CHEMISTRY COMMUNITY

This is a pretty general question, but how do you know how to tell whether you are given an empirical formula or a molecular formula in a problem?

The molar mass of the molecule will be given if you are to find the molecular formula.

Typically, the empirical formula can be found when you are given the mass of each element present in the sample (ex. a sample of a compound has 4.75 g C, 2.03 g N and 8.24 g O) or the mass percent composition of a molecule (ex. if you were told you had 43% C, 37% H and 20% O in a given sample) With this information you can find the number of moles present in each element and compare from there.

If you are asked for the molecular formula, they will provide you its molar mass. You first have to find the molar mass of the empirical formula so you can find the ratio between the two (divide the molecular molar mass / empirical molar mass). You then multiply each subscript in the empirical formula by this number to get your molecular formula.

I find the easiest way to remember is just to think of molar mass first whenever I hear molecular formula.

As Lavelle had said in the lecture, the molar mass will be given if it is asking to distinguish between the molecular and empirical formulas. To decide which is which, you would need to find the molar mass of the formula on hand and compare it to the given molar mass of the substance on hand. If they are equal, you have the molecular formula, but if the given molar mass is more, then you likely have the empirical formula and need to add a subscript to multiply the atoms to match the molar mass of the given substance.

Referring to Lavelle's first module, the empirical formula shows the relative number of atoms that a molecule has, and the molecular formula shows the actual number of atoms. By this, we can understand that the empirical formula is the simplest ratio of the elements. Therefore, if the formula given can be reduced, it most likely will be the molecular formula.

The empirical formula is the smallest integer ratio of atoms in a compound. If you calculate the molar mass for the basic compound and it is smaller than the given molar mass, you know that the empirical and molecular formulas are different.

In the simplest terms, the empirical formula is the relative number of atoms, while the molecular formula is the actual number of atoms.

Compare the formula to the given molar mass. It is also possible it could be both the empirical and the molecular formula if the molar masses match up!

Empirical formulas can be the same as molecular formulas, but in general, an empirical formula gives the simplified ratio of the molecules present rather than the actual formula for a given compound.

Generally if you're given a molecular formula, you will be able to reduce the number of moles of each element by a certain factor. If you're given an empirical formula, you can't divide by a number to reduce the umber of moles of the element in the formula.

If the total molar mass of the molecular formula coincides with the empirical formula, then the empirical formula is the molecular formula. Otherwise, the empirical formula is just a ratio in the form of the lowest possible whole numbers of a molecule. The molecular formula is the actual molecule and should match with the molecular mass that's given for the molecule.

If you are given a formula and a given mass you can calculate the molar mass of the compound and see if the two masses are equal. If they are equal then the given formula is empirical. If not, then it is a molecular formula. You also may be able to tell by checking if the ratios are all in their smallest form.

If the problem gives you the molar mass of the compound, then you can find the molar mass of the formula that's given and compare that to the given mass, and if it's the same, then that's the molecular formula, otherwise it is the empirical formula.