The empirical formula of an organic compound is${\mathrm{CH}}_2\mathrm{O}$, its vapour density is 45, then its molecular formula is ____?

Empirical formula weight = 12 + 2 + 16 = 30, 

            Molecular wt = 2 × V.D = 2 × 45 = 90, n = 90/30 = 3

            The molecular formula = E.F × n= ${\left({\mathrm{CH}}_2\mathrm{O}\right)}_3$