[Don't panic, and no need to 'talk' in CAPS, WE WILL TEACH YOU HOW TO DO IT!]From Wikipedia: "Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions."
I personally think of it as just another way to test us on moles, but that's not the point. Moving on!
If the Wikipedian definition wasn't clear, Stoichiometry, for us, is simply finding a quantity of a product given some prior information. This information can come in the form of mass of a certain substance, molarity, moles, and even volume. Whatever the question may be, we first need a balanced equation. If you don't recall, click here for a refresher.
Let's use the combustion of methanol as an example.
2 CH3OH + 3 O2 → 2 CO2 + 4 H2O
As we can see, this equation is already balanced. This is important, because if we don't balance it, EVERYTHING is going to be buggered; proper ratios are essential for doing stoichiometry.
From the reaction above, we can see that one of the ratios, namely Oxygen to Carbon Dioxide, is 2:3. That means for every 2 Carbon Dioxide molecules (or moles), there are three Oxygen molecules (or moles). These ratios are called molar ratios, and are needed to solve questions in this unit.
Example: How many moles of Oxygen Gas are required to completely react with 6.00 moles of methanol?
Solution: 6.00molCH4O*(3 mol O2)/(2 mol CH4O)
=9.00 mol O2 (Don't forget your sig figs!)
Alright, that was child's play. Let's do a real question.
How many grams of Carbon Dioxide gas is created when sufficient Methanol is reacted with 10.4L of Oxygen gas at STP?
Solution: Recall that 1mol of any gas at STP is 22.4L, the ratio of Oxygen to CO2 is 2:3.
10.4L O2 * (1 mol O2)/(22.4L O2)*(2 mol CO2)/(3 mol O2)*(12.0+16.0*2)g CO2/(1 mol CO2)
=13.6gCO2
In essence, it's just what we've been doing all year, conversions. Conversions with moles, units, volume, and mass. If you get the hang of it, no problem!
Below is a Javascript calculator that gives the amount of moles and grams of a substance relative to another substance in the reaction. This is a helpful checker, but it SHOULD NOT be used to do your homework for you. It doesn't do sig figs and only goes to 5 decimal digits, anyway.
Disclaimer: The creator of this calculator and the writers at PinchOfKCN are NOT responsible for the consequences that occur from using this program, including but not limited to low grades, suspension, or setting your computer on fire.
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