How to Calculate The Molarity of a Solution

All good chemists must know what it means when a fellow scientist says they have a 2 Molar solution of something. A good chemist should also know how to prepare a solution of a specific molarity. Here I describe how to calculate the molarity of a solution and how one would prepare a solution at a certain molarity.

Things You'll Need

• Periodic table
• Pencil
• Paper
• Calculator (optional)

Instructions

1. 

   Let's start with an example problem. We want to calculate the Molarity of a solution prepared by mixing 58.44 grams of NaCl in 1.0 Liters of Water. Remember, Molarity is simply defined as the number of mols of solute divided by the number of liters of solvent. I see we have 1 Liter of solvent, but we don't know how many moles we have, just the number of grams, so, let's convert! The first thing we need to calculate is, how many mols are in 58.44 grams of NaCl?

2. 

   If you need help converting grams to moles, please see my other article "how to convert grams to moles". Here's the crash course: Consulting the periodic table we see that Na is 22.99 atomic mass units and Cl is 35.45 atomic mass units. If we add these two numbers together, we get 58.44 atomic mass units. AMU= grams/mol, in other words, NaCl is 58.44 grams per mol. Now let's set up our dimensional analysis (see attached image)
   58.44 grams/ 1 Liter * 1 mol/ 58.44 grams = 1 mol/ 1 Liter.

3. 

   Remember, make sure your units cancel! We end up with 1 mol/ 1 Liter. since 1/1 =1, we have a 1 Molar solution of NaCl.

Tips & Warnings

• When using dimensional analysis, ALWAYS MAKE SURE YOUR UNITS CANCEL!

• Molarity is defined as mols per liter. Don't be tempted to calculate grams per liter-- always convert your grams to mols!