 Okay, one of the things that you'll find in the lab is that sometimes we deal with stock solutions and working solutions. So your stock solution is a concentrated version of the solution. So that's what you make up ahead of time. It's much more highly concentrated than what you're actually going to use when you're working with things in the lab. When you're working with things in the lab, you're going to work with a working solution. That's going to be your working strength. So that just allows us to have a stock solution made up so that we don't have to weigh out and make up a lot of solutions every time we need a working strength solution, we'll just be able to dilute the stock solution. So one of the ways that you need one of the equations that's important when you're working with stock solutions and working solutions is V1C1, V stands for volume and C stands for concentration. Equals V2C2. So in this case with stock solutions and working solutions, you can think of this using S's for stock, B stock, C stock. So the volume of stock solution and the concentration of stock solution is equal to the volume of the diluted solution and the concentration of the diluted solution. So let's go through a quick example. How would you prepare 400 mLs of a 1.5 molar sodium chloride solution from a 5 molar stock? And so we're going to start with our VSCS equals VdCd equation. We're going to fill in what we know. We're going to want 400 mLs of a 1.5 molar and we're starting with a 5 molar stock. What we don't know is the volume of stock we need to make this 400 ml solution. So we're just going to solve for the volume of stock by dividing both sides by 5 molar. 5 molar, not 5 moles. And so just use your calculator. 400 times 1.5 divided by 5 is 120. So your volume of stock is 120 mLs because here we have the moles cancelling, but we still have mLs. So it's 120 mLs. That's not your final answer, of course, because you still have to tell us how to prepare the 400 mLs. So you're going to dissolve 120 mLs of stock and bring to volume of 400 mLs. And then you'll have your 400 mLs of working strength solution.