Concept
A mass balance is the accounting statement that, at steady state, the mass entering any defined boundary equals the mass leaving it — for the total and for every component — because matter is conserved. It’s the primary check that a model’s streams are physically consistent: a flowsheet that doesn’t balance is wrong somewhere, even if every figure looks plausible.
The conservation law.
in = out + accumulationAt steady state accumulation is zero, so in = out — for total mass and, separately, for each component, which makes it a real constraint rather than one equation. A reacting component carries a generation/consumption term. Total mass is always conserved; moles are not (a reaction can change the number of molecules), so a molar balance must carry the stoichiometry while a mass balance need not.
A solvable system on a declared basis. Given feed flows and composition, per-pass conversion, and separation split fractions, the equations (the total plus one per component, around each unit operation and the whole plant) solve for the unknown flows and compositions — at the maturity anchor by deterministic arithmetic, often shortcut with a conserved “tie” component, not a flowsheet simulation. A balance can be drawn around one operation or the whole plant via the system boundary, and nested balances must reconcile (the unit balances sum to the plant balance). Every balance rides on a basis (per hour, per tonne of product, per 100 mol of feed) applied uniformly; recycle couples balances into a loop solved together, not in one forward sweep.