SUMMARY

This unit built, step by step, the quantitative toolkit that the rest of Chemistry relies on.

Matter and mass. Matter (anything with mass that occupies space) splits by physical state into solids, liquids and gases, and by chemical composition into pure substances (elements, made of one type of atom; and compounds, made of two or more different elements chemically bonded, with properties unlike either constituent) and mixtures (homogeneous or heterogeneous, with no chemical interaction between components). Because atoms are too small to weigh directly, IUPAC fixed carbon-12 as the mass standard, defining the unified atomic mass unit (1 u ≈ 1.6605 × 10⁻²⁷ kg) as one-twelfth its mass. Relative atomic mass is the ratio of an atom's average mass (averaged over its natural isotopes) to 1 u; relative molecular mass is the sum of the relative atomic masses in a formula.

Counting atoms: the mole. A mole is the amount of substance containing as many elementary entities as there are atoms in 12 g of carbon-12 -- a fixed number, the Avogadro number, 6.022 × 10²³ (no unit). Molar mass is the mass of one mole (g mol⁻¹); molar volume is the volume of one mole of gas (22.4 L at 273 K/1 atm; 22.71 L at STP as defined here; 24.47 L at SATP). The parallel gram equivalent concept is built on equivalent mass instead of molecular mass: gram equivalent mass = molar mass ÷ equivalence factor, where the equivalence factor is the acid's basicity, the base's acidity, or the number of electrons an oxidising/reducing agent transfers. Because equivalents always react 1:1 without needing a balanced equation, this concept is especially useful for redox reactions.

From composition to formula, and reaction to yield. Elemental analysis gives mass percentages, from which the empirical formula (simplest atom ratio) is calculated, and then -- using the molar mass -- the molecular formula (actual atom count, always a whole-number multiple of the empirical formula). Stoichiometry gives the numerical mole relationship between reactants and products in a balanced equation, freely convertible between moles, mass and gas volume; when reactants are not supplied in exactly this ratio, the reactant consumed first (the limiting reagent) determines how much product actually forms. …