The mass of an atom is due to the masses of protons and neutrons in the nucleus. The relative masses of protons and neutrons are almost equal to one. Therefore, the atomic mass of an element should be nearly a whole number. But in many cases the atomic masses are fractional. The main reason for these fractional atomic masses is that these elements occur in nature as a mixture of several isotopes. The atomic mass of an element is the average of the atomic masses of these isotopes in the ratio of their proportion of occurrence.
An element occurs in two isotopic forms with atomic masses 10 and 11. What is the percentage abundance of two isotopes in the sample having atomic mass 10.80?

Options

A. 20,80
B. 50,50
C. 25,70
D. 60, 40

Answer

A

Explanation

Let the percentage of isotope with atomic mass 10 = x
` therefore ` Percentage of isotope with atomic mass 11 = 100 - x
Average atomic mass = 10.80
`10.80 = ( x xx 10 +(100 -x) xx 11)/(100 ) `
`1080 xx 100 = 10 x + 1100 -11 x`
` x=1100-1080 =20`
Abundance of isotope - 10 = 20% Abundance of isotope -11 = 100 - 20 = 80%