Let's look at another scenario. You go into a shop, and you purchase an item whose price is #£23-30#. You give the clerk a #£50-00# note. The clerk gives you the item and #£17-70# in change. Would you know whether you had been short-changed? I think you would know, and you would have cause for complaint.
In the banking and finance caper, there is a saying, #"for every credit there must be a corresponding debit"#. What does this mean? It means that if you credit an account, some other account must be debited, or there must be cash on hand (cash, so I am told, is a debit item).
What applies in the banking and finance industries, applies even more so in chemistry, because there are more particles to balance. This illustrates the fundamental principle of #"stoichiometry"#. Mass is ALWAYS conserved. If you start with #10*g # of reactant, at most you are going to get #10*g# of product. In practice, you are not even going to get that, because losses invariably occur on handling. So bear this in mind the next time you balance a chemical equation: if the reactants were money, would you be ripped off? In honesty, I do not think you would be.
So (finally!) a problem. I combust #16*g# of methane, #CH_4#, according to the following rxn:
#CH_4(g) + 2O_2(g) rarr CO_2(g) + 2H_2O(l)#
What is the mass of oxygen gas that I need? What mass of carbon dioxide will I get? Note that mass (and atoms) are necessarily conserved in every chemical reaction.
Try the process with another example; the combustion of hexanes, #C_5H_12#. This combusts according the equation:
#"Pentane + oxygen "rarr" carbon dioxide + water"#
#C_5H_12 + xO_2(g)rarryCO_2+zH_2O#
How to balance this? Remember, if it were money you would do it automatically. Balance #C#, then #H#, and then #O#.
