Atoms and molecules are so tiny that they need to be counted in very large quantities in order to result in a “workable” number.
The mole (abbreviation: mol) is used to count particles. 1 mole of anything is 6.02 ´ 1023 of that thing.
We cannot count out a mole of particles in the lab, therefore a connection between moles and mass is needed. The molar mass is the mass (in grams) for 1 mole of the particle. Molar mass (also often called Molecular mass, formula weight, etc.) is found by adding the atomic masses (multiplied by the number of that atom) for each atom in the molecule. Molar mass can then be used in dimensional analysis conversions as the equality between grams and moles. The molar mass value always goes with the “grams” in such conversions.
“Percent” is part / whole ´ 100. The “part” is the mass of an individual element while the “whole” is the mass of the whole molecule or sample. These calculations can be done with lab data—grams of both the individual element and the whole sample—or with chemical formulas. In the case of chemical formulas, the molar mass is used in the calculations.
The empirical formula is the lowest possible ratio of atoms in a molecule. The ratio of atoms is the same as the ratio of moles of atoms. The mass of each element is converted to moles and then all the moles are divided by the smallest to reach the lowest possible whole number ratio. That ratio is used as the subscripts in writing the empirical formula.
The molecular formula is the actual ratio of atoms in a molecule. The molar mass of the empirical formula is compared to the given molar mass of the molecular formula. The comparison results in a factor that is used to multiply the subscripts of the empirical formula to reach the subscripts of the molecular formula.