mass balance
Atoms are not created or destroyed when a substance dissolves and reacts — they just rearrange into different forms. So whatever you put into a solution must still all be there, merely scattered across the various species it can become. Mass balance is the act of writing that conservation down: the total of all the forms a substance takes equals the amount you originally added.
Concretely, you pick an element or a chemical group and account for every species it ends up in. If you dissolve a weak acid, some stays as the intact acid and some becomes its conjugate base, and the mass balance says those two add up to the total amount you weighed out. Each form is weighted by how many of the tracked unit it contains.
Mass balance is the second pillar of the systematic treatment of equilibrium, working alongside the charge balance to supply enough equations to solve a multi-equilibrium problem exactly. Its honest limitation mirrors the charge balance: by itself it tells you the forms must add up, but not how the total splits among them — for that you still need the equilibrium constants that govern each interconversion.
Dissolve 0.10 mol of acetic acid per litre. Its mass balance is: [acetic acid] + [acetate] = 0.10 — whatever does not stay as the acid must show up as acetate, nothing is lost.
Every form a substance takes must sum back to the amount added.
Coefficients in a mass balance come from stoichiometry — how many of the tracked atom or group appear in each species — which is a different bookkeeping rule than the charge balance, where coefficients are the ionic charges. Mixing up the two is one of the most common errors in setting up the equations.