Question

carbon 85.6% and H 14.4 % is their composition of mass.
what is their molecular formula

examples below
C2H6
C3H4
CH4
C2H4
C3H8

Answer 1

With this information one cannot determine the molecular formula. The empirical formula can be determined.
Take 100 g sample which will give you
85.6g C and 14.4g H.
Convert to moles. moles = grams/molar mass
85.6/12 = ??
14.4/1 = ??

Now find the ratio of the two elements to each other with the lowest number being 1.00. The easy way to do that is to divide the smaller number by itself. If you know the molecular formula is one of those listed, you can pick one of them. Post your work if you get stuck.

Answer 2

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