When finding molecular formulas, one typically uses atomic mass rather than atomic weight. The term "atomic weight" refers to the average atomic mass of an element, taking into account the natural abundance of its isotopes. On the other hand, "atomic mass" refers to the mass of a single atom of an element.
To determine the molecular formula of a compound, one needs to know the relative number of atoms of each element in the compound. This information is obtained from the balanced chemical equation or experimental data. The atomic mass of each element is used to calculate the molar mass of that element, which is then used to determine the ratio of atoms in the compound.
For example, consider glucose (C6H12O6). To find the molecular formula, you would use the atomic masses of carbon (C), hydrogen (H), and oxygen (O) to calculate the molar mass of each element. Then, by comparing the molar mass of each element with the molar mass of the compound, you can determine the ratio of atoms and deduce the molecular formula.
In summary, when finding molecular formulas, atomic mass is used to calculate the molar mass of each element and determine the ratio of atoms in the compound. Atomic weight is not typically used for this purpose.