M.Ed.,San Francisco State Univ.
Jonathan has been teaching since 2000 and currently teaches chemistry at a top-ranked high school in San Francisco.
Here are some tips and tricks for dilution calculations. So as you know dilution, you can actually use the calculations m1, v1 equals m2v2, where these are your initial conditions; molarity, and volume. Then these are your final or your desired ones. All these equals the moles of solute. So the solute never change, because all you're adding is the solvent to change the volume of the solution.
So we have an example here. Calculate how much volume of 12 molar Hydrochloric acid would be needed to make 600mL of 2.5 Molar HCl. So basically what we want to do is we want to find V1. So it's nice to always label which variables you have.
So here are some tips. So if you have V1 here, this must be your M1, because that's what you started with. This has to be your V2, and this has has to be your M2. Note, they always match up. If you get it mixed up then your calculations well you're wrong, so make sure you match them up.
So let's plug in M1. So we have 12 Molar. I like to write Molar, Moles per liter just for the sake of showing you something; a shortcut. Times V1, because that is what we want to calculate. Then we have M2 which is 2.5 Moles per liter of HCl, times 600mL. Normally we would calculate in liters, because Molarities in liters, but I'll show you a shortcut here.
So if you take a look here, what cancels out here is actually the moles per liter on both sides. They cancel out. So what are you left with? You're left with milliliters. So you actually didn't need to convert the liters, and then convert back. So if you do the Math, then your V1 ends up being 125 mL. Well, what does that mean? Well, it means you needed 125mL of 12 Molar Hydrochloric acid in the beginning to start with.
Now, sometimes you might be asked, well how much water did you need to add to make that new volume? So that means if you started off with 125mL of 12 molar Hydrochloric acid, and you got 600mL, all you do is you calculate solvent added to make dilution.
Usually the solvent is usually water, H2O liquid. So to calculate the solvent added, you basically take your final volume of your solution, then you minus your initial volume of your initial stock solution. Then that's equal to how much water added. So in this case, we would take 600mL minus 125mL, so that equals 475mL. That's how much water would be added for this dilution.
So just a couple of tricks. You don't ever need to convert the milliliters for the volume, if you want the volume in the same units. We didn't even convert from milliliters to liter, and it saved us a couple of steps.
Also make sure you label your variables, because often times my students get mixed up. Sometimes they accidentally label something V1, when it should have actually been V2, and vice versa.
Hopefully these tips, and tricks will help you with dilutions, and make them a lot faster for you too. Have a good one.