The molar mass off o2 is 32.0 g/mol. what mass in tye grams of 02 is required to react completely with 4.00 mol mg

The balanced reaction is: 

2Mg + O2 = 2MgO 

We are given the amount of  magnesium to be reacted with O2. This will be the starting point of the calculations.

4.00 g C3H8 ( 1 mol Mg/ 24.31 g Mg ) ( 1 mol O2 / 2 mol Mg ) ( 32.0 g O2 / 1 mol O2 ) = 2.63 g O2

if you mean hydrogen and oxygen then water is the product