

    Medicines, fertilisers, new materials, industry and the environment, with NCERT’s history of chemistry.

    Class 11 only NCERT Section 1.1 11 min read 3 solved examples 6 practice questions

  2. Nature of matter

    The three states and the classification of matter into mixtures, elements and compounds, with solved examples.

    NCERT Section 1.2 12 min read 3 solved examples 6 practice questions

  3. Properties of matter and their measurement

    Physical and chemical properties, SI units, mass, volume, density and temperature scales, with solved examples.

    Class 11 only NCERT Section 1.3 14 min read 4 solved examples 7 practice questions

  4. Uncertainty in measurement

    Scientific notation, significant figures, precision, accuracy, rounding and the unit factor method.

    Class 11 only NCERT Section 1.4 16 min read 5 solved examples 7 practice questions

  5. Laws of chemical combination

    Conservation of mass, definite and multiple proportions, Gay-Lussac and Avogadro.

    NCERT Section 1.5 13 min read 4 solved examples 7 practice questions

  6. Dalton’s atomic theory

    The four postulates, how they explain the laws of chemical combination, and where the theory fails.

    NCERT Section 1.6 13 min read 3 solved examples 6 practice questions

  7. Atomic and molecular masses

    The carbon-12 scale, average atomic mass from isotopes, and formula mass, with solved examples.

    NCERT Section 1.7 12 min read 4 solved examples 6 practice questions

  8. Mole concept and molar mass

    Avogadro number, molar mass and the formulas linking mass, moles, particles and gas volume, with solved examples.

    NCERT Section 1.8 12 min read 5 solved examples 7 practice questions

  9. Percentage composition

    Mass per cent of elements, empirical and molecular formulas, with NCERT examples worked step by step.

    NCERT Section 1.9 12 min read 4 solved examples 6 practice questions

  10. Stoichiometry and stoichiometric calculations

    Balancing equations, mole ratios, the limiting reagent, mass per cent, mole fraction, molarity and molality.

    NCERT Section 1.10 19 min read 6 solved examples 10 practice questions

Key formulas in this chapter

Importance of chemistry

  • Cisplatin: cis-Pt(NH)Cl (cancer therapy)
  • AZT: CHNO (against HIV)
  • N + 3H ⇌ 2NH (ammonia for fertilisers)
  • Urea: CO(NH)
  • Ozone loss: Cl + O → ClO + O, then ClO + O → Cl + O
  • Greenhouse gases: CO, CH

Nature of matter

  • Metal lattices: Na, Cu; molecular elements: H, N, O
  • Compounds: HO (2 H: 1 O), CO (1 C: 2 O)
  • Water: H: O by mass = 2.016: 16.00 (about 1: 8)
  • Water is about 11.2% hydrogen by mass

Properties of matter

  • density = mass/volume (kg m; 1 g cm = 1000 kg m)
  • °F = (9/5)(°C) + 32
  • K = °C + 273.15
  • 1 L = 1 dm = 1000 cm = 1000 mL = 10 m
  • 1 kg = 1000 g
  • weight = mass × g (in newtons)

Uncertainty in measurement

  • Scientific notation: N × 10, with 1 ≤ N < 10
  • (a × 10)(b × 10) = ab × 10; (a × 10)/(b × 10) = (a/b) × 10
  • Add or subtract: same exponent first, then the digit terms
  • Sums and differences: fewest decimal places
  • Products and quotients: fewest significant figures
  • Unit factor: (2.54 cm)/(1 in) = 1

Laws of chemical combination

  • Conservation: mass of reactants = mass of products
  • Water: H: O by mass = 2: 16 = 1: 8
  • Multiple proportions: O per 2 g H in HO and HO = 16 g: 32 g = 1: 2
  • Gay-Lussac: 2 vol H + 1 vol O → 2 vol HO(g)
  • Avogadro: V ∝ number of molecules (same T and p)

Dalton’s atomic theory

  • 2H + O → 2HO: 4 H and 2 O atoms on each side
  • Water: 2 H atoms per O atom, so H: O by mass = 2.016: 16.00
  • CO and CO: 1 and 2 O atoms per C atom, so O masses 1: 2
  • Isotopes: Cl and Cl, same element, different masses

Atomic and molecular masses

  • 1 u = (1/12) × mass of one C atom = 1.66056 × 10 g
  • Average atomic mass = Σ (fractional abundance × isotopic mass)
  • Molecular mass = Σ (number of atoms × atomic mass)
  • Formula mass of NaCl = 23.0 u + 35.5 u = 58.5 u
  • CH = 16.043 u; HO = 18.02 u; CHO = 180.162 u

Mole concept

  • 1 mol = 6.02214076 × 10 entities (exact)
  • Avogadro constant = 6.022 × 10 mol
  • n = m/M
  • n = N/NA
  • Molar mass in g mol ≈ formula mass in u (same numerical value at school precision)
  • Gas at 273.15 K and 1 bar: 1 mol ≈ 22.7 L (22.4 L at 273 K, 1 atm)

Percentage composition

  • Mass % of element = (mass of element in 1 mol of compound ÷ molar mass) × 100
  • Moles of element in 100 g = mass % ÷ atomic mass
  • n = molar mass ÷ empirical formula mass
  • Molecular formula = n × empirical formula
  • Water: H 11.19%, O 88.79%

Stoichiometry

  • Mass per cent = (mass of solute ÷ mass of solution) × 100
  • xA = nA/(nA + nB); xA + xB = 1
  • Molarity M = moles of solute ÷ volume of solution in L
  • Molality m = moles of solute ÷ mass of solvent in kg
  • MV = MV (dilution)
  • CH + 2O → CO + 2HO; CH + 5O → 3CO + 4HO

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