 So, as I stare out over that pretty much empty senior parking lot, and have marked 40 some kids absent already today with one class still to go, I better see a lot of views on this video. The topic is molality. So prom on Saturday, molality on Sunday, molality. It's another way that scientists measure the concentration of a solution. Again, we can't just run around saying concentrated or dilute. We need to know numbers. We need specifics so that we can control our reactions better and control them more carefully. Now molarity from the earlier video is a capital M. Molality will be a lowercase M. That's how we tell the difference between the two in questions. If you're reading a question and it tells you something about the solution, it tells you the concentration of that solution, you have to pay attention. If it's a capital M, they've given you molarity. If it's a lowercase M for that unit, they've given you molality. It's two different equations. We're still going to have moles of solute, just like we did with the molarity equation. But this time, instead of putting it over liters of solution, the total, we're going to put it over kilograms of solvent. It's the two parts this time, so the whole is not even brought up in here. Here's the deal. When you do a molarity, it's really easy to make mistakes when you're making your solution. You figure out that you need to put 300 grams of some solute in one liter of water. If you measure out one liter of water and you put all that solute in there, the final volume of your solution is going to be over one liter. That means your molarity is not going to be correct. You really can't measure out the solvent before you go to mix it. So what you end up doing most of the time is taking an empty container, putting your solute in it, and then adding enough water to get you up to the volume that you need. Now, the problem is, what if you need five liters of solution and you don't have any equipment in your lab that will measure up to five liters? I mean, that would be a pretty big graduated cylinder. That would be a pretty big beaker. Your solution made me to do it, no pun intended, to do molality instead. Because in molality, you could use a balance to measure out the kilograms of the solvent you need. In fact, with water, it's super easy to do because you could either measure out one kilogram of your water, or you can measure out a thousand milliliters. It's all easy. It's very straightforward and it makes mixing the solution easier when you have large amounts. You could measure out what solute you need. You could measure out what water you need, then just mix them together and not worry about what the final solution is. That's really the upside of this whole thing. Now, mathematically, it ends up being pretty similar to what you do with molarity. So those are your myths. You're fortunate in that respect that a lot of the stuff we have to do here is the same as we do for molarity. There's formula masses to do. There's metric conversions to do. It's all a similar process. So let's take a look at a sample question. In this question, I have 0.5 moles of Hc2H3O2. And I have 0.125 kilograms of water. I have moles of a solute and I have a kilograms of a solvent. I have exactly what I need for my equation, moles of solute and kilograms of solvent. So all I've got to do in this one is substitute and solve. So molality, lowercase m, moles solute over kilograms, solvents. So moles go on top, 0.500 moles. Kilograms go on the bottom, 0.125 kilograms. Divide them out, 0.5 divided by 0.125 is 4. Now what I would do here is I would abbreviate it just like I did with molarity. But this time I abbreviated it with a lowercase m. Capital M is molarity, lowercase m is molality. And just like with molarity, I would want to write down what that is. Again, don't want any unlabeled solutions sitting in a lab anywhere. So when you do it, you calculate it, you label it, you put the molality on it, and you also put in the chemical formula, or at least the name, so that everybody knows what substance you're dealing with. Now that was the easy one. That's one where there were no conversions to do. It was just the nice straightforward, easy one. You know me, I'm not going to make it easy on you all the time. I'm going to make you do some other stuff. Let's see what other stuff I want you to do. Ooh, there's some fun other stuff. Let me do question number four. That way we can do a bunch of conversions and practice a lot of things. The final exam's coming up and you want to do well on it, so we need to practice some stuff. So what have we got here? We've got 0.5 kilograms of C2H508. You can tell when I wrote this. I was teaching organic chemistry just before it. So I put a bunch of organic compounds on here, things that you're not likely to see here because we don't even touch organic chemistry in summer though. I've given you this kilograms of what is probably going to be our solute. It does say dissolved, so that is the solute. And I've given you three molar solution. I've already given you a molality solution. I've already given you molality. So this we're going to do the algebra with. Molality is equal to moles of solute over kilograms solution. I've got that number. I want to figure out that number, mass of water. I want to figure out that number. I'm going to have to convert this to moles first to get that done. So let's do that. Let's find the formula mass first. C2H5OH. Carbons, I have two of them. Times 12 is 24. Hydrogens, I've got six of them, five here, one there. Times one is six. Oxygen, I have one. Times 16 is 16. At them all up, 24. Plus six. Plus 16 is 46. So one mole of that equals 46 grams of it. First conversion is not a mole mass conversion. First conversion is to change those kilograms into grams. So back to King Henry died by drinking chocolate milk. Kilo hectadeca, base, desi, semi, milli. King Henry died by drinking chocolate milk. I'm at kilo. I need to get it to grams so I can work with this. One, two, three places over to the right. One, two, three places over to the right. So I'm going to use 500 grams. 500 grams of that, C2H5OH. It's an alcohol. That one is ethanol. Set it up. So the grams cancel. Grams here means it has to go on the bottom. Again, that's just the formula mass we calculated. And the one mole goes on top. Grams cancel. It's 500 divided by 46. That is 10.8695655 blah, blah, blah. Let's round it. 10.9 moles. Now the next step, the final step. I've got this. That's right there. My molality is 3. I've got this. Just calculated it. 10.9. I'm ready to find my kilograms of solution. Variables in the denominator. So the best thing to do in the variables in the denominator is to cross-multiply. So 3 times x equals 10.9 times 1. Because remember, there would be a 1 down there. Divide both sides by 3 to isolate my variable. The amount of water that I would need, 10.9 divided by 3, is 3.6. And it would be liters, or not liters, kilograms. Leaders is the molarity equation. See what I said about paying attention and making sure you're using the right equation? This is a kilogram solution. So the answer there would be kilos. Well, there you have it, molarity and molality. Keep them straight. I'm sure they'll be on the final exam. I know I'm putting them on my test. Make sure you learn it. Next week, we have a whole other set of equations to learn for percent concentration. I'll do that on Monday. And then on Wednesday, we'll be learning net ionic equations. I'll probably give you Thursday as a study guide day. And to answer questions and practice some more, I'll probably move the test off the Friday.