Posted: Oct 21, 2013 2:24 pm
by trevp
Stagman wrote:I remember when I was studying chemistry that one of my teachers mentioned that the maximum concentration of an acid in water is conveniently the same value as its molecular weight. There was no mention of this only being limited to those under 100g/mol, so this may only be the case for a limited ammount of acids.
E.g. H2SO4 has a molecular weight of 98g/mol, and happens to at its max. conc. at 98%
E.g. HCl has a molecular weight of 36g/mol, and happens to at its max. conc. at 36%
Values not exact, but the point is clear. According to various tables I found on the interweb this appears to be a coincidence with a few acids. In any case, I remember there being an actual reason for this and it was explained but I have since long forgotten - despite having a bloody good memory.
Can any of the chemistry wizards here offer an explaination? Or is the above all bollocks at the end of the day?

It's a coincidence. Sulphuric acid can be concentrated to remove all of the water and sulphur trioxide can be dissolved in it to produce an apparent concentration of more than 100% - it's called oleum. However, when concentrating an aqueous solution of sulphuric acid by evaporation (a common technique), it forms a constant boiling mixture (called an azeotrope) at 96% sulphuric acid. Hydrochloric acid is formed by dissolving hydrogen chloride gas in water and the maximuim solubility at atmospheric pressure is 35% by weight (more will dissolve at increased pressure). Similar arguments apply for nitric acid. Although it's only available commercially at 70%, it is possible to produce higher concentrations. It's not related to molecular weight.