A sample of lanthanum is found to have 99.91% atoms with a mass of 138.9 amu and 0.09% atoms with a mass of 137.9 amu. what is the average atomic mass of lanthanum

The average atomic mass of the lanthanum is 137.63 amu

Explanation:

Average atomic mass is defined as the sum of masses of it's isotope each multiplied by the it's natural abundance .

The formula of calculating average atomic mass is= (f= fraction of the natural abundance of the isotope,M=mass number)

Given,

Lanthanum contain  99.91% atom with mass 138.9 amu

and, 0.09% atoms with mass of 137.9 amu

(Converting the percentage into decimal form we will get,

99.91%=  and 0.09%=

   Then, the average atomic mass of lanthanum is, amu