To determine the moles of O2 required, we need to know the balanced chemical equation for the reaction between chlorine and oxygen to produce dichlorine heptoxide. The balanced equation is:

Cl2(g) + 7O2(g) -> 2Cl2O7(g)

From the balanced equation, we can see that for every mole of chlorine (Cl2) that reacts, 7 moles of oxygen (O2) are required. Therefore, to produce 69.1 moles of dichlorine heptoxide (Cl2O7), we need to react:

(69.1 moles Cl2O7) * (7 moles O2 / 2 moles Cl2O7) = 241.85 moles O2

Therefore, 241.85 moles of O2 are required to react with elemental chlorine to produce 69.1 moles of dichlorine heptoxide.