The chemical equation below shows the burning of magnesium (mg) with oxygen (o2) to form magnesium oxide (mgo). 2mg + o2 right arrow. 2mgo the molar mass of o2 is 32. 0 g/mol. What mass, in grams, of o2 is required to react completely with 4. 00 mol of mg? 2. 00 64. 0 128 256.