 All right, so let's talk about how to deal with multi-step problems And the way that I'm going to do this with you is by taking you in steps starting with two step problems And working our way up to three step problems these questions are More difficult because what happens in these questions not only do you have to figure out what kind of conversion to do But you got to figure out Multiple conversions to do and you got to figure out what order to do them in in order to get the answer That you're looking for it really comes down to reading questions carefully Really looking at what it is you're trying to to accomplish there making yourself a road map and then following it You show you what one of these might look like we're going to convert 37.5 grams of water To molecules a two-step problem. I Really have to look at what I've got here to figure out what to do I'll start with the given I've got 37.5 grams of water. So I'm starting with grams of water. I Look at what I'm trying to find. I'm trying to find molecules of water And I think about all those different conversions. I know I know how to convert from grams to moles. I Know how to convert from Moles to molecules. I have nothing though that will take me directly from here to there I have no equivalents in my bag of tricks that will take me directly from grams to molecules So that's telling me I'm going to have to do this in two steps I know how to convert from grams to moles and then once I have those moles I can convert from moles To molecules so that's how I'm going to have to do it I'm going to have to go from grams to moles and from moles to molecules and again There's nothing in the question that tells you that that's just your ability to reason through a problem That's just your ability to think through a problem. That's why research says that your your score in stoichiometry Is the number two indicator of how well you're going to do in college? I don't think it's because the math is hard and I don't think it's because the The concepts are so abstract. I think it's because of this problem-solving process It's you have to figure out what to do on your own And that's something that you definitely have to do a lot of in college You have to do a lot of independent thinking and you have to learn a lot of things independently Each of these arrows represents a conversion that we have to do So in conversion number one, we're going to take our grams and convert it to moles And then in conversion number two, we're going to take our moles and convert it to molecules So let's get started when I convert from grams to moles. I know I'm going to need a formula mass so I take the water and I break it down I've got two hydrogens each one with a mass of one point zero one Gives me two point zero two from my hydrogen. I Have oxygen. There's no subscript on it. So I know that means there's one of them Oxygen is sixteen point zero zero So our total is sixteen point zero zero add them up Eighteen point zero two is my formula mass from that formula mass. I can make an equivalence one mole of water Equals eighteen point zero two grams of water Now I'm ready to do my conversion again whenever I do a conversion I always start with the number that I was given in the question 37.5 times and then the conversion factor And again remember when setting up these conversion factors we look over here grams of water I've got to put grams of water on the bottom My one mole of water goes on top. It's all about unit cancellation Grams water disappear Moles water left now. That's not what I was asked to find in my question But that's where I need to be to get things done. That's the middle ground In my conversion here 18.02 is on the bottom So I'm gonna divide by 18.02 works better for the calculators on So that's 2.08 Again, that's not the answer to my question. That's just my middle ground. I move on to step two And step two is going to be convert those moles into molecules Whenever I'm converting from moles to molecules. I know I'm dealing with Apogadro's number So I make my equivalence based on that and I know that one mole of water Will be equal to six point zero two times ten to the twenty-third molecules Pardon me I'm going to carry over my answer from this step and use that as my given and then times my conversion factor Since I have moles of water here, I'll put the moles of water on the bottom And I'll put my six point zero two times ten to the twenty-third on top again The idea is to cancel out the units moles of water cancel out Now I've got to put this in my calculator the way I enter it is two point zero eight Times I put the six point zero two in first so six point zero two I hit the exponential notation key and that takes care of the times ten And then 23 so six point zero two exponential notation key 23 And when I'm done the calculator spits out one point two five Times ten to the twenty-fourth Every molecules water, which is what I was asked for in my original question Many molecules it was I've got molecules of water. That's how I know I'm done I'm always taking care to look at what I'm asked to find. I'm always taking care looking at what I'm given to work with keeping track of my units along the way making sure that I Get where I need to be and once I get where I need to be I'm dumb and again It's all about writing out these units. You can't shortcut this. I can't say that enough You can't shortcut this. Yeah, it takes you an extra two seconds to write out a unit But do it because if you watch those units, you'll always know what you're doing You always know where you're at. You'll know when you've got to your answer You'll know when you still have more work to do if you just write down numbers It will honestly leave you clueless and and you won't know where to stop And I'll see people doing three-step problems where they're only supposed to be doing two or three-step problems When they're only supposed to be doing one when it comes time to take the test so Take my advice. I Didn't when I was in high school again. I did not do well on this test when I was in high school I remember getting yelled at by my parents I didn't do well in this because I'd refuse to write everything out. I refuse to look at units I refuse to do the unit cancellation and it didn't work out very well, and I've been teaching this stuff for 20 years I've seen plenty of students do the exact same thing and it's always the same problem They don't know what they're doing because they don't write down their units They don't do the unit cancellation so take your time. Make sure you're doing things right