Question

The CRC Handbook, a large reference book of chemical and physical data, lists two isotopes of indium (). The atomic mass of 4.28% of indium atoms is 112.904 u. Through a typographical oversight, the atomic mass of the second isotope is not printed. Calculate that atomic mass, taking the tabulated atomic mass of indium as 114.818 u.

Answer 1

Answer: The mass of second isotope of indium is 114.904 amu

Explanation:

Average atomic mass of an element is defined as the sum of masses of each isotope each multiplied by their natural fractional abundance.

Formula used to calculate average atomic mass follows:

  .....(1)

Let the mass of isotope 2 of indium be 'x'

• For isotope 1:

Mass of isotope 1 = 112.904 amu

Percentage abundance of isotope 1 = 4.28 %

Fractional abundance of isotope 1 = 0.0428

• For isotope 2:

Mass of isotope 2 = x amu

Percentage abundance of isotope 2 = [100 - 4.28] = 95.72 %

Fractional abundance of isotope 2 = 0.9572

Average atomic mass of indium = 114.818 amu

Putting values in equation 1, we get:

$114.818=[(112.904\times 0.0428)+(x\times 0.9572)]\\\\x=114.904amu$

Hence, the mass of second isotope of indium is 114.904 amu