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Back to the Basics: What is Molarity? | ChemistryTwig

Back to the Basics: What is Molarity?

what is molaritySometimes it’s easy for those in the science profession/majors to forget that molarity isn’t something everyone sees on a daily basis. In fact, unless you’re in a science profession, it’s likely that a person will see the term “molarity” only a few times in his or her life. So, for those non-science profession/major people (or those who just need to brush up!), what is molarity?

Molarity is simply a way of expressing concentration. The higher the molarity, the more concentrated the solution is. Molarity is basically a fancy way of saying concentration of something on a mole per liter basis.

For example, if you have one mole of table salt (NaCl) in a liter of water, that would be a “one molar” solution.

Molarity is useful because it tells you the concentration of a solution based on a particular substance. Thus, it can easily be converted to a mass per liter basis if you just know the molecular weight of the substance dissolved in the solution. However, it’s a little more work to convert to a mass percent basis.

In order to convert molarity to a mass percent basis you would need to know two things:
1. The molecular weight of the substance dissolved in the solution
2. The specific gravity or density of the solution

Lets again take the example of a solution containing table salt (NaCl), the solute, and water (H2O), the solvent. Imagine you have a one molar solution. That is, one mole of NaCl per one liter of solution. Assume the specific gravity of a one molar solution of NaCl in water is about 1.04.

Okay, here are the calculation steps to get from molarity to mass percent:

1. Calculate the mass of one mole of table salt (NaCl)

Molecular weight of NaCl = (22.99 g/mole Na + 35.45 g/mole Cl) = 58.44 g/mole

1 mole NaCl * 58.44 g/mole = 58.44 g

2. Calculate mass of one liter of solution with a specific gravity of 1.04

Mass of one liter of water at standard conditions (25 C and 1 atm) = 1000 g

Mass of solution = specific gravity * 1000 g/L = 1.04 * 1000 g/L * 1 L = 1040 g

3. Calculate the mass percent of NaCl in one liter of solution

Mass percent NaCl = mass of NaCl/mass of solution = 58.44 g NaCl / 1040 g solution * 100% = 5.62% by mass NaCl

Cool! Now you know how to convert from molarity to percent mass. Note that you can also skip the specific gravity and just use the density of a solution in step 2. For example, assume you know the s=density of one liter of one molar NaCl solution was 1040 g/L. Then you don’t need to know the specific gravity!

Hope this helps clear up some questions!

[Photo Credit] Clay Shonkwiler

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