How does one determine the molecular formula from the empirical formula?

By first determining the approximate molecular mass of the compound in question, usually by measurements of osmotic pressure, freezing point depression, or some other colligative property that depends on the number of molecules present. The approximate molecular mass of the substance in question is then calculated from the colligative property measurements, and this value is then divided by gram molecular mass corresponding to the empirical formula. The nearest integer to the resulting quotient is then applied as a factor by which to multiply each subscript of the empirical formula to yield the molecular formula.