Question about Buffers and Use of a Salt

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PaulPatel
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Joined: Fri Sep 25, 2015 3:00 am

Question about Buffers and Use of a Salt

Postby PaulPatel » Sat Nov 28, 2015 7:32 pm

Hello!

When talking about buffers, I noticed that the usage of a salt was employed for the weak acid (conjugate base in form of salt), and for the weak base (conjugate acid in form of salt). Is the salt employed because it does not drastically fluctuate the pH, and you want to get the pH of the solution to a certain value by adding the conjugate base/acid in form of a salt?

Thank you!

Anne Cam 3A
Posts: 71
Joined: Fri Sep 25, 2015 3:00 am

Re: Question about Buffers and Use of a Salt

Postby Anne Cam 3A » Sat Nov 28, 2015 8:28 pm

Salts with conjugate acids/bases are used because these conjugates are usually in the form of ions and would not be found freely in solution (unlike the weak acid/base which is already a compound). When a salt with the required conjugate acid/base ion is mixed with the buffer solution, it would dissociate and provide the required conjugates to maintain the desired pH.

Hannah Markovic 3C
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Joined: Fri Sep 25, 2015 3:00 am

Re: Question about Buffers and Use of a Salt

Postby Hannah Markovic 3C » Sun Nov 29, 2015 9:43 pm

The salt is there so that you can have a reaction that is trying to reach equilibrium. If you just add, say, HCl to water, you will have an acidic pH but this pH can be easily changed by adding more OH- or H+ ions because HCl is a strong acid and the H+ ions won't react with the Cl- ions when in water. But if you added a weak acid that had an equilibrium point at the same pH, you would have a solution that would be harder to change the pH of. This is because the weak acid at this pH would have both H+ ions in the water, salt ions, and the H+ and salt bonded together. If you add H+ to this solution, the reactions will shift to the oposite direction and more H+ bonded with the salt will form because of Le Chatelier's principle. If you add OH-, it bonds with the H+ in solution which causes the concentration of H+ in solution to decrease and so the H+ and salt break apart also because of Le Chatelier's principle. In both cases, the pH changes very little, whereas it would change drastically if the salt was not there because there would be no equilibrium to reach and Le Chatelier's principle would not apply. This is the point of a buffer: to stabilize pH.

Daniella Ching 4C
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Joined: Fri Sep 25, 2015 3:00 am

Re: Question about Buffers and Use of a Salt

Postby Daniella Ching 4C » Mon Nov 30, 2015 10:05 pm

Since the conjugate of a strong base is a weak acid, does this mean that the salt (when dissolved) is a strong base? Would there be less salt then in a buffer solution than weak acid?

Anne Cam 3A
Posts: 71
Joined: Fri Sep 25, 2015 3:00 am

Re: Question about Buffers and Use of a Salt

Postby Anne Cam 3A » Wed Dec 02, 2015 6:13 pm

Yes, the conjugate base of a weak acid would be stronger than that of a strong acid, though it wouldn't be as strong a base as the ones listed in the course reader. In a buffer system it is optimal to have [HA]=[A-] to keep the pH constant, because if the amounts were different the addition of an acid or base could change the pH easily.


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