posted by Sheryl .
I won't try to draw one here but take for example, CH3CH2COO-. This has two resonance structures but they look the same to me. The CH2-C single bond can rotate and it looks like you can just flip the COO- section and it looks like the same structure. Maybe that's the point. Any enlightenment?
The two resonating structures of the acid (which you haven't addressed) are non-equivalent and they add some stabilization to the molecule. However, in the anion, the two structures are equivalent, as you point out, because of the rotation of the bond. But equivalent resonating structures stabilize MORE than non-equivalent resonating structures and the anion has more stability (due to resonance) than does the acid.
And as you point out, "That is the point."