pH of a buffer changes after dilutions

[brownhamster]

I'm looking at mcat question a day Jun 24, 2010.
The explanation says that adding water to a buffer is going to bring the pH of the solution closer to 7.
This makes sense, because if you have very little molar quantities of conjugate acid and acid in an infinite amount of water, it should be undetectable, bringing the pH to 7.
However, from doing problems from TBR, I got this question wrong a few times, and the explanation is that looking at pH = pKa + log (A-/HA), dilutions should have no effect on pH, since you are diluting both the conjugate base and acid, so the liters part of (mol/L) cancel out, leaving just the moles part.
This makes sense also.
So which one is the best answer, for the purposes of the mcat?

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[starbaduk]

interesting....... this almost seems paradoxical to me... I know that buffer equation is an approximation but... still.

 

[brownhamster]

.

 

[seraphkz]

Interesting... I also have the impression that water doesn't change the pH of a buffer.

 

[Dartmouth2005]

Interesting... I also have the impression that water doesn't change the pH of a buffer.

Can't water ionize into OH- and H+? If so, then it is possible that it would lower the pH of the basic buffer solution since the pH of the water is 7. It won't be 7 exactly since the solution is slightly basic. That is why I chose B when I did it. I admit that I could be completely wrong, but that is the rationale that I employed.

 

[TieuBachHo]

I'm looking at mcat question a day Jun 24, 2010.
The explanation says that adding water to a buffer is going to bring the pH of the solution closer to 7.
This makes sense, because if you have very little molar quantities of conjugate acid and acid in an infinite amount of water, it should be undetectable, bringing the pH to 7.
However, from doing problems from TBR, I got this question wrong a few times, and the explanation is that looking at pH = pKa + log (A-/HA), dilutions should have no effect on pH, since you are diluting both the conjugate acid and acid, so the liters part of (mol/L) cancel out, leaving just the moles part.
This makes sense also.
So which one is the best answer, for the purposes of the mcat?

There are limitations to H-H equation. Dartmouth2005 is right about the auto-ionization of water that accounts for the skewed H-H application. When the solution is overwhelmed by diluting with water, ionization of water protons take precedent place which gives the pH of being close to 7. Dilutions may also depend on the solvents and cases. As a general approach, if the concentration of protons increases then the pH decreases as well. H-H equation fails short for strong acids/bases, changes in equilibrium concentrations, or conc. ratios.

PS: You mean to say acid and its conjugate base, right?

 

[Chocolatebear89]

I think water dilution only lowers raise pH for strong acids, because since strong acids dissociate completely, there is more volume, so less concentration of H+.

For weak acids, I think water dilution does not change pH.

Someone has to verify this though.

 

[Rabolisk]

Dilution changes pH for all acids/bases. They converge to 7.