[oque]

The label on a bottle containing a dilute aqueous solution of an acid became
damaged. Only its concentration was readable. A pH meter was nearby, and a quick
measurement showed that the hydrogen ion concentration is equal to the value on the
label

a)Give the formulae of four acids that could have been in the solution if the pH
changed one unit after a tenfold dilution.

I don't understand why tenfold dilution should be added.

a.1)Could it be possible that the solution contained acetic acid?
Acetic acid: pK a = 4.76
If yes, calculate the pH (or at least try to estimate it) and show your work.

So I came up with these

HA  H⁺ + A^-

If the acid were just put on water then always [A]=[H], so [HA]+[A] = [H] could never happen. But as it turns out the answer is yes. So how does this happen? If a dilution took place it shouldn't have an impact on anything because every single concentration would have the same denominator so it becomes meaning less. Another possibility i thought of was that what took place was a buffer or something like that but if that possibility was true then question a) becomes meaningless because every acid could meet the criteria established. Could it be that the equilibria H2O H⁺ + ^-OH played some role somehow?

Hope I made myself clear, i'm not that good at english

[Borek]

What if the concentration of HA is negligible?