How do you know what equation to use when finding the pH?

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Zsanielle Moncayo 1E
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Joined: Wed Nov 18, 2015 3:00 am
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How do you know what equation to use when finding the pH?

Postby Zsanielle Moncayo 1E » Fri Dec 02, 2016 3:35 pm

I was looking over question 7 in the second to last practice final and I don't understand how it was known to use CH3COO- + H20 = CH3COOH + OH-? Knowing what chemical equation to use is something I still don't understand, if anyone has some pointers on knowing what equation to use I'd really appreciate it!

Simon Kapler 3I
Posts: 15
Joined: Mon Sep 26, 2016 3:01 am

Re: How do you know what equation to use when finding the pH?

Postby Simon Kapler 3I » Fri Dec 02, 2016 4:09 pm

So, the question is asking you to identify which indicator you should use in a titration (which we didn't go over in lecture), but basically you're looking for the pH at the stoichiometric point, when moles of acid = moles of base. Since we are told that we're titrating CH3COOH with NaOH (weak acid and strong base), we know that the pH will be slightly basic at the stoichiometric point. This is because only H2O, Na+, and CH3COO- are present at the stoichiometric point (since all the H3O+ and OH- has neutralized/cancelled itself out). We know that Na+ has no effect on pH, but CH3COO- does, since it is the conjugate base of a weak acid (conjugate seesaw). Therefore, CH3COO- reacts with H2O as shown in the reaction you typed, producing some OH-, making the solution somewhat basic at the stoichiometric point. Hope this helps!

Zsanielle Moncayo 1E
Posts: 13
Joined: Wed Nov 18, 2015 3:00 am
Been upvoted: 1 time

Re: How do you know what equation to use when finding the pH?

Postby Zsanielle Moncayo 1E » Fri Dec 02, 2016 8:26 pm

Thank you so much! This helped clarify the problem


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