buffer solutions (acids/bases)

[yestomeds]

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Am blanking out a bit as it is late hehe, so thought I'd quickly ask for advice before spending 30 mins looking for info only to forget what I was looking for.

Ok, here is a Q & A:
The correct statement concerning 50 mL of basic solution composed of 0.2 M NaH2PO4 and 0.2 MNa2HPO4 is the pH of the solution will not change with the addition of 20 mL of water.
Answer: This solution is a buffer. The pH of a buffer does not change with the addition or removal of water.

Ok why is the solution a buffer again?

 

[theonlytycrane]

A buffer solution consists of an acid (NaH2PO4) and it's conjugate base (Na2HPO4). It resists change in pH as addition of a bit of base will be neutralized by the acid and addition of a bit of acid will be neutralized by a bit of base. Water can act as a weak acid or weak base, but is neutral (pH = 7). So the addition of water wouldn't be expected to change the pH of the solution.