MHNO3=1+14+3.16=63-------->63g/moli
-se afla masa de HNO3 din 3moli
n=m : M [ n=nr.moli ; m=masa subst. M=masa molara]
m= n . M
m=3moli . 63 g/moli=189 g HNO3
63 g HNO3------------14 g N
189 g ----------------------xg N
x=189 .14 :63=42 g N [ 42 g : 28g/moli=1,5 moli azot ]