In this problem we're going to find the mass in grams of hydrogen found in a sample of calcium dihydrogen phosphate. And the first thing we want to look at is kind of come up with our plan of how we're going to approach this problem. We see that we have our grams of our compound. And I want to go from grams of my compounds to moles of compound because I know there's a relationship between the moles of the compounds and the moles of hydrogen in that compound. And what we're, that's going to allow us to do is use that ratio to find moles of hydrogen and then we can find grams of hydrogen. So this is the plan we're going to take. The first thing we're going to have to do is in order to get from grams to moles is we're going to have to know the molar mass. So we look at calcium, which has a molar mass of 40.01, and there's one of those in the compound. We have hydrogen, and there are four of those, each with a mass of 1.08. For our phosphorus, there are two phosphorus, each with a mass of 30.97. And our oxygen and there are 8 of those times 16. And so what we can actually do is add all those up and we get a molar mass of 233.98 grams per mole. So now we know the relationship for this compound, for grams and moles. So we're able to get from grams of the compound to moles of the compound. So, now let's look and see how we're going to set this up now that we have our molar mass. And we said we're going to start with our grams of our compound, which is given in the problem, and so that is 8.92 grams of Ca(H2PO4)2. And we're going to use our molar mass, our 233.98 grams per mole of the compound. Then we're going to say, well for every one mole of Ca(H2PO4)2 we have, we have 4 mols of hydrogen. And where this comes from is these subscripts. Here we have two mols of hydrogen for one of these ions, and we have two of those ions per unit of the compound. So I know I have four mols of hydrogen for every one mol of the entire compound. Now we can use our molar mass of hydrogen, which is 1.008 grams per mole of hydrogen. And I'm going to go back and check through my units, just to make sure everything is canceling out correctly. Here I have grams of the compound. Cancel that with grams. Mols and mols cancel. Mols of hydrogen, mols of hydrogen cancel, and I've got grams of hydrogen remaining. Make sure when you're cancelling out mols that you cancel out mols of hydrogen with mols of hydrogen or mols of the compound with mols of the compound. They have to be like units and like substance. Now, I can do the calculation, and what I end up with is that I have 0.1537 grams of hydrogen. And because my initial number here only has three significant figures, I need to round that to 0.154 grams of hydrogen found in my 8.92 grams of calcium dihydrogen phosphate.
