How many moles of elemental oxygen gas (o2) must react with elemental chlorine (cl2) to produce 60.7 moles of dichlorine heptoxide?

The balanced chemical reaction that would occur here would be:
7O2 + 2 Cl2 --> 2 Cl2O7 
To determine the moles of oxygen that is needed in the system, we use the given amount of the product that is needed to be produced. Then, we use the relation from the balanced reaction to relate the product and one of the reactants. We do as follows:

moles Cl2O7 = 60.7 mol

moles O2 = 60.7 mol Cl2O7 ( 7 mol O2 / 2 mol Cl2O7 )
moles O2 = 212.45 mol O2

Therefore, about 213 moles of oxygen gas is needed in order to producr 60.7 moles of dichlorine heptoxide.