-deduc moli Mg din ecuatia chimica
2mol.....1mol
2Mg + O2----> 2MgO
x..............7,2mol
x=n,Mg= 14,4mol
-masa de magneziu pur
m= nxM----> m= 14,4gx24g/mol=345,6g Mg pur
-masa de magnaziu impur, cu p= 98%
p=m,purx100/m,impur
inlocuieste si calculeaza m,impur !!!!!