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An easy guide to mmols

Every nurse is familiar with seeing mol, mmol and occasionally µmol written on vials and infusion packs.

But it appears that many are unfamiliar with what they actually mean. Here is a small crash course in the mole!

Let’s take any medication; in this case let us look at something simple like a sodium chloride solution.

Sodium chloride (NaCl) is a salt made up of molecules which contain one sodium (Na) atom for every one chlorine (Cl) atom (a molecule is a collection of atoms to make a substance).

Because molecules are very small and atoms even smaller we cannot measure their weight in grams or even milligrams. To overcome this, the weights of atoms are measured in Atomic Mass Units (amu), molecular weights are a combination of the weight of all the atoms in that molecule. An amu is equivalent to 0.000000000000000000000000001g so using grams to express the weight of molecules would be a nightmare!

In the example here, NaCl, the molecular weight of one molecule is the sum of 23 (average weight of sodium atoms) and 35.45 (average weight of chlorine atoms) to give a weight of 58.45 amu. But we are never going to be dealing with only one molecule at a time.

This is where the concept of a mole comes from.

One mole is used to describe the number of atoms that are required so that the weight of the sample is equal to the atomic weight in grams, i.e.: one mole of NaCl weights 58.45g. This quantity of atoms is 602,214,150,000,000,000,000,000 (let’s call this number N). These figures are getting bigger and harder to handle so can you see why we refer to things in moles?

While mole describes the physically number of atoms we have to hand, mol is a unit to describe the concentration. If we have one mole of NaCl (58.45g) dissolved in one litre of water, this gives us a concentration of one mol. Similarly, if we only dissolve 5.845g of NaCl in one litre than this has a concentration of 0.1 mol (or 100 mmol, a mmol is a thousandth of a mol).

Finally, let’s put it all together at look at a saline solution. The bag will often say 0.9% w/v, this means that for every one litre of water there is 9 g of NaCl dissolved in. If one mole is 58.45g, then 9g is 0.154 moles (0.154 x N molecules). Now that we dissolve this into water, it becomes a concentration - 0.154 moles in 1 litre is a 154mmol solution.

This principle does not just apply to administered medication, but to anywhere you see a concentration written. Another example where this unit “mmol” is seen is blood sugar results. What exactly does that “3.9 mmol” reading mean? It means in each litre of blood there are 3.9 millimoles of glucose. If one mole of glucose is 180.16g, then the concentration of glucose in the blood is 0.7g/ litre.

Keep an eye out for any mol/mmol labels you see today and just think of the billons of trillion of atoms floating in that solution!

Dr. Julie Ann Lough has a Ph.D. in chemistry from the University of Warwick, specialising in potential anticancer agents (cisplatin derivatives). She has an interest in publicising the science of everyday life and has previously presented on BBC radio and television.

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