Stoichiometry is chemistry's bookkeeping, and atoms never go missing
A chemical reaction can rearrange atoms but never create, destroy or transform them. Every atom of carbon, hydrogen or oxygen that goes in must come out. That strict accounting, called stoichiometry, lets chemists work out exactly how much of each ingredient a reaction needs and how much it will produce.
Jeremias Benjamin Richter coined the word in 1792 in a German book on the art of measuring chemical elements, building it from the Ancient Greek words for element and measure. It rests on the conservation of mass and on the discovery that substances combine in fixed proportions. Because molecules are built from whole atoms, reactants always pair up in ratios of whole numbers.
Burning methane shows how it works. Written crudely, one methane plus one oxygen molecule giving carbon dioxide and water does not add up: four hydrogens and two oxygens go in, but two hydrogens and three oxygens come out. Putting a 2 in front of the water fixes the hydrogen, and a 2 in front of the oxygen fixes the oxygen. The balanced version says each molecule of methane consumes two of oxygen and yields one of carbon dioxide and two of water, and the same ratio holds for moles.
Turning those ratios into weights needs molar masses. By definition carbon-12 weighs exactly 12 grams per mole, and since the 2019 revision of the SI a mole contains exactly 6.02214076×1023 particles. Natural elements are mixtures of isotopes, so the figures are rarely neat: 17.031 grams of ammonia holds 14.007 grams of nitrogen rather than a tidy 14, because some nitrogen-15 and deuterium are mixed in. The same logic converts units, so 2.00 grams of table salt divided by 58.44 grams per mole comes to 0.0342 moles.
For gases at a known temperature and pressure, volumes combine in the same simple ratios as the molecules themselves, though mass ratios still have to be calculated. A catalyst is the exception to the ledger: it joins in one step and is regenerated in another, so it is not used up.
Source: Stoichiometry
