How do you find how much of something is produced if you know the MW and reacting w/ excess reagent?

You will need to start with a balanced reaction. From this, you can determine how many moles of product obtain from each mole of the limiting reagent. Given a specific quantity of the limiting reagent, you can use its molecular weight to determine the number of moles of the stuff, and then the number of moles of product, and applying its molecular weight, get the weight of the product.