The given reaction is,
2N2O5(g)→4NO2(g)+O2(g)
The initial concentration of N2O5 is 3molL−1 and it is 2.75molL−1 after 30 minutes.
Now,
−ΔtΔ[N2O5]=−30−0(2.75−3)molL−1min−1=300.25
21×ΔtΔ[N2O5]=41Δt−Δ[NO2]Δt−Δ[NO2]=300.25×2=1.66×10−2M/Min
≃17×10−3M/Min