Question

What mass of oxygen is needed to combust and produced 6.4 moles of carbon dioxide gas?

C8H12 + 11O2= 8CO2 + 6H2O

Answer 1

You have already gotten the balanced equation. And the ratio of mol number of reactants and production is the ratio of coefficient. So there is 6.4/8*11=8.8 mol oxygen needed. The mass is 8.8*32=281.6 g.