Buffers!?!

[pjk77]

For the MCAT, can we just assume that anything added to a buffer is not going to change the pH? I'm so confused by this... I figured if you added a strong base to a buffer solution, the pH would increase significantly since its a strong base, but apparently it increases only insignificantly. What equation can you use to calculate the change of pH in a buffer solution? I'm so confused. Thanks!

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[brood910]

If you add water, pH will NOT change at all as you dilute both acid + base equally.

However, even if buffers resist pH change, according to TBR, there are still slight changes initially when you add acids/bases.
Regardless of whether it is strong or not, the change is only slight, not significant. Buffer RESISTS the change very very well!

There's no equation to solve the pH of buffer directly unless []s are given. (pH = pKa + log [A-]/[HA]). REMEMBER THAT pH = pKa at half point!!
pH of the buffer should be in +-1 pKa range as well. We can use this info easily to predict the pH.
Ex: For a weak acid, below half point, pKa-1<pH<Pka. Above half-point, it's pKa<pH<Pka+1

Also, know that pH at eq is in +-1 range of the pKa of the indicator added.

 

[pjk77]

ok great thanks! So I guess if I see something added to a buffer on the MCAT the answer is going to be a small change in pH. thanks!