%SO2 = 20 = 2x/75*100=>2x=15=>x=7,5
2 SO2 + O2 ---> 2 SO3
I: 75 m 25 m -
C: 2x x 2x
E: 75-2x 25-x 2x
nr moli SO2=75-15=60 moli
nr moli O2=25-7,5=17,5 moli
nr moli SO3=2*7,5=15 moli
=> nr moli total = 92,5 moli
%SO2=60/92.5*100=64,865%
%O2=17,5/92,5*100=18,919%
%SO3=15/92.5*100=16,216%