The law of definite proportions, the law of multipleproportions and this law form the basis of stoichiometry.
The operating lines on a McCabe-Thiele diagram are drawn to obey this principle. This law is exploited bythe lever rule, which gives the ratio of concentrations at equilibrium on a phase diagram. This law applied to acontrol volume sets the partial derivative of concentration with time equal to the negative gradient of flux,which reduces to the continuity equation at steady state. The law of definite proportions, the law of multipleproportions and this law form the basis of stoichiometry. The classic example of this principle involvesburning magnesium in air, a variant of Lavoisier's original experiments that discovered it. For 10 points, namethis principle which implies that matter is neither created nor destroyed. 
The operating lines on a McCabe-Thiele diagram are drawn to obey this principle.
 law of conservation of mass [or the law of conservation of matter; or the law of conservation of moles]