How does mass balance apply in a chemical equation?

1 Answer
Mar 10, 2016


By the principle of conservation of mass: GARBAGE IN EQUALS GARBAGE OUT.


Mass is CONSERVED in every chemical reaction. If I start with 10 g of reactant (from all sources) AT MOST I can get 10 g of product; in practice, I can't even get that; I am going to lose something on handling.

Because elements and compounds have definite masses, their products on reaction are going to have those same masses.

Let's take a simple example, the combustion of methane, on which our industrial society depends:

#CH_4(g) + 2O_2(g) rarr CO_2(g) + 2H_2O(g)#

This balanced equation says in fact that 16 g of methane gas reacts with 64 g dioxygen gas to give 44 g of carbon dioxide gas and 36 g water vapour. (From where am I getting these numbers?) The mass of the products is PRECISELY EQUAL to the mass of the reactants. If the masses were not equal, I would know that I have made an error in my arithmetic, and would have to try again.

Every chemical reaction ever performed is known to follow the principle of conservation of mass; which is, I suppose, why educators insist that chemical equations be stoichiometrically balanced. #"Stoichiometry"# is a fancy way of saying #"balanced"# or #"in the right measure"#. Stoichiometry also applies to the banking and finance industries: for every credit to someone's account, there must be a corresponding debit to someone else's account. If there is not, then someone (the bank or you) has made a mistake. A banker or an accountant would not disagree with what I have said.