 All right time to do a three-step problem similar to the two-step problems, but with another step So we have a little bit more thinking to do we have a little bit more planning to do before we jump into the calculations Again, we're gonna start just like we do the two-step problems by looking at what we're given 250 grams of water and What it is we're trying to find here the number of grams of hydrogen fluoride I'm going to try to figure out how we're going to get from here to here So we start off with the grams the 250 grams of water and we ask ourself What can we do with 250 grams of water and the answer is the only thing we can do is Convert to moles of water and then I kind of jump over here and look at that grams of hydrogen fluoride And I ask myself a what am I going to need? In order to find the number of grams of hydrogen fluoride and the answer that question is I'm gonna have to know how many moles I've got as if I have moles of hydrogen fluoride. I convert that to grams Then I look at what I've got here in the middle moles of water moles of hydrogen fluoride Do I have a balanced equation that would allow me to go back and forth between the two of those and by golly? I do So I can make that conversion. I've got a three-step problem here But the first step will be going from grams of water to moles of water The second step will be going from moles of water to moles of hydrogen fluoride using that balanced equation And my third step will be going from moles of hydrogen fluoride to grams of hydrogen fluoride I'm a mass conversion at the end So let's get started first conversion Grams of water to moles of water to do the mole mass conversion for a compound. I need to know its formula mass Water is composed of hydrogen and oxygen There are two hydrogens Each one has a mass of 1.01 Giving me a total mass of 2.02 H2O has one oxygen Oxygen is 16.00 times 1 is 16.00 Add the two together the formula mass of water is 18.02 grams So we can say one mole of water Equals 18.02 grams of water Just like that Now we can do our conversion Always start with what we were given We were given 250 grams of water to work with Write that down first All of these equations all this math always starts with the given times and Then the conversion factor that fraction that will cancel out the grams of water It's made from these two numbers and the equivalence that we just put together We've got grams of water as are given. So we have to put the grams on the bottom It's all about unit cancellation. We need to get rid of the grams of water and convert it to moles so we can move on In these types of problems. We can ignore the ones are 18.02 is on the bottom. So we'll divide by it 250 divided by 13.02 is 13.87 if I'm using significant figures I'd run that to 14 First step done now time for step two and in step two we decided we're going to convert from moles of water to moles of Hydrogen fluoride do that we need the equivalence from the balanced equation. We find water To two in front of it So you write two moles of water hydrogen fluoride Find it in the balanced equation There's a four in front of it So four moles of hydrogen fluoride It's a ratio Too bad for them. At this point you could reduce that to one to two if you want But why do extra math especially if you're already struggling and Really focused on how to set this stuff up. Just leave it the way it is Now carry this number over as the starting point for your next calculation So we're starting with the 14 moles of water Times and then our conversion factor now We can't look at moles this time to decide what to put where because they're both moles This is why I'm always getting on your case when you don't write down the formulas. You got to write down the Unit and the formula In this case, it's the formula. We're trying to cancel. It's the water. We're trying to cancel So we got to put the water on the bottom and we'll put the hydrogen fluoride on the top moles of water cancel out Just like we wanted them to We're left with moles of hydrogen fluoride is what's just where we need to be when this step is done Now we have numbers on the top and on the bottom and again will multiply by numbers on the top We'll divide by numbers on the bottom So we'll take 14 times 4 and then we'll divide by 2 And that gives us 28 28 moles of hydrogen fluoride now. We are ready to wrap this thing up We want grams of hydrogen fluoride. We have moles of hydrogen fluoride. We have what we need to do that final conversion So on to step three moles of hydrogen fluoride to grams and As always whenever we're doing a mole mass conversion We need to have a formula mass There's hydrogen and fluorine in hydrogen fluoride There's no subscript on the hydrogen. So that's one Times one point zero one There is no subscript on the fluorine. So that's times one Nineteen point zero zero is its mass from the periodic table Add those up twenty point zero one grams. So one mole hydrogen fluoride Equals twenty point zero one grams of it and we're ready to move on get that math rolling take the answer from the previous step 28 moles hydrogen fluoride Times and then the conversion factor Remember when we make a conversion factor the numbers come from the equivalence. We just made I Have moles of hydrogen fluoride. So I'll put the moles of hydrogen fluoride in the bottom I'll take the grams and put it on top moles cancel So 28 times twenty point zero one again multiplying because the number that we're interested in is on the top ignoring the one again 560 grams Hydrogen fluoride look at all that work But we're done That's a three-step problem Now sometimes there's more mass conversions all the way through it like in this case sometimes there's more volume conversions to throw in there Sometimes there's more particle conversions ago in these things too, but again the thought process is the same Look at what you're given and ask yourself. What can I do with that? Look at what you're trying to find and ask yourself. What do I need to get that? Odds are that'll take you right down the path you need to figure out how to do your three-step problem in True stoichiometry problems the middle step will be your mole ratio conversion Whenever you're you're doing a three-step problem. So once you figured out that these things become much much easier All right, that's it enjoy. I know you will