I'm modeling a cylindrical combustion chamber with inlet velocity B.C and outlet pressure B.C. in the finite rate chemistry mode. Do i need to set the mass fraction of species at the outlet? If yes, how should i know them? Fluent tutorial_11 has set the B.C. at the outlet 0.23 for O2 and 0.77% N2, something seems to be wrong! where are CO2 and H2O?

you do not need to set the specie mass fractions at the outlet, they will be calculated in the simulation instead

The specified values are those to be used for cases of reverse flow on outlets. In such cases some faces become inlets and need corresponding inlets species fractions.