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How to Calculate the Molarity of a Solution

Updated on April 21, 2017
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Molarity is a rather important concept in chemistry, and one that can make you life easier if you properly understand it. Pretty much, Molarity is the measure of a solute in a solvent. What does that mean? Well, Molarity is mostly used when talking about aqueous solutions. Solvents are the liquids that are doing the dissolving. The most well known and most used solvent is water! Think about how many things are dissolved in water. Salt and sugar are the two most common things dissolved in water. The salt and the sugar would be the solutes! For the most part, you'll have more of a solvent than a solute. The way I remember the difference between the two is that the word solvent is bigger than solute, and you normally have more of a solvent than a solute. Back to Molarity. So Molarity is defined as Moles per Liter, or

M= moles    
    Liter.

(If you have no clue what a mole is, go check out my other hub real quick, How to Calculate the Molar Mass.) Basically, you can take one mole of any substance and mix it with the same amount of a solvent and you would get a molarity of 1, or 36.46g (36.46g is the amount in one mole of HCL) of HCl mixed with 36.46 Liters would give you a molarity of one.

1M=   36.46g HCL
       36.46 L

In essence, the Molarity is very important in finding the pH of a solution, as well as any reaction equations, but we will not be going over that right now

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A Little Practical!

So if 10g of HCl is added to 350 mL of a solvent, what would be the molarity? First, make sure that you're using the right formula. Molarity equals Moles over Liters. Right now, we have milliliters! So we need to fix that first.

350mL x   1L
         1000mL

We divided 350 by 1000 and got .35L. Now that we have our amount in Liters, we need to find out how many moles of HCl we have. To do that we divide 10g by 36.46g/mol, to get .274mol. Now we can use our formula, finally!

M= .274mol
    .35L

You should've gotten a Molarity of 0.782. If you didn't, that's okay! Run through the steps again, and make sure everything was converted to the right units.

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A Slightly Harder Practical

Here's a slightly harder one. We want to have a solution with a molarity of 2, using HCl. We have 25g of HCl, what volume is needed? This problem uses the exact same formula, so don't panic! This time, it's just set up a little differently. First, we find how many moles we have. 25g/36.46 = .686mol. So now we have 2 of the three variables that we need! Let's set it up.

.5M= .686mol
        ?L

From here, it's just basic algebra! Solve for Liters! You should've gotten about 1.37 L. Easy Peasy, right? Hopefully? I will say that with all things regarding chemistry, practice makes perfect! The more you practice, the easier this will become, until you can calculate the molarity of a solution in your sleep! I will say that it really does get easier. All you need to do is push through and keep trying. It's a beautiful feeling when it all clicks into place!

Wrapping Up!

So that's all there is to calculating the Molarity of a solution! Just remember that you need to have your measurements in Moles and Liters. Also, make sure that when solving a word problem that you are solving for the end amount Liters. For example, if .5L of one solution is being added to another .5L of solution, make sure you are solving for 1L, not .5L. Otherwise, you’d get the completely wrong answer! If you want more practice with any chemistry problem, I would recommend looking into the 1001 Chemistry Problems for Dummies. The online course is excellent, and they make sure that they have very clear and concise explanations for the solutions of all the problems. That’s what I’ve been using to help me in my Chemistry class, and so far I have absolutely no complaints. If you liked this hub, or found anything wrong with it, please let me know in the lovely comment section below! Happy Calculations!

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