moles of O2 reqiured for the complete combustion of one mole of CnH2n+2

The general combustion equation of alkanes is: 
C_nH_2n+2 +  (3n+1/2)O₂  $\to$ nCO₂ + (n+1)H₂O

So, according to the number of carbon atoms ,number of moles of oxygen will be decided. Example :-
For n = 1 i.e. CH₄ , 
number of moles  of oxygen required would be (3$\times$1+ 1/2)
                                            = 7/2
Similarly you can solve for other hydrocarbons.