Question

# which of the following yields a buffer solution when equal volumes of the two solutions are...

which of the following yields a buffer solution when equal volumes of the two solutions are mixed? a) 0.050 m h3po4 and 0.050m hcl b) 0.050m h3po4 and 0.025 m hcl c) 0.050m nah2po4 and 0.025m naoh d) 0.050m na3po4 and 0.050m m naoh e) 0.050m na3po4 and 0.025m naoh the answer is C, but why?

SOLUTION :

1) A buffer must contain some of a weak acid and some of its conjugate base. So if those two components are mixed together directly, clearly a buffer will result.

2)However, one must realize that if a strong acid or strong base is one of the substances being mixed a reaction may occur to generate one of the components of the buffer that is not “there” initially.

In case of option C

0.050 M NaH2PO4 is present and 0.025M of NaOH is present

So when they react Na2HPO4 is formed = 0.025 M

but we also have NaH2PO4 present = 0.025 M

so we can see that both the salt and the acid is present

For a buffer solution the pH should not change

Applying Handersons equation

pH = pKa + log [Csalt/Cacid]

Cacid = C salt = 0.025M

so the pH will remain same and it will be a buffer solution