p=100mp/mi , unde mp-masa de substanta pura si mi-masa de subtanta impura
=>mp=mi x p/100=10x98/100=9,8g S pur(0.306 moli S)
=20x88/100=17,6g Fe pur(0.314 moli Fe)
reactie : Fe + 2S -> FeS2 => Fe exces (Fe:S=2:1-rap molar)
2moli S..........1 mol FeS2
0.306moli S.......x moli FeS2
=>x=0.153 moli FeS2