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Topic: [H3O+] = [OH-]  (Read 3980 times)

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Offline greggo

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[H3O+] = [OH-]
« on: April 26, 2008, 08:30:45 PM »
This is a question on a practice exam for Gen Chem 2 that I cannot figure out...

"If you prepared 0.010M solutions of each of the following salts in pure water, for which one would the [H3O+] be equal to the [OH-]?

a. NaOCl b. CsI c. NaF d. KCN e. NH4Cl"

I know the answer is CsI because that is what the key says, but I have no idea about how to solve the question. I know that when [H3O+] = [OH-] the solution is neutral, meaning pH is 7. I know that NaOCl will create a weak base solution since OCl is a weak base. This also applies for KCN since CN- is a weak base in solution as well. Then there is NH4Cl, which will make NH4+ in solution which is a weak acid so the solution will not be neutral there, either. So I am left with NaF and CsI, which I cannot distinguish how they will react in solution. Anyways, any help would be much appreciated. Thanks.

Offline Yggdrasil

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Re: [H3O+] = [OH-]
« Reply #1 on: April 26, 2008, 10:27:58 PM »
Is HF a strong or weak acid?  Is HI a strong or weak acid?

Offline greggo

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Re: [H3O+] = [OH-]
« Reply #2 on: April 26, 2008, 10:40:35 PM »
I know that HF is a weak acid because F- is too electronegative to allow HF to dissociate completely in water, and HI is the strong acid, as it is a hydrohalic acid and it dissociates completely in water.

I am still confused as to why CsI would not react with the water leaving it as neutral...

Is it because since HI is a strong acid and dissociates completely, if it formed it would re-dissociate into H30+ and I- ions as the I- ions from the CsI takes H+ from the H20, leaving OH- ions that would be in equal amounts with the H30+ ions that re-dissocaited?

Unlike the F-, which would form a few HF ions and not completely re-dissociate which would create cause there to be an imbalance just as all the other weak bases would.

Offline Yggdrasil

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Re: [H3O+] = [OH-]
« Reply #3 on: April 26, 2008, 11:17:28 PM »
Yes.  HI is a strong acid meaning that I- anions do not bind H+ ions in solution.  Therefore, the presence of I- does not change the concentration of H+ ions in solution.  The conjugate "base" of a strong acid will always be a neutral anion.

In contrast, HF is a weak acid, meaning that F- will associate with H+ ions.  Therefore, addition of F- ions changes the concentration of H+ in solution (due to the formation of HF).  The conjugate base of a weak acid will always be a weak base.

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