Atoms and molecules are far too small and far too numerous to count individually in any real laboratory sample, so chemists invented the mole, a fixed counting unit representing roughly six hundred billion trillion of whatever's being counted, the same way "dozen" fixes a count at twelve regardless of what's being counted. The mole's real power is that it lets a chemist convert directly between a substance's measurable mass, something an ordinary laboratory scale can weigh, and the actual number of atoms or molecules that mass contains. Stoichiometry is the branch of chemistry that uses exactly that mole-based conversion to predict how much product a chemical reaction will actually yield.
The mole bridges the huge gap between weighable mass and countable atoms
A single atom's mass is far too small to weigh directly on any practical scale, but a mole of atoms, that same fixed, enormous count, has a total mass that's easily weighable, and that total mass, a substance's molar mass, is fixed and known for every element and compound. This relationship is exactly what lets a chemist go from a weighed mass in the lab to a known number of moles, and from moles to the actual atom or molecule count, moving fluidly between the microscopic scale chemistry actually happens at and the macroscopic scale a laboratory can actually measure.
Stoichiometry uses mole ratios from a balanced equation to predict reaction quantities
A balanced chemical equation states the exact ratio of moles of each reactant that combine to form moles of each product, and stoichiometry uses that fixed mole ratio, combined with the mass-to-mole conversion the mole concept provides, to predict exactly how many grams of product a given mass of starting reactant will actually yield. This is exactly the calculation underlying real chemical work, from a school laboratory experiment to industrial-scale chemical manufacturing: knowing how much of each ingredient to combine, and how much product to actually expect, depends entirely on this mole-based bridge between mass and molecular count.
What we're still unsure about
That the mole provides an exact, reliable bridge between measurable mass and molecular count, and that stoichiometry correctly predicts reaction quantities from balanced equations, is extremely well established, foundational chemistry confirmed across more than a century of precise laboratory and industrial practice. What's more genuinely a matter of practical, ongoing difficulty is exactly how closely a real reaction's actual yield matches the stoichiometric prediction, since real reactions rarely go to full completion or perfect purity, side reactions, incomplete conversion and practical losses during the process all reduce actual yield below the theoretical maximum, and chemists continue to develop better techniques for pushing real yields closer to that theoretical stoichiometric prediction, rather than the prediction alone guaranteeing what a lab bench will actually produce.
This sits inside Stoichiometry & the Mole Concept, one of eight topics in General Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.