Mole Concept: Atomic and Molecular Mass

Key Concepts

• Mean atomic mass

• Molecular mass

• Grain molecular mass

• Molar mass

• Grain molar volume

• Average molar mass

• Fractional molar mass

Mole concept:  

One mole is the collection of entities that are equal to the number of atoms in 12gms of C-12 isotope. 

The concept that is related to a single-atom is called atomic mass, whereas molecular mass means which relates to the molecule. A substance is something with mass that takes up space. The sum of the total mass in grams of the atoms present in a molecule per mole is the molar mass/molecular weight. The molar mass unit is grams/mole.  

Atomic mass: The atomic mass is the mass of a single atom, which can only contain one isotope at a time and is not a quantity. 1/12 of the mass of a carbon-12 atom in its ground state is the best definition. The total masses of protons and neutrons, which is nearly equal to the atomic mass, can account for the mass of an atom.  

When an atom’s atomic weight is divided by unified atomic weight or Daltons, it becomes a dimensionless number. The atomic masses of elements range from 1.008 amu for hydrogen to 250 amu for extremely high-atomic-number elements. The average atomic mass of each atom in a molecule can be used to calculate the mass of the molecule.  

Example: The atomic mass of sulphur – 32.065u 

The atomic mass of iron – 55.845u 

The atomic mass of Calcium – 40.078u 

The atomic mass of an atom is a feature that is closely related to its mass number. A single atom’s atomic mass is just its total mass, which is usually given in atomic mass units, or amu. An atom of carbon with six neutrons, carbon-12, has an atomic mass of 12 amu by definition. For reasons that are outside the scope of this article, other atoms do not have round-number atomic masses. However, in most cases, an atom’s atomic mass will be fairly near to its mass number, with minor variation in decimal places. 

Molecular mass: 

Earlier we have discussed the concept of atomic mass. This concept can be extended to calculate the molecule mass. The molecular mass of a substance is the total atomic weight of the molecule. So, it’s the relative mass of molecules expressed in atomic mass (u).  

Molecular mass is the mass of one molecule of a substance expressed in amu. It is obtained by multiplying the atomic mass of each element by its atomic number and by adding them. 

The molecular mass of ethane: 

Methane has one carbon atom and four hydrogen atoms. 

= 12.011u + 4(1.008u) 

             =16 

The molecular mass of Glucose 

The glucose molecular formula is

, where it has 6 carbon atoms, 12 hydrogen atoms and  
6 Oxygen atoms. 

Molecular weight =

                                   = 72.066+12.096+95.994 

                                   = 180.156 

The molecular weight of a compound is determined by using the following methods: 

1. Mass spectroscopy: Mass spectroscopy is used for calculating mass to charge for one or more molecules. Such a type of measurement is also used to determine the molecular weight of the compound. This is one of the analytical methods to find the molecular mass and which  indirectly helps to prove the identity of the isotope. 

2. Hydrodynamic method: The weight of a molecule is determined according to the Mark-Hou wink relations. This is also described as the relative molecular mass as it requires calibration. 

3. Static light scattering: This method is used to determine the amount of light scattered which is used by the Zimm method. 

Average atomic mass: 

The average atomic mass is the average weight of the isotopic masses of the element that is present in the isotopes of the element.  

Or 

Average atomic mass is the atomic mass of the elements is fractional due to the presence of isotopes. 

Average atomic mass =

The average atomic mass of unknown isotopes(X) (A_x) =

Where: a₁, a₂,………. is the atomic mass of isotope and x₁, x₂,……… is the mole percentage of isotopes 

For example: Chlorine isotope ³⁵Cl and ³⁷Cl  

Average atomic mass of Cl =

=35.5 

Molar mass unit:  

Molar mass is the amount of atom present in 12gm of carbon-12 isotope is the number of particles present in one mole of the actual substance. One mole of a substance must be equal to the equal number of entities that are present in the substance. 

To know the mass of C-12, it is calculated by using one of the methods for determining the molecular mass which is by mass spectroscopy and the value is found to be 1.99 x 10²³gm. 

Mass of substance containing 1 mole of particles is equal to the molar mass. 

Mean molar mass or molecular mass: 

The mean molar mass of the substance is the average molar mass of the different substance which is present in the container. 

Average molecular mass=

                                               =

Where n₁,n₂,……….n_i is the number of moles of the compound and m₁,m₂,………..m_i is the molecular mass of the compound. 

Difference between molar mass and molecular weight 

Gram molecular mass:  

Gram molecular mass is the mass of one mole of a molecular compound which is measured in grams or the substance which is expressed in grams is also called as the gram-molecular mass of the substance. 

Gram molecular mass is the same as the molar mass. 

GMM is also defined as

molecules. 

For example:  

1. Molecular mass of ‘N₂’ molecule = Mass of one N₂ molecule 

                                                                            = 2 × mass of N atom 

                                                                            = 2 × 14amu 

                                                                            = 28 amu 

Gram molecular mass = mass of 6.02 × 10²³ ‘N₂’ molecule= 32 amu × 6.02 × 10²³ 

                                                         =

 

                                                         = 32 g 

2. Mass of one Oxygen molecule = 32amu = 32NA32NA grams 

Mass of

molecule of oxygen =

32/N_A

× N_A g = 32 g 

Fractional unit mass:  

The formula mass unit of a substance is the sum of the atomic mass of all elements in the formula of a compound. It is calculated the same as we calculate for molecular mass. The difference we could recognise is that we use the word formula unit for that substance whose constituent particles are ions.  

Formula mass is also known as molecular mass or molecular weight. 

For Example: For NaCl  

Formula unit mass:

Summary

• The atomic mass is the mass ofa single atom, which can only contain one isotope at a time and is not a quantity. The total masses of protons and neutrons, which is nearly equal to the atomic mass, can account for the mass of an atom.

• The average atomic mass is the average weight of the isotopic masses of the element that is present in the isotopes of the element.

• A single atom’s atomic mass is just its total mass, which is usually given in atomic mass units, or amu.

• The formula mass unit ofa substance is the sum of the atomic mass of all elements in the formula ofa compound.

• Molar mass is the amount of atom present in 12gm of carbon-12 isotope is the number of particles present in one mole of the actual substance.

• Grain molecular mass is the mass of one mole ofa molecular compound which is measured in grams or the substance which is expressed in grams is also called as the gram-molecular mass of the substance.

• The mean molar mass of the substance is the average molar mass of the different substance which is present in the container.