[Computerized]

I'm reading the Princeton Review SAT Chem book, and I'm confused about buffer pH calculations.

The example it gave was:

Given Ka of acetic acid = 1.8*10^-5, what is the pH of a solution of 0.1 M acetic acid and 0.01 M of sodium acetate?

So the equation is HC2H3O2 (aq) -> <- H(aq) + C2H3O2 (aq)

Now, isn't the conj. acid HC2H3O2? So then plugging into the Henderson Hasselbalch equation we get

pH = pKa + log [HC2H3O2] / [C2H3O2] ??

However, in the answers it said that the formula is supposed to be pH = pkA + log [C2H3O2]/[HC2H3O2]

So I'm really confused here. Is there something I'm missing or not understanding?

[plankk]

Yours Henderson-Hasselbalch equation has a mistake. The proper Henderson-Hasselbalch equations are:
pH=pKa + log[A-]/[HA] or pH=pKa - log[HA]/[A-]

[Computerized]

Yeah, didn't I use pH = pKa + log[A-]/[HA+] ?

[A-], the conj. acid is HC2H3O2
[HA], the conj. base is C2H3O2

Please correct me if I'm wrong. I'm still confused.

[plankk]

Your are right in saying what is an acid and what is a base, but... you have got the wrong signs. HA means an acid, and A^- is a base.

[Computerized]

Oh, wow. That's a pretty dumb mistake I made.

I guess what confused me was that the book said another way to write the equation is pH = pKa + log[conj.acid]/[conj.base]

Is the book wrong there?

[ryzhang]

pH = pKa + log [HC2H3O2] / [C2H3O2]
it is right

[Borek]

pH = pKa + log [HC2H3O2] / [C2H3O2]
it is right

Judging from the equation adding acid you will increase pH - which is obviously wrong.

And don't ignore charges in the formulas.