How many moles of o2 are required for the complete reaction of 88.9 g of c2h4 to form co2 and h2o?

The molecular mass for C2H4 should be: 2*12 + 4*1= 28 gram/mol. Then the amount of mol in 88.9 grams C2H4 should be: 88.9g/ (28g/mol)= 3.175 mol.

To answer this question, you need to know the reaction formula for the problem. The formula should be: 
C2H4 + 3O2 --> 2CO2 + 2H2O 
In the formula, every 1 C2H4 will be reacted with 3 O2. Then the amount of oxygen needed would be: 3/1 * 3.175 mol= 9.525 mol