Molecular Weight

For students who are learning Chemistry, the term molecule is going to be one of the most frequently used terms from the day they start to learn the subject. Molecular weight can be defined as the average mass of a molecule of a compound compared to ¹/₁₂ the mass of carbon 12 (C - 12) and is calculated as the sum of the atomic weights of the constituent atoms, It is also called as molar mass. Eg: The molecular weight of water containing two atoms of hydrogen and one atom of oxygen, is 18 (i.e. 2 + 16). Also called molecular mass. An important aspect of molecules is the way their weight or mass is calculated in the form of molecular weight also referred to as molar mass. 

The main question arises, What do we do by knowing the mass of such tiny atoms and molecules? Well, the significance of these atomic mass and molecular mass of a substance is huge in chemistry. In this topic, we have discussed molecular weight meaning, Let’s understand what is molecular mass.

Molecular Mass

The molecular mass of a compound is defined as the mass of one molecule. The molecular mass of a compound differs due to the presence of isotopes. Molecular mass is measured in AMU which stands for atomic mass units. In the above topics, we have discussed the molecular weight definition and molecular mass lets understand some key differences between them.

Difference between Molar Mass and Molecular Mass

Molar Mass Formula and Molecular Mass Formula 

In this topic, we will discuss the difference between Molar Mass Formula and Molecular Mass Formula 

Molecular Mass

• Molecular mass is equal to the sum of the individual atomic mass of each atom in the molecule. By adding the atomic weight of the constituent elements of a molecule we can get molecular weight formula as:  

Molecular Mass (AB2) = Atomic Mass of A + 2 × Atomic Mass of B 

Molar Mass

• Molar mass is equal to the sum of the individual atomic mass of each substance (in grams) in the molecule. By adding all the atomic weight of the elements given in grams we will get the molar mass equation as:

Molar Mass (ab2) = Atomic Mass (in Grams) of a + 2 × Atomic Mass of B (in Grams)

The Molecular Mass of the Molecule  Can be Calculated by the Following Steps:

• First Identify the formula of the compound or the molecule given.

• Use the formula to determine the number of atoms present in each element of the compound or molecule.

• Now multiply the atomic weight of each element by the number of atoms of that particular element. Similar to this do it for all the elements in the molecule or compound.

• Now sum all the values obtained in the above step.

• Then add the unit as grams/mole and we will get the molecular mass of the substance given.

Solved Examples 

Q1: Find the molecular mass of the water molecule.

Solution: We know that the formula of the water molecule is H2O.

The atomic mass of hydrogen = 1 AMU

The atomic mass of oxygen = 16 AMU

So, Molar mass of water = 2 x 1 + 16 = 18 AMU.

Q2: Calculate the relative molar mass of the hydrogen molecule.

Solution: The hydrogen molecule is represented by H2.

The relative molecular mass of hydrogen molecules can be calculated as the sum of the relative atomic masses of two atoms of hydrogen.

i.e 1U + 1U = 2U.

Conclusion

Molar mass, molecular mass, and relative molar mass differ from each other in terms of the unit they are expressed in. It can be easily noticed that the atomic mass and the molar mass of carbon-12 are numerically equal. Molar mass represents the weight of a one-mole molecule in g/mol, and molecular mass represents the weight of one molecule in AMU and relative molar mass represents the weight of a single atom in terms of ununified mass. 

This same relationship is between atomic and molar masses of elements, between molecular masses and molar masses of molecular substances, and between formula masses and molar masses of ionic compounds.