How can you describe the pH of a strong acid?

Answer 1

Generally speaking, the pH scale is good at relaying the concentration of the hydronium ion in an aqueous solution. I can tell you that a STRONGLY ACIDIC solution would give you a pH of roughly 0 or maybe even as far as just under -1.

The pH of concentrated sulfuric acid (18M) is about -1.2.

Still although a pH of less than 0 is certainly strongly acidic, I wouldn't necessarily call it a strong acid. I can tell you that hydrofluoric acid (HF) is a weak acid while hydrobromic acid (HBr) is strong but if I add a drop of HBr to a barrel of water and add a two cups of HF to a gallon of water, the HF solution will have a lower pH than the HBr solution and would be more strongly acidic (even though HF is a weaker acid).

Instead of using pH to characterize acid strength, I'd probably look at pKa. pKa is a measure of a molecules intrinsic ability to give up a proton...the lower the pKa, the more strongly acidic a molecule is.

Because we're so water based in our thinking, we usually say any thing that is a stronger acid than the hydronium ion (H3O+) is a strong acid. The hydronium ion is the strongest acid that can exist in water and has a pKa of roughly -1.7 so anything that has a pKa of less than -1.7 would be considered a strong acid and better at protonating a base than the hydronium ion.

HSbF6, fluoroantimonic acid, is THE strongest acid with a pKa of roughly -25.

That makes it about 100,000,000,000,000,000,000,000 more acidic than the hydronium ion and a superacid.

Let me know if you want to know what pKa quantitatively means.