 Okay, so let's do another one of these practice exam problems. It says, if you don't mind, it says in the following reaction iron metal was reacted with excess oxygen gas forming iron 3 oxide as the product. How many grams of iron 3 oxide will form if we start with 5 grams of iron? The cool thing about this is that they give you the balance chemical equation. Okay, balance chemical equation. So that really helps us. But we're trying to look for the mass of iron 3 oxide and we're only given the starting material iron. Okay, so in order to do this we're going to have to go through converting grams to moles then use the molar ratio moles to moles and then from iron to iron 3 oxide and then from there we get the moles of iron 3 oxide. We're going to have to figure out the mass using the molecular weight or the molar mass of iron 3 oxide. So first we start by converting grams to moles. How do we do that? Using the atomic mass. Okay, so when we look up there the atomic mass of iron is 55.85 grams of iron or one mole like that. So that cancels, cancels. So now we're at moles of iron. So we don't want moles of iron. We want moles of iron 3 oxide. So we have to use the balance chemical equation here to get that molar ratio. So what's the molar ratio? For every 4 moles of iron we've got 2 moles of iron 3 oxide like that. Okay, so cancel, cancel. So we're now at moles of iron 3 oxide but we want mass of iron 3 oxide. So we're going to have to use the molar mass of iron 3 oxide. So for every 1 mole of iron 3 oxide what's the mass of iron 3 oxide? Well we'll have to figure it out. So there's 2 iron atoms so 55.85 times 2 plus 16 times 3. So we get 159.7 iron 3 oxide. Okay, for every mole. Cancel, cancel. And look what we're in now. We're in mass of iron 3 oxide. Isn't that what we wanted? Okay, so we're there. So all we've got to do is do this calculation here and we get the number that we want. Okay, to 3 signals. Okay, so 5 divided by 15.85 9 times 2 divided by 4 times 159.6. And the final answer to 3 sig figs is going to be 7.15 grams iron 3 oxide. Okay, any questions on that one? Pretty straight forward, right? Got that one. You got that one? Can I kill it?