Some Basic Concepts of Chemistry – Class 11 Notes

Some Basic Concepts of Chemistry – Class 11 Notes

1.1 Importance of Chemistry

  • Chemistry is the study of matter, its properties, and transformations.
  • Plays a vital role in medicine, agriculture, industry, and environmental science.

1.2 Nature of Matter

  • Matter: Anything that occupies space and has mass.
  • States of matter: Solid, liquid, gas
  • Composed of atoms and molecules.

1.3 Properties of Matter and their Measurement

  • Physical properties: Can be observed without changing composition (e.g., density, color, melting point).
  • Chemical properties: Involve change in composition (e.g., flammability, reactivity).
  • Measurements: Must be precise and accurate; expressed in SI units.

1.4 Uncertainty in Measurement

  • All measurements have uncertainty.
  • Significant figures: Indicate precision.
  • Example: 12.34 → 4 significant figures
  • Error analysis helps improve reliability of results.

1.5 Laws of Chemical Combinations

  1. Law of Conservation of Mass: Mass of reactants = mass of products.
  2. Law of Definite Proportions: A compound always contains elements in fixed proportion by mass.
  3. Law of Multiple Proportions: Elements can combine in more than one way to form different compounds with small whole-number ratios.

1.6 Dalton’s Atomic Theory

  • Matter is made of indivisible atoms.
  • Atoms of same element are identical; different elements have different atoms.
  • Compounds are formed by combination of atoms in simple ratios.
  • Chemical reactions involve rearrangement of atoms.

1.7 Atomic and Molecular Masses

  • Atomic mass: Average mass of an element’s atom relative to 12C isotope.
  • Molecular mass: Sum of atomic masses in a molecule.
  • Units: amu (atomic mass unit)

1.8 Mole Concept and Molar Masses

  • Mole: Amount of substance containing 6.022×10236.022 \times 10^{23}6.022×1023 particles (Avogadro’s number).
  • Molar mass: Mass of 1 mole of a substance = molecular mass in grams.

1.9 Percentage Composition

  • % Composition of an element in a compound:

%element=Mass of elementMolar mass of compound×100\% \text{element} = \frac{\text{Mass of element}}{\text{Molar mass of compound}} \times 100%element=Molar mass of compoundMass of element​×100

1.10 Stoichiometry and Stoichiometric Calculations

  • Stoichiometry: Relationship between reactants and products in a chemical reaction.
  • Calculations involve:
    • Mole-to-mole ratios
    • Mass-to-mass conversions
    • Limiting reagent determination