-se afla md sol. de Na2CO3
md = c.ms : 100
md= 37 .572 :100=211,64 g Na2CO3
-se calculeaza masa moleculara
MNa2CO3=2.23 + 12 +3.16=106------>106g/moli
106 g Na2CO3------------48 g oxigen
211,64 g-------------------------x
x=211,64 .48 : 106=95,83 g oxigen