14.7 grams of magnesium reacts completely with 9.7 grams of oxygen to form magnesium oxide (mgo). what is the percent composition of the magnesium oxide?

The answer is 60.3% magnesium, 39.7% oxygen.
Solution:
The chemical equation for the reaction is 2 Mg + O2 → 2 MgO.
Since magnesium reacts completely with oxygen, it is the limiting reactant in the reaction. Hence, we can use the number of moles of magnesium to get the mass of MgO produced:
     moles of magnesium = 14.7g / 24.305g mol-1
                                       = 0.6048 mol

     mass of MgO = 0.6048mol Mg(2 mol MgO/2mol Mg)(40.3044g MgO/1 mol MgO) 
                           = 24.376g MgO

We can now solve for the percentage of magnesium:
     % Mg = (14.7g Mg / 24.376g MgO)*100% = 60.3%

We also use the number of moles of magnesium to get the mass of oxygen consumed in the reaction:
     mass of O2 =  0.6048 mol Mg (1mol O2 / 2mol Mg) (31.998g / 1mol O2) 
                        = 9.676g

The percentage of oxygen is therefore
     % O2 = (9.676g O2 / 24.376g MgO)*100%
               = 39.7%
Notice that we can just subtract the magnesium's percentage from 100% to get 
     % O2 = 100% - 60.3% = 39.7%