To calculate the molecular formula from percent by mass of each element in a compound, you need additional information about the molar mass of the compound.

The molar mass is the sum of the atomic masses of all the atoms in the compound. Once you have the molar mass, you can use the percent by mass of each element to calculate the number of moles of each element in the compound.

Finally, you can use the number of moles of each element to determine the simplest whole number ratio of atoms in the compound, which gives you the molecular formula.