Molecular Mass and Formula Mass

Formula Mass and Molecular Mass are two types of terms used to find out the mass present in a molecule. Formula mass gives us an idea regarding the mass of a molecule only when the empirical formula of the molecule is considered. On the other hand, Molecular mass gives the idea regarding the mass of a molecule when only the molecular formula of the molecule is considered. So in both the processes, the mass is thus calculated by adding the masses of each atom of the molecules.

Thus, the main difference is that- Formula mass is calculated by simply adding the masses of atoms present in the simplest formula. But, the Molecular mass is calculated using the actual numbers of atoms present in a molecule.

More about Formula Mass

Formula mass is the mass of a molecule, where it is calculated based on the empirical formula

The empirical formula is the formula of a molecule, where it shows all types of elemental atoms present in a molecule along with their corresponding ratios. This represents the simplest formula that can be formed for a molecule. Sometimes, this and the molecular formula are the same for all smaller molecules.

Example of Formula Mass (Calculation)

Formula mass is the sum of the masses of the atoms, which are present in the empirical formula of the molecules. It is calculated using the unit ‘amu’ (Atomic Mass Unit). Every atom has a mass given by amu units.

Through some examples, we will be looking into the formula mass of some compounds.

MgCl₂:

The empirical formula for MgCl₂ is the same as the molecular formula. Therefore, the formula mass can be calculated as follows:-

The mass of the Mg atom is  = 24.305 amu

The mass of a Cl atom is = 35.453 amu

Therefore the formula mass is = (24.305 amu) + (2 x 35.453 amu) = 95.211 amu

So, the formula mass of MgCl₂ comes out to be 95.211 amu.

C₄H₁₀ :

The empirical formula for C₄H₁₀ is C₂H₅. So, the formula mass can be calculated as follows:-

The mass of the C atom is = 12.0107amu

The mass of an H atom is = 1.0079 amu

Therefore the formula mass is = (2 x 12.0107amu) + (5 x 1.0079 amu) = 29.0609 amu

So, finally, the formula mass of C₄H₁₀ is considered to be 29.0609 amu.

More about Molecular Mass:

The molecular mass of a molecule is the calculation of mass obtained through a mole of that molecule. It means that the sum of the masses of molecules present in a mole can be calculated using the molecular mass formula. The atomic weights of each atom are in the g/mol unit, which is added to get the molecular mass.

One mole of a molecule is composed of 6.023 x 1023 molecules. Therefore, the molecular mass is the weight of those combined 6.023 x 1023 molecules. Therefore, the atomic masses of each element are easy to calculate for the molecular mass rather than calculating about 6.023 x 1023 number of molecules.

Examples of Molecular Mass (Calculation):

Now, let us look at some examples.

1. Molecular mass of HCl:

The atomic weight of H = 1.0079 g/mol

The atomic weight of Cl = 35.453 g/mol.

The molecular mass of HCl = (1.0079 g/mol) + (35.453 g/mol) = 36.4609 g/mol.

So, the molecular mass of HCl is 36.4609 g/mol.

2. Molecular mass of C₃H₆ :

The atomic weight of C = 12.0107 g/mol

The atomic weight of H = 1.0079 g/mol.

The molecular mass of C₃H₆ = (3 x 12.0107 g/mol) + (6 x 1.0079 g/mol)= 42.0795 g/mol

So, the molecular mass of C₃H₆ is 42.0795 g/mol.

Conclusion:

The formula mass is easily calculated by using the empirical formula, whereas the molecular mass is calculated by using the actual formula. Sometimes, the empirical and molecular formulas are the same for a molecule. This is because the formula mass and molecular mass are the same. The main difference is that the formula mass is calculated by adding the masses of atoms present in the simplest formula that can be given for a molecule. In contrast, the molecular mass is calculated using the number of atoms present in a molecule.