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What exactly is molarity, and how do I find it?  ~Ashley E.

1/25/2014

2 Comments

 
First off, molarity is just a way of expressing the concentration of a solution.  The molarity is the number of moles per liter of solution.  A mole is defined as 6.022 x 1023 (also called Avogadro’s constant) molecules—that’s basically a six with 23 numbers behind it!    

Secondly, every element or molecule has a molecular weight, or a mass per molecule of that compound. 
For example, sodium chloride—NaCl—has a molecular weight of 58.443 grams per mole.  This is, conveniently, also the number of grams per mole of this compound. Therefore, one mole of sodium chloride will weigh 58.443 grams.  

Finally, if you know how much of your sodium chloride is in the solution, and you know how much liquid it is dissolved in, you can find the molar concentration.  
 
Here’s an example:  
 
You have 26 grams of NaCl dissolved in 0.6 liters of water.  What is the molarity?


We know that NaCl is 58.443 grams per mole, and that we have 0.6 liters of water.  We need to find
how many moles of NaCl there are per liter of water to get the molarity.  Here is how we set the problem
up:
Picture
So the final answer is 0.742 molar (M) NaCl.
2 Comments
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11/20/2017 06:03:33 am


Good post but I was wondering if you could write a litte more on this subject? I’d be very thankful if you could elaborate a little bit further. Appreciate it!

Reply
Dr. E
11/20/2017 09:08:52 am

Basically, finding the molarity of a solution is just understanding how many molecules are present per unit of volume. If we know the molecular mass of a compound (which we can find by adding up the atomic masses on the periodic table for each atom in the molecule), and we know the mass that was added in solution, we can calculate the molarity as shown above.

Hope that helps!

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