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What Do You Need to Know to Calculate Molarity?

by Kay Santos

In chemistry, concentration is expressed in several ways. One of these expressions is molarity. Molarity is very important because it is used to calculate concentrations in other expressions, such as mole fraction and molarity. Calculating molarity is fairly simple and requires only two components: the total volume of the solution and the moles of solute. If you don't immediately know the number of moles of solute, you can find it using either the mass of the solute or the number of atoms in the solute.

Liters of Solution

Molarity is a ratio of the moles of solute to the total volume, in liters, of the solution, the final product after combining the solvent and solute. Once the solute is dissolved into the solvent, the volume usually increases. You can then find the molarity by dividing the moles of solute by the total volume of the solution.

Moles of Solute

The solute is the compound or substance that is being dissolved into something else, called the solvent. If you know the number of moles of solute being dissolved, you can easily find the molarity by dividing this number by the total volume of the solution, in liters. For example, if you dissolve 50 moles of sodium, the solute, into water, the solvent, to create a 100-liter solution, you have a 0.5 molar sodium solution.

Mass of Solute

You can also calculate molarity using the mass of the solute. This can be done by converting the mass of the solute into moles. In order to do this, you must know the molar mass of your solute and multiply it by the mass of the solute in grams. As an example, the molar mass of sodium is 23 moles per gram. If you have three grams of sodium, this is 69 moles of sodium. If you dissolve those 23 grams in water to make a 100-liter solution, this is a 0.69-molar sodium solution.

Atoms of Solute

Another way to find the moles of solute and the molarity of a solution is by using the number of atoms of solute. In a mole of any substance or element, there are 6.022 times 10^23 atoms. Therefore, if you know how many atoms of solute you have, you can find the number of moles by dividing that number by 6.022 times 10^23. Continuing to use the example of sodium, if you have 3.011 times 10^25 atoms, that is 50 moles. If you dissolve those 3.011 times 10^25 atoms in water to make a 100-liter solution, that is a 0.5 molar sodium solution.

About the Author

Kay Santos is a freelance writer specializing in math and science. She holds Bachelor of Science degrees in physics and health science, both from Clemson University.

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