How to Calculate Osmolarity in 4 Easy Steps

Osmolarity is a measure of concentration. Concentration, regardless of the different units that can be used, is the ratio of mass and quantity. When concentration is expressed as osmolarity, mass is the number of osmoles and quantity is volume in liters.

When is Osmolarity Useful

Osmolarity permeates many aspects of medicine:

Homeostasis

The concept of osmolarity is relevant in medicine because it influences how fluids moves through the body. Whether its intravascular to extravascular, intracellular to extracellular, osmolarity helps to maintain homeostasis. It keeps fluids where they need to be, in the appropriate quantities throughout the body.

Oral Compounding

The osmolarity of oral solutions must be considered as hyperosmotic oral solutions can cause adverse gastrointestinal effects like diarrhea and cramping.

Plasma

Plasma is one of the most important “solutions” we work with in medicine. The osmolarity of blood is maintained within a narrow range of 275-290 mOsm/ml.

Osmotic Pressure

Osmolarity influences all of those facets of medicine just discussed because of osmotic pressure. This pressure is what determines how water will or will not move across the semi permeable membranes that creates compartments throughout the body.

The pressure is the results of the differences in the amount of solute on either side of the membrane. Water will always move in the direction of higher osmolarity i.e. it will move to the side of the membrane that has a higher number of solutes. The higher the concentration of solutes, the higher the osmotic pressure.

The amount of pressure the solute will create is also dependent on how the solute interacts with each other once in solution. The interactions can be ionic or non-ionic.

Ionic versus Non Ionic Solutes

Whether a solute is ionic or non-ionic has to be determined before osmolarity can be calculated. Ionic solutes will require the use of an osmotic coefficient. Non-ionic solutes have an “ideal” osmotic coefficient of 1 and therefore not necessary for the calculation of osmolarity.

Osmotic Coefficient

The osmotic coefficient tells us the degree to which particles of a solute will interaction in solution. It ranges from 0 to 1.

A osmotic coefficient of 1 indicates the “ideal” of particles existing independently in solution i.e. there is minimal, near zero, interaction between the particles. You can think of an osmotic coefficient of 1 meaning that the particles are providing their maximum osmotic pressure.

Any deviation from 1 means that there are other factors, like electric charges and attraction, that are affecting the “ideal” osmotic pressure. This deviation is represented by the osmotic coefficient moving away from 1. It must be accounted for when calculating osmolarity.

Non-ionic solutes like dextrose or albumin maintain the covalent bonds that hold them together when in solution. There is minimal interaction between the solute particles. The osmotic coefficient for non-ionic solutes are closer to 1 or assumed to be 1.

Ionic solutes dissolve in solution for form ions. These ions maintain some degree of interaction with each other due to their opposing charges. The osmotic coefficient on ionic solution will be less than 1.

Osmotic coefficients are calculated experimentally and can be found in various reference sources. The Journal of Physical and Chemical Reference Data is a useful resource for osmotic coefficients.

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What are Osmoles?

An (os)mole is the osmotic pressure produced by each mole of a substance in solution.

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Moles to Osmoles

In the unit Calculating Milliequivalents we explain the concepts of mole and equivalent weight. If you are unsure about either of those you will need to review that unit to track along with calculation of osmolarity.

Briefly, 1 mole is a surrogate marker for a very large number of particles, 6.002 x10²³ to be exact.

It would be very hard to use such a large number in practice so we use 1 mole instead.

Equivalent weight is a way to standardize moles. It accounts for the differences in mass and valence when elements interact with each other. This affects their behavior including how they interact with each other in solution which affects osmolarity.

When elements interact with each other, the goal is to form a stable atom or molecule that has a low level of reactivity. They do this by sharing electrons in a way that allows all atoms involved to achieve a stable outershell. If any of this sounds foreign to you please stop and review the unit on How to Read Valence. The stability of valence electrons is the basis of formation of osmoles when calculating osmolarity for ionic solutions.

In this example using sodium chloride (NaCl):

In an “ideal” solution the particles exist independently of each other, there is little to no interaction between them i.e. an osmotic coefficient of 1. In reality differing molecular weights and valence contribute to the attraction that is maintained between ions in solution.

NaCl has an osmotic coefficient (OC) of 0.93 (less than 1) indicating that there are interactions that cause a less than “ideal” solution.

The "ideal" osmole for Na-Cl would be: 2 osmoles x 1 OC

The "true" osmole for Na-Cl would be: 2 osmoles x 0.93 OC

Calculate Osmolarity in 4 Steps

We can organize all this foundational information into 4 easy steps to calculate the osmolarity of any solutions. There are slight differences when the solute is ionic versus non-ionic.

Calculations

Let’s look at some sample questions. One example uses an ionic solute and the other a non-ionic solute.

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I hope this unit has provided clarity on the concepts involved in calculating osmolarity. These concepts are often taught in silos of chemistry, physics and physiology. Tying them together leads to true understanding rather than memorization.

If you’ve found this unit helpful I would love to hear from you. Leave a comment or question below!

The information on this website is intended to be used solely for educational and informational purposes. While the content may be about specific medical and health care issues, it is not a substitute for or replacement of personalized medical advice and is not intended to be used as the sole basis for making individualized medical or health-related decisions.