Molar mass
C8H18 => 114.23 g/mol
O2 => 31.99 g/mol
2 C8H18 + 25 O2 ---> 16 CO2 + 18 H2O
2 x 114.23 g ---------------- 25 x 31.99 g
10.0 g ----------------------- ( mass of O2)
mass of O2 = 10.0 x 25 x 31.99 / 2 x 114.23
mass of O2 = 7997.5 / 228.46
= 35.00 g of O2
hope this helps!