Average Atomic Mass/weight or Relative Atomic Mass:

Elements are found in different isotopic forms (atoms of same elements having different atomic mass), so the atomic mass of any element is the average of all the isotopic mass within a given sample. 

(A) If an elements exists in two isotopes having atomic masses ‘M1’ and ‘M2’ in the ratio   X: Y, then

or

(B) If an elements exists in two isotopes having atomic masses ‘M1’ and ‘M2’  and their abundant  percentage (%)  are  X % and Y % then

Since the atomic mass is a ratio, it has no units and is expressed in amu.

Illustrative Examples:

(1) Calculate the average atomic mass of silicon if relative abundance is 92.23 ,isotope Si^28, 4.77 % isotope = Si^29 and 3% isotope = Si^39 .

(2) Silver contains two isotopes Ag 107 and Ag109 and average atomic weight of silver is 108.5 find percentage abundant of isotopes of silver.

(3) Boron have two isotopes B11 and BA and their relative abundant % are 80% and 20% respectively and average atomic weight of Boron is 10.8 find the value of “A”

(4) Natural hydrogen gas is a mixture of 1H and 2H in the ratio of 5000:1. Calculate the atomic mass of the hydrogen.

(5) Carbon occurs in nature as a mixture of carbon 12 and carbon 13. The average atomic mass of carbon is 12.011. What is the percentage abundance of carbon 12 in nature?

(6) Calculate the molar mass of naturally occurring argon. Using the date given in the following table to ..