 Okay, let's do this problem. It says how many iron 2 ions are there in 5 grams of iron 2 sulfate? Mass of iron 2 sulfate. So you're going to have to remember the formula unit of iron 2 sulfate. If you're not able to do that, then you probably can't do this problem. Okay, so iron 2 sulfate is going to be FeSO4. So it wanted to know, well, how many iron 2 ions are there, right? So you can think about it like this. So for every FeSO4, you make 1 Fe2 plus and 1 SO4 2 minus. So it's just looking for the number of iron 2 ions. So we have the mass of iron 2 sulfate here. So we're going to have to figure out the number of moles of iron 2 sulfate. So let's figure that out. So the number of moles of iron 2 sulfate is going to be 5.0 grams. And we're going to have to know the molar mass of iron 2 sulfate. So let's calculate that. The periodic table here. So we've got 55.85 plus 32.07 plus 4 times 16. So the molar mass of iron 2 sulfate is 151.92 grams per mole. Okay, so we've got that many grams of iron 2 sulfate. We'll multiply that by the inverse of the molar mass. Because we want to cancel out the grams and get moles. So 151.92 grams of iron 2 sulfate per one mole of iron 2 sulfate. So that gives us the number of moles there, right? Let's look for, well, the number of atoms. Or sorry, the number of formula units of iron 2 sulfate. So for every one mole of iron 2 sulfate, right? So remember Avogadro's number. One mole equals 6.022 times 10 to the 23rd of anything. In this case formula units. So one mole of iron 2 sulfate is 6.022 times 10 to the 23rd formula units. You have iron 2 sulfate? Like that? Okay, but we don't want the number of formula units of iron 2 sulfate, right? We want the number of iron 2 ions, right? So for every one formula unit of iron 2 sulfate, you've got one iron 2 ion, okay? So we're not looking for that anymore. We're now looking for the number of ions of iron 2 sulfate, okay? Now all we've got to do is multiply these things together and divide them. So 5 divided by 151 times 022 equals times 1 divided by 1. We're going to get two three significant figures. 1.98 times 10 to the 22th F e 2 plus. In order to do your stoichiometry for this, you use your imbalance chemical equation. Any questions on that one? No. Okay.