An empirical formula is the simplest ratio of atoms in a chemical compound, it gives the lowest whole number ratio of atoms in a compound. A molecular formula is the actual number of atoms of each element in a molecule.
In this case, the empirical formula XY3 gives the ratio of atoms in the compound, but not the actual number of atoms. The molecular formula of the compound can be a multiple of the empirical formula.
A possible molecular formula for the compound with the empirical formula XY3 is:
b. X2Y6: This is a possible molecular formula for the compound, as it is a multiple of the empirical formula, and it matches the ratio of atoms in the compound.
This is because, in order to obtain the molecular formula, you need to know the molecular weight or the molar mass, which can be obtained by isotopic analysis or other methods, once you have the molecular weight, you can calculate the number of atoms of each element in the compound.