How do you convert from the empirical formula of a compound to the molecular formula of a compound?

use the mole ratio

Two experiments are required to arrive at the molecular formula for a compound.
1. Do an experiment to find the empirical formula
2. Do an experiment to find the molar mass.

Suppose one experiment gives an empirical formula of CH2. Suppose a second experiment reveals that the molar mass of the compound is 28 g/mol.

To find the molecular formula, simply divide the molar mass by the empirical formula mass. This should give a simple integer ratio, by which you then mulltiply the subscripts on the empirical formula.

For CH2, the empirical molar mass is 12 + 2 = 14 g/mol
28/14 = 2, so multiply the subscripts on the empirical formula to get the true molecular formula, which in this example gives C2H4.