Why is this sp2 hybridized

Hi guys,

so I'm going through my organic chem 1 as a second language book and got to an example where in the molecule it has a hydrogen atom bonded to two carbon atoms. This means that there are two lone pairs on the oxygen atom. I originally thought this would be sp3 hybridized and bent, however the back of the book says it is actually sp2 hybridized due to being in an ester and to check the resonance structures. One carbon it is bonded to is double bonded to another oxygen, so I'm thinking that if you draw the resonance structure and push a lone pair from the original oxygen onto the double bond and then a pair of electrons from the double bond onto the oxygen to make it (-1), the original oxygen would now be sp2 hybridized. Is that right?

Can anyone help explain this to me?

Your explanation sounds correct to me. You have O=CR-O-R <---> O(-)-CR=O(+)-R. The latter is a minor resonance structure, but does establish that the ester group forms a pi system. If one of the oxygen's p orbitals is used in a pi system, that leaves 2 p orbitals left, so the hybridization is sp2.

As for "sp3 hybridized and bent", an sp2 hybridized oxygen is also bent, but at a different angle.