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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: lexi105 on November 21, 2006, 09:05:44 PM

Title: Why does the pH sof hydrogen phosphate buffer remain constant when...
Post by: lexi105 on November 21, 2006, 09:05:44 PM

This question will be marked out of 10.

Use Le Chatelier'd principle to explain why the pH of a hydrogen phosphate buffer system remains constant when:
a) Small amounts of acid are added
b) small amounts of bases are added

Heres what I have:
a)   If small amounts of acid (hydrogen ions) are added to the hydrogen phosphate buffer solution, the H3O+ concentration will increase. It increases in order to maintain equilibrium. The equilibrium will shift to the left to adjust the inequality of the concentration on the one side compared to the other. So, the H3O+ reacts with HPO42-. The added H3O+ is then consumed by the hydrogen phosphate buffer solution and there is little to no change in the pH.
b)   If small amounts of base (hydroxide ions) are added to the hydrogen phosphate buffer solution they react with H2PO4- and produce HPO42-, which shifts the equilibrium to the right.

Any thoughts? Thanks:)

Title: Re: Why does the pH sof hydrogen phosphate buffer remain constant when...
Post by: DevaDevil on November 21, 2006, 10:06:17 PM

Le Chatelier's Principle:
the position of equilibrium will move in such a way as to counteract the change

So you should start writing the equilibrium out:

HPO₄^2- + H₃O⁺ <--> H₂PO₄^- + H₂O (for acid)
HPO₄^2- + H₂O <--> H₂PO₄^- + OH^- (for base)

Then it is easy to see your answer is correct:
Add acid (H₃O⁺), move the equilibrium away from H₃O⁺ to keep this concentration (and thus the pH) almost equal, hence you will have less HPO₄^2- and more H₂PO₄^- in solution.

For base add OH^-, and shift the equilibrium the other way