How do we identify the molecular formula from the empirical formula of a completely unknown substance? [closed]

Suppose you are given a sample of new (undiscovered) hydrocarbon with 600/7% $\ce{C}$ and 100/7% $\ce{H}$. (From combustion or any other method) We can find the empirical formula to be CH. Then how the we figure out whether it will be $\ce{C2H2}$ or $\ce{C4H4}$ or even something else?

Now we have identified the different compounds and can identify it through various reactions, but what would we do if we had something entirety new? How did chemists initially differentiate between these compounds?

closed as too broad by Mithoron, Todd Minehardt, Nilay Ghosh, airhuff, Jon CusterDec 15 '17 at 19:22

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