Zep said:
.... would it be possible for a chemist someone to provide a simple explanation of how we obtain the number of molecules in a given mass/volume of substance? For example, earlier in this thread we were given an approximation for the count of molecules in 1ml of water (33,460,000,000,000,000,000,000, or say 3.35x10E22). To (re)learn how this is obtained would be something I, at least, would appreciate (and should have remembered from my school days!).
Donks did the sum for water.
In principle, start by remembering that a mole is a gramme molecular weight. So, for a substance with a molecular weight of 1 dalton [edited to add, that's hydrogen, obviously], 1g will contain a mole (Avogadro's constant) of molecules. For something larger, say cortisol, with a molecular weight of 362.5 daltons, then 362.5g contains a mole of molecules.
Knowing that, if you know the molecular weight of the compund you're dealing with, you can work out how many molecules are in any given weight of the substance. For water, we assume that 1ml = 1 g.
Actually, this is at the heart of the disagreement you may have seen me have with the Merikan medics. In virtually all countries of the world except the USA, it has been standard practice for about 30 years to report clinical biochemistry results in molar units (e.g.mmol/l) rather than gravimetric (e.g. mg/100ml). This makes it so much easier to understand what is going on, as you are comparing molecule to molecule, rather than being distracted by weights. However, for reasons best known to themselves, the Merikans have refused to make the change. Which is why we count our cholesterols in numbers that look like 5 (mmol/l) while the Merikans do it in numbers that look like 200 (mg/100ml).
Because of this, when referring to Merikan publications, I often have to do this calculation to figure out what the bloody blue blazes they're actually talking about. They're reporting in weight, and I want to know number of molecules.
Rolfe.