[mslee]

So... given that the following is the proposed mechanism:

O₃<-->O₂+O... k₁ and k_-1 for forward/reverse
O+O₃-->2O₂... k₂

If I had to use the Steady state approximation to find the rate law for the overall reaction... well, I feel like I did it right but I'm not quite getting what I was expecting. So first I said that d[O3]/dt=-k1[O3]+k_-1[O2][ O ]-k2[ O ][O3]... and w/ SSA find that [ O ] is equal to k₁[O3]/{k_-1[O2]+k₂[O3]}... and just substitute in and try to simplify the resulting rate law--> I get that:

rate=-2k₁k₂[O₃]² DIVIDED BY k_-1[O₂]+k₂[O₃]

And the teacher told us to expect, without any assumptions of relative rates or concentrations: (expression of various k's)[O₃]²[O₂]^-1

And I think I get an expression of that sort only if I assume that k2[O3] << k_-1[O2]


Am I just making a stupid algebra mistake somewhere?? 

[Yggdrasil]

And I think I get an expression of that sort only if I assume that k2[O3] << k-1[O2]

This sentence tells me that you are understanding everything well.  This is the assumption that you are supposed to make.  Think about what goes into the steady state approximation and why the SSA makes this assumption valid.