SOME BASIC CONCEPTS IN CHEMISTRY
├── LAWS OF CHEMICAL COMBINATION
│   ├── Conservation of Mass (Lavoisier, 1789)
│   ├── Definite Proportions (Proust, 1799)
│   ├── Multiple Proportions (Dalton, 1803)
│   ├── Gaseous Volumes (Gay-Lussac, 1808)
│   └── Avogadro's Law (1811)
│
├── MOLE CONCEPT
│   ├── 1 mol = $6.022 \times 10^{23}$ entities
│   ├── Molar mass = mass (g) per mol
│   ├── 1 amu = $1.66054 \times 10^{-24}$ g
│   └── Molar volume at STP = 22.4 L/mol
│
├── MOLE CONVERSIONS (hub)
│   ├── Mass ↔ Moles (÷ or × molar mass)
│   ├── Moles ↔ Particles (÷ or × Nₐ)
│   └── Moles ↔ Volume at STP (÷ or × 22.4)
│
├── PERCENTAGE COMPOSITION & FORMULAE
│   ├── % element = (n × at. mass / molar mass) × 100
│   ├── Empirical formula — simplest ratio
│   │   └── Steps: divide by at. mass → ÷ by min → simplify
│   └── Molecular formula = n × EF (n = Mr / EF mass)
│
├── STOICHIOMETRY
│   ├── Balanced equation → mole ratios
│   ├── Limiting reagent — smallest (moles / coefficient)
│   └── Theoretical yield from limiting reagent
│
└── CONCENTRATION TERMS
    ├── Molarity (M) — mol/L — T-dependent
    ├── Molality (m) — mol/kg solvent — T-independent
    ├── Mole fraction (x) — dimensionless — T-independent
    ├── Normality (N) — eq/L — N = M × n-factor
    ├── Mass % — T-independent
    └── ppm — mg/kg — for trace concentrations