Respuesta :
Answer:
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= [tex]f_1M_1+f_2M_2+f_3M_3.............+f_nM_n[/tex] (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%=[tex]\frac{99.91}{100} =0.99[/tex] and 0.09%=[tex]\frac{0.09}{100} =0.0009[/tex]
Then, the average atomic mass of lanthanum is,[tex](0.99\times138.9)+(0.0009\times137.9)=137.63[/tex] amu
