
Theoretical yield
I think that the theoretical yield is the max amount of the products that can be formed. But I don't see how to find it.
The problem that is stumping me is this:
Given the equation $\displaystyle Cu_2 + O_2 \to 2Cu + SO_2$, what is the theoretical yield of $\displaystyle SO_2$ if $\displaystyle 8.20 g$ of $\displaystyle 0_2$ are used ?
Will someone simply and clearly explain how to go about solving this?

You have 8.20 g of $\displaystyle O_{2}$ so we must "convert" it into terms of $\displaystyle SO_{2}$. However, remember that a chemical reaction is representative of each species in moles. So that is where we will start off with:
$\displaystyle \text{8.20 g } O_{2} \times \frac{\text{1 mol } O_{2} }{\text{32.00 g }O_{2} } = \text{0.25625 mol } O_{2}$
Now, we know that for every mole of $\displaystyle SO_{2}$ produced, 1 mole of $\displaystyle O_{2}$ was used up. So we have a 11 ratio.
$\displaystyle \text{0.25625 mol } O_{2} \times \frac{\text{1 mol } SO_{2} }{\text{1 mol } O_{2} } = \text{0.25625 mol } SO_{2} \text{ produced}$
Note how I set up the ratio so that the units "mol $\displaystyle O_{2}$" cancels out. That is pretty much the key in solving stoichiometry questions.
So you have the moles of $\displaystyle SO_{2}$ produced. Now all that is left is converting it into terms of grams and that should be your theoretical yield.

that makes complete sense! thank you. :D