What information do we need to determine the molecular formula of a compound from the empirical formula?

Molecular mass

Explanation:

Molecular formulas is the actual number of atoms of each element in the compound while empirical formulas is the simplest or reduced ratio of the elements in the compound.

Thus,

Molecular mass = n * Empirical mass

Where, n is any positive number from 1, 2, 3 ...

Thus if the molecular mass is known, then we can find the value of n which results to molecular formula.