Atomic and Molecular Structure

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.

Difference Between Molar Mass and Molecular Weight

 Basis Molar Mass Molecular Weight
Definition It is the mass of a mole of a particular substance It is the mass of a molecule
Unit Kgmol-1 Atomic mass units
Calculation It is calculated by dividing the mass of a substance by the amount of the substance It is calculated as the mass of substance relative to the 1/12th of the mass of the carbon-12 atom.
Example Water has 3 atoms, two hydrogen atoms, and one oxygen atom. Also, the atomic weight of hydrogen is 1 gram and oxygen is 16 grams. We calculate the molar mass of one water molecule, when we add (2 x 1) + 16 = 18 grams. 18 grams per mole is the molar mass of water. Water has 3 atoms, two hydrogen atoms, and one oxygen atom. In addition, the atomic weight of hydrogen is 1 gram and oxygen is 16 grams. Molecular weight of one water molecule is calculated, when we add (2 x 1) + 16 = 18 grams. The molecular weight of water is 18 grams.

 

Solved Question for You

Q. Calculate the number of moles of water in 600 grams of water (H2O)?
Ans. Let the number of moles in water be x
So the equation is:
x = \(\frac{1 mole H2O}{18 grams H2O}\)  x 600 grams H2O
After solving the equation we get,  3.33 moles of H2O in 600 grams of H2O.

 

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