puritatea = mpur/mimpur*100 => mpur=puritatea*mimpur/100=90*25/100=22,5g Fe2O3
MFe2O3 = 2AFe+3AO=2*56+3*16=160g/mol
MAl2O3 = 2AAl+3AO=2*27+3*16=102g/mol
Fe2O3 + 2Al = Al2O3 + 2Fe
160g Fe2O3................102g Al2O3
22,5g Fe2O3.................x=Calculeaza !!