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

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SUMMARY

The discussion centers on the relationship between the concentrations of the conjugate base [A-] and hydronium ions [H3O+] in a 1M solution of a highly acidic weak acid (HA). It is established that the dissociation of HA in water produces equal concentrations of H+ and A-, leading to the conclusion that [H3O+] = [A-] under complete dissociation conditions. The confusion surrounding the role of hydronium ions is clarified, noting that H+ ions associate with water to form H3O+, which is a common representation in acid-base chemistry.

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HazyMan
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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.
 
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What is an acid?

What does it mean "highly acidic" in the context of water solution?
 
Borek said:
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?
 
HazyMan said:
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?
 
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Borek said:
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.
 
HazyMan said:
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.
 
HazyMan said:
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?
 
  • #10
There is more than one source of hydronium in the solution. Told you that in the other thread.
 
  • #11
Borek said:
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?
 
  • #12
Yes.
 

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