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Video transcript

- [Instructor] In this video, we're gonna talk about one of the most common ways to measure solute concentration in a solution. And that is molarity. And molarity is defined as the number of moles of solute, the thing that we are dissolving in a solvent, that divided by the liters of solution. So let's just do an example and see if we can calculate the molarity of a solution. So let's say that I have, this is a container here. And I'm going to dissolve some sodium sulfate in water. So sodium sulfate is the solute and water is the solvent. Together, they give us this solution. And let's say we have a total, the total volume of solution is 250 milliliters of solution. And that solution is made up of just to give ourselves a bit of a refresher. We have the solvent, which is H2O, it is water in this situation. And you might say, do we have 250 milliliters of water? And the answer would be not quite because the 250 milliliters that's the volume of the water plus the sodium sulfate. And so we're gonna have some sodium sulfate in here. And let's say we know that we have 35.5 grams of sodium sulfate. That is the formula for sodium sulfate. And so given this information, how do we figure out molarity? Well, the first thing you might say is okay, I know the number of grams of sodium sulfate. I need to figure out the number of moles. And to figure out the number of moles, you'd have to look at the molar mass. You could figure that out from a periodic table of elements. But just to speed us along, I will help you out a little bit here. The molar mass sodium sulfate, molar mass is 142.04 grams per mole. So given everything I've now told you, see if you can pause this video and figure out the molarity of this solution. What's the molarity of the sodium sulfate in this solution? All right, now let's work through this together. So first, we wanna figure out the number of moles of solute. So we can start with the mass of solute that we have right over here. So we have 35.5 grams of sodium sulfate. And now if we wanna figure out the number of moles, see I'm gonna multiply this times something that would cancel out the grams. And so I don't want grams per mole. I want moles per gram. And so I could write this. I could multiply this times for every one mole of sodium sulfate, NA2SO4, we have 142.04. I'll write this way, grams of sodium sulfate. And you can see very clearly that that will cancel with that and we're left with moles of sodium sulfate. So we'll get a calculator out in a second and just take 35.5 and divide that by 142.04. And then to figure out molarity, we wanna divide by the liters of solution. So up here we have a calculation for number of moles and then the liters have solution, 250 milliliters is the same thing as 0.250 liters of our solution. And now we can just use our calculator to figure out what this is. 35.5 divided by 142.04 equals that, and then we divide that by 0.25. I could just throw a zero in there. And then that gets us that right over there. And then we can think about how many significant figures we have. So we have three over here. We have one, two, three, four, five over here. We have three over here. So, I would say that we have three significant figures. And so we would round this right most or this nine right over here. Well, if we round that up, we get 1.00, if we were to go to three significant figures. So that gets us 1.00. And then you might say what is the units here? And what people will normally say is this is 1.00 molar. When you see this capital M right over here, that is the unit for more molarity but they're really talking about the number of moles of solute per liter of solution. And we are done.