Tuesday, 19 February 2008

What is the difference between osmoles and moles?

We've already answered the question, "What is a mole?". Now it's time to steer the session more distinctly towards biology and medicine: what is an osmole?

Osmosis depends on the number of solute particles in a solution. In calculating the number of particles, it is important to consider that a salt (like NaCl - table salt) may well dissociate in the solution. If it does, then you have to consider the number of moles of Na+ and the number of moles of Cl- when thinking about osmosis. If you just considered the number of moles of NaCl, you would underestimate the osmotic force. In other words, 1 mol of NaCl gives 2 moles of osmotically-acting particles.

Note that many molecules don't give you this sort of headache, since they don't dissociate. 1 mol of, say, lactose still only gives 1 mol of osmotically-acting particles.

To encapsulate this in a concept, biochemists often talk of osmoles, which are the number of moles of osmotically-acting particles in a solution.

Putting that altogether, you could say that 1 mol of NaCl contains 2 osmoles.

Usually, a full osmole is too great a number for convenient use with reference to the human body, and so milliosmoles (mOsm) is often used instead - one milliosmole being one thousandth of an osmole of course.

(Whew! That was dry, wasn't it? Sorry...)


  1. ei man, tnx for making this blog, really helps in med school

  2. Thanks for the info.

  3. really loved it , thank you :)

  4. really loved it , thank you :)

  5. Very helpful, thank you! But I'm still confused about something, I'm reading Guyton & Hall's and it says," 180 grams of glucose, which is 1 gram molecular weight of glucose, is equal to 1 osmole of glucose."
    I get the osmole part, but when it says 180 grams of glucose, to what weight unit are they referring to? I mean what's the difference between 180 g of glucose and molecular weight of 1. I think I forgot my chemistry...

    1. Mole is molecular weight expressed in grams. NaCl has molecular weight of 58, so 1 mole of NaCl = 58 grams. Glucose molecular weight is 180, so 180 grams of glucose is 1 mole of glucose. 1 mole has this number of particles: 6 × 10 raised to the 23 power. 58 grams of NaCl has this number of particles, so thus 180 grams of glucose. So thus 1 gram of hydrogen or 12 grams of carbon.