M(fe2o3)=2*56+3*16=160г/моль

w(o)=3M(o)/M(fe2o3)=3*16/160=0.3*100%=30%

M(bao)=137+16=153г/моль

w(o)=M(o)/M(bao)=16/153=0.1046*100%=10.46%

M(n2o5)=2*14+5*16=108г/моль

w(o)=5M(o)/M(n2o5)=5*16/108=0.74*100%=74%