Question

Why must a chemical equation be balanced to solve stoichiometry problems?

Answer 1

Because the chemical equation is the shorthand, or map if you like, of complex molecular rearrangement, which will always conserve mass.

Explanation:

Take the oxidation of hexanes, `C_6H_14`, the which reaction drives our motors.

I can represent its combustion reaction simply:

`C_6H_14(l) + (19/2)O_2(g) rarr 6CO_2(g) + 7H_2O(l)`

This reaction scheme tells me that for each 86 g of hexanes I burn, I am going to get approx. 264 g carbon dioxide as a by-product. This reaction scheme tells me a little bit more than this, in that this combustion will release a certain and measurable amount of energy, which I could also put in the reaction scheme (as an amount in Joules!). If the equations are not balanced correctly (or stoichiometrically) I can make no such predictions.

Please note that you practise such stoichiometry all the time; for instance, when you buy stuff at the supermarket; the value of the goods must equal the value of money that you give to the shop. These amounts must be stoichiometrically balanced, otherwise someone has been ripped off; make sure it's not you!