what is the lewis structure for C2O4 2-?

Answer 1

try using the equations

S = N - A

N = the need bonds
N = (2 x 8) + (4 x 8) + 2 (for the -2 charge) = 50 bonds need

A = the available bonds
A = (2 x 4) + (4 x 2) + 2 = 18 bonds available

S = 32 bonds shared

Answer 2

You have two (CO2)- fragments joined together by a carbon-carbon single bond. You can probably work it out from there.