= Mass balance
A <mass balance> accounts for material entering, leaving and accumulating within a stated system. For total mass in ordinary nonrelativistic process calculations, $dM/dt=\sum\dot m_{\rm in}-\sum\dot m_{\rm out}$. A balance for one <chemical species> also includes its net production or consumption by <chemical reactions>.
For a well-mixed tank of constant volume $V$ flushed at flow $Q$ with solute-free liquid, the solute concentration satisfies $V\,dC/dt=-QC$. Thus $C(t)=C(0)e^{-Qt/V}$. With $V=100$ litres and $Q=10$ litres per minute, the concentration falls by a factor $e$ in ten minutes. The tank is at steady total inventory while its composition changes, showing why a total-mass balance and a species balance answer different questions.
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