What is stoichiometry?

Answer 1

Ratios in Balanced Chemical Reactions

When you write a balanced chemical equation, the number of atoms that go into the reaction (as reactant) must equal the number of atoms that come out as products (for each type of atom). To balance a chemical equation, you must use coefficients in front of molecules to make these numbers come out right so that the reaction is balanced. Stoichiometry allows you to use these coefficients to predict how much of a certain molecule you will create for a reaction if you put in a certain amount of reactants. Let me illustrate with an example (which is the burning of propane gas in oxygen to form carbon dioxide and water):

C3H8 + O2 --> CO2 + H2O

This reaction is not balanced though (Look at how many C atoms go in and how many come out. Do the same with O and H. More go in than come out, right?). Instead it should be:

C3H8 + 5O2 --> 3CO2 + 4H2O

This balanced reaction tells us then that for each propane molecule, C3H8, that is burned, it will produce 3 molecules of carbon dioxide, CO2, and also 4 molecules of water, H2O. The ratio is 1 to 3 for carbon dioxide, and 1 to 4 for water. I can also say that if I know that I burned propane, and I produced 8 molecules of water, I know that I must have burned 2 molecules of C3H8.

The ratio is always 1 to 4, just like in balanced reaction above. It is all based on the ratio of the coefficients. I haven't mention oxygen yet, but it's the same thing. If I burn 1 molecule of propane, I'll need 5 of O2 in order for the reaction to work. The ratio here is 1 to 5. I can even say that if I burned propane and I get 5 molecules of water out, than I must have used 5 molecules of oxygen (and 1 molecule of propane, and also I got out 3 molecules of carbon dioxide along with the water!). The ratio is then 1:5:3:4, which is just like in the balanced reaction above. Note that we usually don't talk about single molecules burning, but rather moles of molecules (which is just a whole lot of molecules). The same rules work in exactly the same way with moles. If I burn 5 moles of propane, I know I will produce 15 moles of carbon dioxide and 20 moles of water. Or if I use up 5 moles of oxygen burning propane, I'll have burned 1 mole of propane, and made 3 moles of CO2 and 4 moles of H2O.
it is the study of relative proportions in which substances react or in which elements form compounds.