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

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Discussion Overview

The discussion revolves around the question of whether the concentration of the conjugate base [A-] is greater than the concentration of hydronium ions [H3O+] in a 1M solution of a highly acidic weak acid (HA). Participants explore definitions of acids, the implications of high acidity, and the dissociation of acids in solution.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants express confusion about the role of hydronium ions in the context of the question and the definition of a highly acidic solution.
  • There is a discussion about the definition of an acid, with some participants noting that pH is a property of a solution rather than the acid itself.
  • Participants propose that the dissociation of the acid HA can be represented by the equation HA -> H+ + A-, suggesting that 1 mole of HA produces 1 mole of H+ and 1 mole of A-.
  • Some participants argue that the concentrations of H+ and A- should be equal, assuming complete dissociation and no change in volume.
  • Others caution that the dissociation may not go to completion and that the presence of water also contributes to the concentration of hydronium ions.
  • There is clarification that H+ and H3O+ can be considered equivalent in some contexts, but the implications of water dissociation are also discussed.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether [A-] is greater than [H3O+]. There are multiple competing views regarding the dissociation of the acid and the contributions of water to hydronium concentration.

Contextual Notes

Some participants express uncertainty about the definitions and relationships between the species involved, particularly regarding the role of water in the solution and the conditions under which the acid dissociates.

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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