I'm confused (Acids/Bases)

  • #1
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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.
 

Answers and Replies

  • #2
Borek
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What is an acid?

What does it mean "highly acidic" in the context of water solution?
 
  • #3
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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?
 
  • #4
Borek
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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?
 
  • #5
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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.
 
  • #6
Borek
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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).
 
  • #7
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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.
 
  • #8
Borek
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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
 
  • #9
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That's approximately correct, but you have also dissociating water.
What exactly does that mean?
 
  • #10
Borek
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There is more than one source of hydronium in the solution. Told you that in the other thread.
 
  • #11
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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
Borek
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Yes.
 

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