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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I know by definition pH = -log10(H+) Most acids and bases fall in the range of 0 to 14. There are some that are negative. Can anyone elaborate more on this and clear up for me why it is hard for an acid to dissociate to a high enough H+ concentration to have a negative pH?
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
OK Let's assume a perfectly dissociated acid (ie onethat's infinitely strong); to be able to provide pH0 it has to ensure that the concentration of H+ is 1 molar. Fair enought, for a 1 M solution of a perfect acid, that gives the concentration of H+ as 1 molar so that's pH 0. To get to pH minus1 we need a concentration of this perfect acid to be 10 molar. Well, hang on . The highest concentrations we can get with real acids are not that high. For HCl the most concentrated solution is about 12 M (37%). More than that and the gas simply doesn't dissolve. In the same way, let's look at pure H2SO4. OK In reality it isn't perfectly ioniosed but imagine that it were. H2SO4 has a density of about 2 (1.84 really) and a molar mass of about 100 (98.something for those who think it wil matter). 1 litre of H2SO4 is about 2Kg, that's abouth 20 moles. So it only has a total available concentration of 20mol/litre of H+ at the best. That would give -1.3ish as a pH. But H2SO4 isn't a really good ionising solvent so it won't be fully ionised; the pH will be rather higher than the calculated value. Similar calculations apply to the other acids. Even if water were fully ionised to H+ and OH - the total concentration is about 56M which would give about -1.8 as a pH. Lower than -1 is a real problem.
Registered Member #690
Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616
Yes, it's never really discussed but 12M HCl has pH of about -1.1, as Bored Chemist said. Since it's a log scale, pH=-2 would be 100 mol/L H+, which is pretty impossible without some type of super-dense (I mean like denser than mercury) extremely strong acid, which does not exist. HCl is probably as close to a perfect acid as you can get, since it is so light and soluble.
There are superacids (fluoroantimonic acid?) which are supposedly like billions of times stronger than conc. H2SO4, but I don't think that changes the pH facts much.
Registered Member #1262
Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451
Actually Fluoroantimonic acid has a pH down to -25 in 100% solutions, it's 2*10^19 (20 quintillion) times stronger then pure sulfuric acid. That stuff can dissolve hydrocarbons
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pH question
