The balanced reaction is:
4Cr(s)+3O2 (g )= Cr2O3 (s)
Since we are not given the amount of any of the reactants, we assume we have one gram of chromium. Calculations are as follows:
1 g Cr ( 1 mol Cr / 52 g Cr ) ( 1 mol Cr2O3 / 4 mol Cr ) = 0.005 mol Cr2O3
0.005 mol Cr2O3 (151.99 g Cr2O3 / 1 mol Cr2O3 ) = 0.7307 g Cr2O3
Therefore, the theoretical yield for 1 gram of Cr is 0.005 mol Cr2O3 or 0.7307 g Cr2O3.