The mass of one atom is called as atomic mass
Atomic mass is given in amu (Atomic Mass Unit).
(AMU): -
It is the mass of C -12 carbon atom. It is also known as unified mass and denoted by u
1 amu =1.660056 × 10-24 g
Atomic Mass of the elements are expressed in terms of amu.
Atomic Mass of element in amu= Mass of atom in grams / 1.660056 × 10-24
AVERAGE ATOMIC MASS
Many natural occurring elements exists as more than one isotope. They occur in various percentage. So, we calculate average atomic mass of the elements using the percentage of the isotopes
Example
Chlorine has two isotopes of atomic mass units 34.97 and 36.97. The relative abundance of the isotopes is 75.5 % and 24.5% respectively. Find the average atomic mass of chlorine. Solution
Average atomic mass = 34.97 x 0.755 +36.97 x 0.245 = 35.46 u
MOLECULAR MASS
Mass of molecule is the sum of masses of elements present in molecule Example
Molecular Mass of CO2 = Atomic mass of C + 2 × atomic mass of Oxygen
=12 +2 × 16= 44 amu or 44 u
Molecular Mass of H2O = Atomic mass of O + 2 × atomic mass of Hydrogen
=16 +2 × 1= 18 amu
Formula Mass-
It is the sum of atomic masses of the elements present in one formula unit of a compound. It is used for the ionic compounds like NaCL
Formula Mass for NaCl = Atomic mass for Sodium + Atomic Mass for Cl atom
=23 +35.5 = 58.5 u
