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Molarity - Molality - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

Molarity and molality are units of concentration. Molarity measures concentration in terms of moles per liter. A one molar solution has one mole of solvent for every one liter of solution. Molality, on the other hand, measures concentration in terms of kilograms per liter. A one molal solution has one kilogram of solvent for every one liter of solution. Molarity and molality are important in reactions in aqueous solutions and affects reaction rates.

Alright. So we're going to talk about different ways you can express concentration, and concentration is the measurement of how much solute dissolves in a specific amount of solvent or solution. So like let's for example say you're talking about getting juice from concentrate. Concentrate means it's very highly concentrated have a little bit of so- sorry a lot of solute compared to the little bit of solvent. But if you want to make it less concentrated, you're going to make it more dilute. The opposite of concentrated. You're going to add more solvent making it the measurement basically a ratio of solute to solvent.

So the different ways you can actually measure that, several different ways are the ways that we're going to talk about today. One being percent by mass and then we're talking about like solid solutions like metal alloys and things like that or you're doing percent by mass and that's mass of a solute over the mass of a solution. Obviously percentage means multiply by 100. And you can also do percent by volume and these are things we're talking about liquids. And you might even see symbols like this, excuse me. You might even see symbols like on like a, on like a product. This means percent by mass. This means percent by volume. You might see like 70 v v that means percent by volume then times the solute over volume solution. Molarity is very very common. You're going to see this a lot in Chemistry class. Talking about mols of solute versus litres of solution. Another way to like abbreviate that or shorten that is you might see a big m. When you see this big m you know it's mols over litres. Another one is molality and they are very easily confused but molality is mols of solute over kilograms of solvent. Very very different. These are just used in different ways. This is little m. Don't mix it up with metres but it is little m.

Lastly we're going to talk about this mol ratio. Mol ratio is an easy way to talk about the ratio of mols of solute versus mols of solute plus solvent. It's an easy way to go from, you know to kind of like go from any one of these. It's a good like converter. So these are the five different ways you're probably going to see and the way you're going to actually use these a lot of times it's through when you're talking about dilutions.

Let's go to dilute solution and what that means. So this could be of highly concentrated solution. Let's say this is acid and 12 molar acid is actually extremely concentrated. If you've got your hands on it like any skin on this, it would burn you to no end. So let's see how this is my stock solution meaning that I'm a Chemistry teacher and the back when, before I do lab demos, this is what I'm going to use. So I don't want to use that with you guys, I don't want to use that with my students because it could hurt them. So I'm going to dilute it. Meaning I'm going to add water. Okay. It means I'm adding liquid but I'm changing the concentration. I'm keeping the same number of mols, my volume is getting bigger. So my concentration is getting smaller. So the mols of this I didn't change the amount of substance I had, I just changed the water. So the number of mols of whatever it is my volume is, is exactly the same. So another way to dilute it, if I rearrange my molarity solution, my molarity equation, I find that the mv because mols are the same, mv of the first one, molarity times volume of the solution times equals the molarity times the volume of the second solution because the mols are the same. Let's do a problem based on that.

Let's say I have a, what is the volume of a two molar solution of a calcium chloride which you use to make a 0.5 litre solution of 0.3 molar calcium chloride. Okay. So, basically I want to make this, I want to make a 0.3 molar solution. I want to make 0.5 litres of it. I only have two molar solution. What I'm I going to do? Well, I take that two molar solution. I'm going to multiply to figure out the volume is. Multiply by 0.3 molar, that's my second molarity times my volume. And I do the math and I end up with, sorry. I end up with 0.075 litres.

So what I'm going to do, is I'm going to take my 0.075 litres or 75 millilitres of my 0.2 solution and add 25 millilitres of water to make my 0.3 molar solution of that. Because I know the number of mols and of calcium chloride's the same in both scenarios. So concentration, the ways that they can use it is numerous but one of the ways is that they can use it in diluting solutions.