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