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[BMEEngineer]

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http://datquestionoftheday.com/chemistry/buffer-solution/

I understand the answer based on Le Chatelier’s Principle, but I have written in my notes and flashcards that buffers are intended to resist changes in pH. Why does the buffer allow the pH to change so much?

 

[KyoPhan]

You are correct that buffers resist change; however, that's not what the statement is really saying.

It is saying that as you add Acetate ion, the acetate will be consumed (react will H+) to form Acetic acid.

H+ + CH3COO- -------> CH3COOH

So if you added acetate ion, you will have less H+. You lose H+, this means pH will go higher. Thus the pH will "approach 7". It'll go up, but not by much due to the buffer.

 

[BMEEngineer]

You are correct that buffers resist change; however, that's not what the statement is really saying.

It is saying that as you add Acetate ion, the acetate will be consumed (react will H+) to form Acetic acid.

H+ + CH3COO- -------> CH3COOH

So if you added acetate ion, you will have less H+. You lose H+, this means pH will go higher. Thus the pH will "approach 7". It'll go up, but not by much due to the buffer.

So, in other words, because you are making the buffer instead of adding something to a mixture that is a buffer already, that is why the pH change is not resisted. For example, if the question was that you had a buffer with 1 mol/mol ratio of weak acetic acid with sodium acetate and you added in more acetic acid, would the pH stay around 7 (at least within the buffer region on a titration curve) or would it get lower?

 

[KyoPhan]

So, in other words, because you are making the buffer instead of adding something to a mixture that is a buffer already, that is why the pH change is not resisted. For example, if the question was that you had a buffer with 1 mol/mol ratio of weak acetic acid with sodium acetate and you added in more acetic acid, would the pH stay around 7 (at least within the buffer region on a titration curve) or would it get lower?

It doesn't really have to do with making it vs being at a buffer.
By adding CH3COO-, the pH will always go higher. Even if we had pure H2O, and added CH3COO-, the pH will go higher! pH of 7 to maybe 8+. It starts at a pH of 2. It will go higher to "approach 7"

It is not saying that the pH will suddenly spike to around 7 with the addition of acetate ion. They key sentence is "the pH of the solution will approach 7."

Even if we were at a buffer, the pH will still go higher!
Let's take your example:
If we are at a 1M 1/1 ratio of acetic and and sodium acetate buffer, lets say the pH is around 4.5. If I add more acetic acid, the pH will go up to maybe 4.8(NOT BY MUCH DUE TO BUFFER). But it will still "approach 7".

 

[BMEEngineer]

It doesn't really have to do with making it vs being at a buffer.
By adding CH3COO-, the pH will always go higher. Even if we had pure H2O, and added CH3COO-, the pH will go higher! pH of 7 to maybe 8+. It starts at a pH of 2. It will go higher to "approach 7"

It is not saying that the pH will suddenly spike to around 7 with the addition of acetate ion. They key sentence is "the pH of the solution will approach 7."

Even if we were at a buffer, the pH will still go higher!
Let's take your example:
If we are at a 1M 1/1 ratio of acetic and and sodium acetate buffer, lets say the pH is around 4.5. If I add more acetic acid, the pH will go up to maybe 4.8(NOT BY MUCH DUE TO BUFFER). But it will still "approach 7".

Thanks for clarifying!