Question

Is a buffer supposed to keep the pH of a solution at 7?

Answer 1

Sometimes, but usually no.

It just keeps the pH from changing much, and is centered around the pKa of the acid used to make the buffer.

Let's say we made an acetic acid buffer, where the concentration of acetic acid was `"0.500 M"` and the concentration of sodium acetate was `"1.00 M"`. The pKa of acetic acid is about `4.76`.

Acetic acid is `"CH"_3"COOH"`, and sodium acetate is `"CH"_3"COO"^(-) "Na"^(+)`.

Using the Henderson-Hasselbalch equation (which you will see often with buffers), we get:

`\mathbf("pH" = "pKa" + log \frac(["A"^(-)])(["HA"]))`

`"pH" = "pKa" + log \frac(["CH"_3"COO"^(-)])(["CH"_3"COOH"])`

`"pH" = 4.76 + log (("1.00 M")/("0.500 M"))`

`"pH" = 4.76 + 0.301029996`

`color(blue)("pH" ~~ 4.79)`

So, with a buffer like this, you should expect the pH to stay generally close to or return to something close to `4.79`, not `7`, if the equilibrium were to be disturbed.

If it were to become `7` for a long time, that would not be a very good buffer.