Osmolarity vs Osmolality

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Can anyone tell me an easy way to remember the difference between Osmolarity and Osmolality? I am a nursing student and we are currently studying fluids and electrolytes. I am having difficulty with these two terms.

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Specializes in Critical Care, ED, Cath lab, CTPAC,Trauma.

Osmolarity and Osmolality are both units of measurements. What these two terms measure and calculate is the osmotic activity. Remember, osmotic activity is when one solution , usually lower in conductivity or mineral content, passes through a semi-permeable barrier for the purpose of diluting the concentration of the solution on the other side, usually containing a higher conductivity or mineral content.

Osmolarity is the number of milliosmoles/liter (mOsm/L) of solution. It is the concentration of an osmotic solution. This is the common bedside calculation used in clinical settings for osmotic activity.

Example: Plasma and other body fluid osmolarity: 270 – 300 mOsm/L

Osmolality is the number of milliosmoles/ kg (mOsm/kg ) of solvent. It is the concentration of the particles that is dissolved in a fluid. This is the clinical laboratory measurement using an osmometer.

Example: Urine osmolality: extreme range of 50 – 1400 mOsm/kg water, but average is about 500 – 800 mOsm. After an overnight fast, the urine osmolality should be at least 3 times the serum osmolality.

Another example for the osmolality and osmolarity of the blood.

osmolality is the weight of the plasma without the weight of the solutes.

osmolarity is the volume of the plasma including the volume of the solutes.

the solutes in the blood are the electrolytes.


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Thank you for your reply to my post. I'm still trying to find a way to remember which is which :) This unit is challenging for me.

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Specializes in Critical Care, Med-Surg, Psych, Geri, LTC, Tele,.

I think of osmolality As concentration/ weight. (Weight is not really accurate, but in my mind, I visualize the particles in the soln as giving it more "weight"

I think of osmolarity as moles.... Because it refers to the mole concentration.

Not sure if this is accurate though, as it is very simplified.

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Specializes in Critical Care, ED, Cath lab, CTPAC,Trauma.
Thank you for your reply to my post. I'm still trying to find a way to remember which is which :) This unit is challenging for me.
osmolality is the weight of the plasma without the weight of the solutes.

osmolarity is the volume of the plasma including the volume of the solutes.

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Thank you, that makes it much easier to remember :)

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