moles=mass of substance 0.500=mass/2[55.8]+3[16] 0.500=mass/111.6+48 0.500 x 159.6=mass mass=79.8g In order to calculate the number of grams in .5 moles of fe2o3, you have to use the above calculations. .5mol= x/2(55.8) + 3(16). then, you can solve the equation for the answer which is 79.8 grams.