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...)

7 comments:

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

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  2. Thanks for the info.

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  3. really loved it , thank you :)

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  4. really loved it , thank you :)

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  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...

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    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.

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