so we have 2moles of Fe and 3 moles of oxygen in ferric oxide
Altogether we have a molar mass of 159.7grams. so one mole of Fe2O3 has 159.7g we need to find how many grams of Fe2O3 we will have in 0.7891 moles. So using simple arithmetic, we have
0.7891 molFe2O3 X 159.7gFe2O3/ 1moleFe2O3 = 126gFe2O3
So, in 0.7891 moles of ferric oxide, we will have 126 grams.
0.7891 mol X 159.7 g/mol = 126.0 grams