设有机物的分子式为CxHyOz
CxHyOz+(x+y/4-z/2)O2→xCO2+y/2H2O
x+y+z=12
6x+y+8z=34
x+y/4-z/2=9/2
解得x=3,y=8,z=1
∴该有机物的分子式为C3H8O