Comparing a hydronium with a hydroxide in a weak acid solution (0.1M)

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Discussion Overview

The discussion revolves around the comparison of hydronium ion (H3O+) and hydroxide ion (OH-) concentrations in a 0.1M weak acid solution (HA). Participants explore the implications of water's autodissociation and the water ion product (Kw) in relation to the concentrations of these ions in the solution.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that in a weak acid solution, the concentration of H3O+ is greater than that of OH-, but expresses uncertainty about this claim.
  • Another participant questions the understanding of water autodissociation and the water ion product, indicating its relevance to the discussion.
  • There is a proposal that if the water ion product is considered, the concentrations of H3O+ and OH- might be equal, but this is challenged by others who assert that in an acid solution, H3O+ will be more concentrated due to the presence of the acid.
  • Some participants express confusion over the application of the water ion product formula, particularly regarding the inclusion of HA in the equation.
  • One participant acknowledges a misunderstanding about the comparison being made between H3O+ and OH- concentrations, clarifying that the OH- concentration arises from water's self-ionization, which is relatively rare.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the relationship between H3O+ and OH- concentrations in the weak acid solution. There are competing views on the implications of the water ion product and the role of the weak acid in determining these concentrations.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about the behavior of weak acids and the application of the water ion product. The participants do not fully resolve the mathematical steps or the implications of including HA in the equilibrium expression.

HazyMan
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TL;DR
I am trying to find out why the hydronium molarity is greater than the hydroxide molarity in this solution of a weak acid "HA".
This question says: An HA weak acid solution with a molarity of 0.1M is dissolved in water. In the new solution, is the molarity of OH- greater than the H3O+ molarity, or the opposite? Or are they equal?

I came up with two possible answers:

1. [H3O+]>[OH-] because there are no hydroxides involved at all, but this leads me to the second answer, which IS wrong according to my solution book but i just wanted to mention it..

2 [H3O+]=[OH-]. I came up with this because if you replace "A" with a hydroxide, you will end up with HOH+H2O<->H3O+OH which is basically water self-ionization.

I'm just not sure if my first answer is correct either, because the solution book simply says that [H3O+]>[OH-], implying that hydroxides are indeed involved.

Is my answer (the first one) correct?
 
Chemistry news on Phys.org
Are you aware of the water autodissociation? Do you know what the water ion product is?

(if not: google them)
 
Borek said:
Are you aware of the water autodissociation? Do you know what the water ion product is?

(if not: google them)
That's what i thought of regarding the second answer i came up with. However if that was the case, wouldn't the hydronium molarity be equal to the hydroxide one? In the book it says that this is not the correct answer.
I didn't think of the ion product. If i substitute that in then i get Kw=([H3O+][OH-])/[HA]
 
HazyMan said:
wouldn't the hydronium molarity be equal to the hydroxide one?

In a pure water - yes. But you have an acid solution, so there is more than one source of H+.

If i substitute that in then i get Kw=([H3O+][OH-])/[HA]

No idea what you did here nor how, but it is definitely wrong.
 
Borek said:
In a pure water - yes. But you have an acid solution, so there is more than one source of H+.
No idea what you did here nor how, but it is definitely wrong.

Isn't the water ion product notated as Kw? Isn't it an equilibrium constant?
 
HazyMan said:
Isn't the water ion product notated as Kw? Isn't it an equilibrium constant?

Yes to both, it still doesn't make the formula correct.

If it is equilibrium just for water, why do you put HA there?
 
Borek said:
Yes to both, it still doesn't make the formula correct.

If it is equilibrium just for water, why do you put HA there?
i forgot that OH was substituted in A. So it's Kw=[H3O+][OH-] ?
 
  • #10
Wait, i think i realized something. All along i thought i had to replace A with OH but now i get what's going on. I'm supposed to compare the overall H3O+ concetration with the OH- concetration which is only produced by the water's self ionization, which as a phenomenon, is known to be quite rare. So it is obvious that H3O has a bigger concetration simply because of the HA acid being dissolved ALONG with the water
 

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