Difference Between Molar Mass and Molecular Weight

Difference Between Molar Mass & Molecular Weight

Molecules are substances formed due to combinations of the same or different atoms in different ratios. These molecules attached to each other to form compounds. Besides, the molar mass and molecular weight are two physical properties of substances.

Almost all chemical reactions are involved the changes occur in molecules or compounds so it is very important to understand the chemical and physical properties of molecules in order to perform chemical reactions and assume the final products.

Moreover, the main difference between both is that molar mass gives the mass of a mole of a particular substance. Whereas molecular weight is the mass of a molecule of a particular substance. While the definition and units are different for molar mass and molecular weight, the value is the same.

molecular weight

Molar Mass

Molar mass is the mass of a mole of a particular substance means the mass of a particular compound divided by the amount of substance. It is a physical property of substances.

Molar Mass = \(\frac{Mass of the substances (kg)}{Amount of substance (mol)}\)

The SI unit of molar mass is kg/mol. In general, we write the unit as g/mol.

The molar mass of an element is the atomic mass of the element. Therefore, the molar mass of a substance calculated by adding the atomic masses of each element present in that molecule.

Calculation of Molar Mass

Let us calculate the molar mass of NaCl compound.

H₂O:

The number of sodium atoms present = 1
Atomic mass of Sodium = 99 g/mol
Number of chlorine atoms present = 1
Atomic mass of Chlorine = 45 g/mol
Therefore, the molar mass of NaCl = (1x 99 g/mol) + (1 x 35.45 g/mol)

= 58.44 g/mol

By the help of this simple calculation, we find that the mass of one mole of NaCl compound is 58.44 g.

Molecular Weight

The term molecular weight defined as the mass of a molecule. It is also known as the relative molecular mass is another because molecular weight calculated as the mass that is relative to the carbon -12 isotopes.

The smallest particle in an element or compound is a molecule that possesses the chemical properties of that element or compound. They are made up of atoms join together by chemical bonds.

Molecular Weight of a substance = \(\frac{Mass of one molecules of substances}{ Mass of one carbon-12 atom}\) x \(\frac{1}{12}\)

The molecular weight has no units because the division has done between two masses that have the same units. Therefore, the unit of the molecular weight taken as atomic mass units or amu.

With the help of the above equation, we can find the molecular weight of an element or a compound. In other words, we can say that the Molecular weight of a particular molecule is equal to the sum of the atomic masses of each element.

Calculation of Molecular Weight

Let us consider the same example NaCl,

The number of sodium atoms present = 1
The number of chlorine atoms present = 1
Mass of a sodium atom = 989770 amu
Mass of an oxygen atom = 45 amu
Therefore the molecular weight of H₂O = (1x 99 amu) + (1 x 35.45 amu)

= 58.44 amu

With the help of the above example, we can say that both molar mass and molecular mass are the same in their values but are different in their units.