Law of conservation of mass (Lavoisier): total mass of reactants = total mass of products. Law of definite proportions (Proust): a compound always contains the same elements in the same mass ratio. Law of multiple proportions (Dalton): when two elements form multiple compounds, the different masses of one element combining with a fixed mass of the other are in simple whole number ratios. Example: CO and CO2 — O masses for fixed C are 16 and 32, ratio 1:2. Gay-Lussac's law: gases combine in simple whole number ratios by volume. Avogadro's law: equal volumes of gases at same T, P contain equal molecules.