Could a buffered solution be made by mixing aqueous solutions of HCl and NaOH? Explain. Why isn't a mixture of a strong acid and its conjugate base considered a buffered solution?

1 Answer
Mar 25, 2017

No, and you've also got two definitions mixed up. Two things:

  • The whole point of a buffer is to resist pH change. You're not going to do that with two things that dissociate completely.
  • #"HCl"# and #"NaOH"# are not conjugates of each other. What is the definition of a conjugate base or acid? What do they differ by?

Any strong acid has a terrible conjugate base that does almost literally nothing in solution (it's a spectator ion), and furthermore, by definition, a strong acid will barely exist in water as itself. With zero components, there is no buffer (why must buffers have two components? Better yet, why can't we have a buffer with zero components?).

Even with a weak acid, we can (initially) only resist strong base. With a weak base, we can (initially) only resist strong acid. But we care about ALL #"pH"# changes, not just decreases OR increases.

Thus, a proper buffer must have components that resist incoming acid AND base.

Why must we not have only strong acids or bases in a buffer? Think about it from the idea of torturing a buffer. Say we're trying to torture it and get it to crack. What do we add? Strong acid or base. Those will react 1:1 easily. If the so-called "buffer" has only a strong acid or base, it "cracks" earlier.

  • To resist strong base, the buffer must have a weak acid (strong acids "crack" too easily).
  • To resist strong acid, the buffer must have a weak base (strong bases "crack" too easily).

We can do even better. If the weak acid and weak base are conjugates of each other, they generate each other upon reacting with incoming strong whatevers.

A weak acid neutralizing an incoming strong base:

#"HA"(aq) + "OH"^(-)(aq) rightleftharpoons "A"^(-)(aq) + "H"_2"O"(l)#

A weak base neutralizing an incoming strong acid:

#"A"^(-)(aq) + "H"_3"O"^(+)(aq) rightleftharpoons "HA"(aq) + "H"_2"O"(l)#

Thus, a good buffer must have a weak acid and its conjugate base.