Osmolarity: Definition, Formula & Calculations

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  • 0:05 What is Osmolarity?
  • 0:52 Definitions and Formula
  • 1:46 Calculation
  • 4:34 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson teaches you the mathematical definition and formula of osmolarity. Then, you'll learn step by step how to apply this definition and formula to calculate osmolarity in a variety of examples.

What Is Osmolarity?

Salt (NaCl) is made out of sodium (Na) and Cloride (Cl). It can be precipitated out of - big surprise here - saltwater. But what happens in the reverse situation? What happens if we put salt into water? It will dissolve and dissociate into sodium (Na+) and chloride (Cl-) particles (ions) to create a saltwater solution.

The sum of the molarities of the dissolved particles of a solution is known as osmolarity, denoted by the unit of osmol. There are other ways by which osmolarity can be defined, but because this lesson focuses on the mathematical aspects of osmolarity, we're going to stick to that definition in order to keep things straight and simple for the rest of this lesson.

Definitions and Formula

In case the definition caught you off-guard with some new terms, let's quickly cover some basic things so we're all on the same page. And in case you didn't know, a solute is the substance being dissolved by the solvent in order to create a solution. Sugar, is the solute when being dissolved by water, the solvent, in order to create a sweet solution.

Molarity, denoted by the unit M, is the number of moles of a solute dissolved in one liter (L) of solution, thus M can be written as moles per liter, or mol/L. Remember, osmolarity can be defined as the sum of the molarities of the the dissolved particles of a solution. Now that we've straightened all of that out, here's a quick and easy formula for osmolarity:

osmolarity (osmol) = (# of moles of dissolved particles) / (# of liters of solution)


This sounds harder than it is, so let's do some practice calculations using our familiar salt and sugar examples. Since we started this lesson with salt, let's start our calculations with salt as well.

Example 1

Let's say you have 4 moles of NaCl in one liter of water: a 4M solution of NaCl. What's the osmolarity, or sum of the molarities of the dissolved particles of this solution? Well, what are the dissolved particles in this salty solution? Yep, the sodium ion (Na+) and the cloride ion (Cl-). Thus, we have two dissolved particles. How many total moles of dissolved particles do we have? The sum of the molarities of the dissolved particles in this solution is:

4 moles of sodium ions + 4 moles of chloride ions = 8 moles in total

Thus, our equation looks like this:

osmolarity (osmol) = (8) / (# liters of solution)

How many liters do we have of this solution? One. So:

osmolarity (osmol) = (8) / (1) = 8 osmol

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