# I'm confused (Acids/Bases)

## Main Question or Discussion Point

The question says: A solution of highly acidic HA is given, with a molarity of 1M. Is it true that [A-]>[H3O+] or not? I simply don't understand why the hydronium is mentioned and i don't know how to find the molarity of these two individually.

Borek
Mentor
What is an acid?

What does it mean "highly acidic" in the context of water solution?

What is an acid?

What does it mean "highly acidic" in the context of water solution?
acid= substance with pH<7

solution of highly acidic HA= a highly acidic substance is solute in water?

Borek
Mentor
acid= substance with pH<7
No, pH is not a property of an acid. It is property of a solution. Oxalic acid is a solid and as such it has no pH, yet it is still an acid.

How is pH defined?

Why is it lower than 7 when there is an acid present?

• HazyMan and berkeman
No, pH is not a property of an acid. It is property of a solution. Oxalic acid is a solid and as such it has no pH, yet it is still an acid.

How is pH defined?

Why is it lower than 7 when there is an acid present?
Sorry for being late, i believe pH is derived as the negative logarithm of the acid's molarity (moles of H+ divided by the solution's volume)

I suppose that the acid is being dissolved in the water, releasing H+ ions. This might be wrong (aswell as the H+/V derivation i mentioned in the previous parenthesis perhaps) because i don't know if these ions will react with the water in the solution.

Borek
Mentor
I suppose that the acid is being dissolved in the water, releasing H+ ions.
Yes. Just that's not something to "suppose", that's a definition of an acid (more precisely: Arrhenius definition, there are other, more general definitions).

Ignoring water (the solvent) for a moment, what can you tell about concentrations of H+ and A-? (think in terms of stochiometry, try to write reaction equation of HA dissociation).

I think the equation is HA->H+ & A- .

Obviously 1 mol of HA gives 1 mol of H+ and 1 mol of A- so the mole number of H+ and A- are equal. Since there is no change in volume, the concetrations of H+ and A- should also be equal. Therefore [H3O+]=[A-].

What confused me about the original question was the hydronium, but i later found out that it's the same thing as H+, something that i still can't understand quite so well but i've accepted it.

Borek
Mentor
I think the equation is HA->H+ & A- .

Obviously 1 mol of HA gives 1 mol of H+ and 1 mol of A-
Only if the dissociation went to completion. That's not always the case.

so the mole number of H+ and A- are equal
That would actually hold always, when there are no other sources of H+.

Since there is no change in volume, the concetrations of H+ and A- should also be equal. Therefore [H3O+]=[A-].
That's approximately correct, but you have also dissociating water.

What confused me about the original question was the hydronium, but i later found out that it's the same thing as H+, something that i still can't understand quite so well but i've accepted it.
To some extent H+ and H3O+ are equivalents. See https://en.wikipedia.org/wiki/Hydronium

That's approximately correct, but you have also dissociating water.
What exactly does that mean?

Borek
Mentor
There is more than one source of hydronium in the solution. Told you that in the other thread.

There is more than one source of hydronium in the solution. Told you that in the other thread.
Is that other source the water solvent?

Borek
Mentor
Yes.