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MCAT Chemistry Question — Buffer Solution

Adding 2L of pure water to a 500mL buffer solution that contains equimolar concentrations of acetic acid and sodium acetate will have which of the following effects on the buffer?


A) Increase pH

B) Decrease pH

C) Reduce buffering capacity

D) No effect




The pH of a buffer solution is determined by the Henderson-Hasselbalch equation, and adding pure water to such a mixture would have no effect on pH. Neither the pH, nor the ratio of CH3COOH to Na+CH3COO would change upon the addition of water. Thus we can eliminated (A) and (B).


Buffering capacity is the ability of a buffer to resist changes in pH, and depends on the moles of acid and conjugate base present. While the addition of pure water would reduce the molarity of these components, it would not change the number of moles. Thus, the ability of the buffer solution to “soak up” a certain number of moles of acid added would not change simply because it’s now a 2.5L buffer solution instead of a 0.5L one.


By process of elimination, we are left with (D).


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