2Mg + O2 = 2MgO
p=80% = masa Mg = 0.8x400 = 320g Mg
randament 90% = 0.9x320 = 288g Mg intra in reactie
masa molara magneziu = 24 g/mol
=> 2x24g Mg................................2 (24+16)
288...........................................x
x=288x2x40/48
=240g MgO