How could you show experimentally that the molecular formula of propene is C3H3 , not CH2

Answer 1

That would be impossible since the molecular formula of propene is C₃H₆, not C₃H₃, not CH₂. The empirical formula of propene happens to be CH₂, so I'm not sure what your question was meant to ask. Maybe it has to do with the empirical formula, but I'm not sure.