buffer

[chiddler]

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If water is added to a buffer solution with some pH, what happens to pH?

Answer: It does not change because the mole ratio of acid and conjugate base is the same.

While this makes sense mathematically using the henderson hasselbalch equation, I don't understand this conceptually. What if a very large amount of water is added? I find it hard to believe that the pH remains the same.

Thanks!

 

[MT Headed]

Correct. The question refers to adding water, not a large amount of water. A large amount would affect the pH.

Conceptually a buffer resists (but doesn't completely prevent) changes to pH when you add modest amounts of extra acid, base, or even water. Most folks learn about the acid or base by rote. This question forces you to think about an old situation in a new way to see if you understand what it really means.

A good mcat style question!