N2+ 3H2= 2NH3
a b 0
c/2 3c/2 c
a-(c/2) b-(3c/2) c
质量分数为17c/[28*(a-c/2)+2*(b-3c/2)+17c]=17c/(28a-14c+2b-3c+17c)=17c/(28a+2b)