2 Mg + O2 = 2 MgO
puritatea = m pură/m impură * 100
m pură = 3000*100/80 = 2400 g Mg
M Mg = 24 g/mol
M MgO = 24+16 = 40 g/mol
M O2 = 2*16 = 32 g/mol
2*24 g Mg........32 g O2.........2*40 g MgO
2400 g Mg............x.......................y
x = 1600 g O2
n = m/M = 1600/32 = 50 moli O2
y = 4000 g MgO
n = m/M = 4000/40 = 100 moli MgO