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Steps on How to Calculate Molarity

By Keren Perles

Figuring out how to calculate molarity is easy - if you know exactly how many moles of a substance you added to a solution. Otherwise things can get more complicated. Read on for a step-by-step guide on how to find molarity.

What is Molarity?

Before you can figure out how to calculate molarity, you have to understand the definitions of two words: "mole" and "solution." A mole is a precise number of molecules, namely 6.02 X 1023, that is used often in chemistry applications. A solution is what results when you stir one substance into another substance and it dissolves completely. For example, salt water is a solution because the salt dissolves in the water.

So now - what is molarity? The molarity of a solution is how many molecules of solute are dissolved in one liter of the solution. For example, if you stir one teaspoon of salt into water to make a solution, the molarity will be much less than if you stir ten teaspoons of salt into the same amount of solution. The unit for expressing molarity is moles per liter, since you are describing the number of moles of a solute in each liter of a solution.

Given Moles of a Solute

Wondering how to calculate molarity for a given substance? Assuming that you have the exact amount of the solute in moles, the calculation is extremely easy. Just take the amount of the moles of solute in the solution and divide it by the number of liters in the solution. For example, if you stir .5 moles of salt into 5 liters of water, you would get .5 moles/5 liters, or 0.1 moles/liters for the substances molarity.

Given Grams of a Solute

If this were the only step in finding the molarity of a solution, no one would ever have questions on how to find molarity. In the science lab, we rarely have a precise amount of how much solute we are adding to a solution in moles. We can, however, measure the solute in grams and then convert those grams to moles.

The first step of converting grams to moles is to find the molar mass of the solute (see the article in this series for how to do so). In short, you would add up the atomic mass of each of the elements in the solute. For example, when the solute is NaCl, or salt, you would add up the molar mass of Na (23 grams/mole) and the molar mass of Cl (35.5 grams/mole) to get the molar mass of NaCl (58.8 grams/mole).

Once you know the molar mass of the substance, you would simply divide the grams of the substance that you used for the solute by the molar mass of the solute. In other words, if you added 10 grams of salt to the salt water solution, you would have added 10/58.8, or 0.17 moles to the solution. You would then use the method given in the previous section to figure out how to calculate molarity.


Mole conversions:

How to calculate the molar masses of chemical compounds: