Question

The element lanthanum has an atomic weight of 139 and consists of two stable isotopes lanthanum-138 and lanthanum- 139. The isotope lanthanum-138 has a mass of 138 amu and a percent natural abundance of 8.90X10-2 %. The isotope lanthanum-139 has a percent natural abundance of 99.9%. What is the mass of lanthanum-1392

Answer 1

The atomic weight is the weighted average of the abundance of isotopes in nature.

so for lanthanum the mass of lanthanum 139 has the mass as given below

(138 X .089%) + ( 139 X 99.9%) = 138.906 amu

for second materials which has two isotopes 113 and 115 e atomic weight of the element is

= (113 X 4.28 %) + ( 115 X 95.7 %) = 114.89 amu

The last question is the repeat of second question