Assuming X is small

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Elias Ruben 1O
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Joined: Wed Sep 21, 2016 2:56 pm

Assuming X is small

Postby Elias Ruben 1O » Tue Nov 29, 2016 9:49 pm

Can you only assume X is small for weak acids/base, or can you also assume it is small for strong acids/bases. It makes sense that you can do it for weak acids/bases, since they don't dissociate well. But I'm unsure about strong acids/bases.

Jonathan Sarquiz 3F
Posts: 32
Joined: Fri Jul 15, 2016 3:00 am

Re: Assuming X is small

Postby Jonathan Sarquiz 3F » Tue Nov 29, 2016 10:09 pm

You only do the assumption that X is small when your equilibrium constant, K is less than 10-4. Weak acids and weak bases have a K value because they do not fully dissociate. Strong acids and strong bases fully dissociate (hence the single sided arrow rather than the equilibrium arrow). Therefore, strong acids and strong bases do not have a K value.

That being said, only weak acids/bases require ICE charts for calculations. Strong acids and strong bases completely dissociate and so the concentration of the strong acid is the same as the hydronium ion concentration. And the concentration of the strong base is the same as the hydroxide ion concentration.

Rosalva_Mejia_41
Posts: 19
Joined: Wed Sep 21, 2016 2:56 pm

Re: Assuming X is small

Postby Rosalva_Mejia_41 » Wed Nov 30, 2016 11:33 am

After finding the value of x in the ICE table, is that the value of H3O+ ions, and whichever molecule that is x?
For some questions, it's shown that x is added to the molar concentration to find the amount of H3O+ ions, How do we determine to do so?

Alexandra_Cooper_4H
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Joined: Wed Sep 21, 2016 2:56 pm

Re: Assuming X is small

Postby Alexandra_Cooper_4H » Sun Dec 04, 2016 12:38 am

I was also wondering about when we can consider x negligible. So just to clarify, it doesn't matter if it's a strong/weak acid or base, as long as the Ka or Kb is less than 10^-4?

Diondraya_Taylor_1C
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Joined: Wed Sep 21, 2016 2:59 pm

Re: Assuming X is small

Postby Diondraya_Taylor_1C » Sun Dec 04, 2016 2:06 am

That was the understanding I got as well but if anyone has a more concrete reason for why we can do that, it would be great because I'm afraid to employ that method for fear of being wrong

Amy_Bugwadia_3I
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Re: Assuming X is small

Postby Amy_Bugwadia_3I » Sun Dec 04, 2016 9:30 am

Generally, just look at the K value and as the above students mentioned, if i'ts less than 10^-4, you can assume that x is negligible. However, if the question asks you to test that assumption, after you solve for x, make sure to calculate the percentage deprotonation or protonation by dividing x by the concentration of the initial acid or base and multiply that fraction by 100%. If that value is less than 5%, then x is negligible and your assumption to eliminate it during the calculation was correct.


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