Is [A-] greater than [H3O+] in a 1M solution of highly acidic HA?

[HazyMan]

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]

What is an acid?

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

 

[HazyMan]

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]

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]

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]

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).

 

[HazyMan]

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]

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

 

[HazyMan]

That's approximately correct, but you have also dissociating water.

What exactly does that mean?

 

[Borek]

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

 

[HazyMan]

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]

Yes.