Atomic Mass Unit

Table of Contents

 Atomic Mass Unit 

One atomic mass unit is defined as a mass exactly equal to one twelfth the mass of  one carbon -12 atom. And 1 amu = 1.66056×10^–24g. 

Today, ‘amu’ has been replaced by ‘u’ which is known as unified mass.

Atomic Mass 

Atomic mass of an element is defined as the average relative mass of an atom of an  element as compared to the mass of an atom of carbon -12 taken as 12. 

Gram Atomic Mass 

The quantity of an element whose mass in grams is numerically equal to its atomic  mass. In simple terms, atomic mass of an element expressed in grams is the gram  atomic mass or gram atom.

For example, the atomic mass of oxygen = 16 amu 

Therefore gram atomic mass of oxygen = 16 g 

Molecular Mass

 

Molecular mass of a substance is defined as the average relative mass of its molecule  as compared to the mass of an atom of C-12 taken as 12. It expresses as to how many  times the molecule of a substance is heavier than 1/12th of the mass of an atom of  carbon. 

For example, a molecule of carbon dioxide is 44 times heavier than 1/12th of the  mass of an atom of carbon. Therefore the molecular mass of CO2 is 44 amu. It is obtained by adding the atomic masses of all the atoms present in one molecule. 

Gram Molecular Mass

 

A quantity of substance whose mass in grams is numerically equal to its molecular  mass is called gram molecular mass. In simple terms, molecular mass of a substance  expressed in grams is called gram molecular mass. 

e.g., the molecular mass of oxygen = 32 amu 

Therefore, gram molecular mass of oxygen = 32 g 

111Formula Mass 

Sum of atomic masses of the elements present in one formula unit of a compound. It  is used for the ionic compounds. 

111Mole Concept. 

Mole is defined as the amount of a substance, which contains the same number of  chemical units (atoms, molecules, ions or electrons) as there are atoms in exactly 12  grams of pure carbon-12.  

A mole represents a collection of 6.022 x10²³( Avogadro's number) chemical units.. 111The mass of one mole of a substance in grams is called its molar mass. Molar Volume 

The volume occupied by one mole of any substance is called its molar volume. It is  denoted by Vm. One mole of all gaseous substances at 273 K and 1 atm pressure  occupies a volume equal to 22.4 litre or 22,400 mL. The unit of molar volume is litre  per mol or millilitre per mol