Some Basic Concepts in Chemistry: Essential Facts

Avogadro's number: $6.022 \times 10^{23}$ $mol^{-1}$ . This is the number of entities in 1 mole of any substance.
1 amu = $1.66054 \times 10^{-24}$ g = 1/12 the mass of one C-12 atom.
Molar mass in g/mol is numerically equal to atomic/molecular mass in amu.
Molar volume at STP = 22.4 L/mol (0 °C, 1 atm only — not applicable at 25 °C).
Empirical formula = simplest whole-number atom ratio. Molecular formula = n × empirical formula, where n = molar mass / EF mass.
Percentage composition: % element = (n × atomic mass / molar mass) × 100.
Limiting reagent = reactant with smallest value of (moles / stoichiometric coefficient); determines theoretical yield.
Molarity (M) is temperature-dependent (involves volume); Molality (m) is temperature-independent (involves mass of solvent).
Temperature-independent units: Molality, mole fraction, mass%, ppm.
Temperature-dependent units: Molarity, Normality.
Interconversion: M = (1000 × d × w%) / (Mr × 100); m = (1000 × w%) / (Mr × (100 − w%)).
Normality = Molarity × n-factor. Equivalent weight = Mr / n-factor.
Sum of mole fractions of all components = 1 (always).
ppm = mg solute / kg solution = mg/L for dilute aqueous solutions.
Laws in order: Conservation (1789) → Definite Proportions (1799) → Multiple Proportions (1803) → Gay-Lussac (1808) → Avogadro (1811).
NEET frequency: 1–2 questions per year; mostly numerical (mole conversions, empirical formula, concentration interconversion).