Problem 101P

Rubidium has two naturally occurring isotopes: Rb-85 with mass 84.9118 amu and a natural abimdance of 72.17%, and Rb-87 with mass 86.9092 amu and a natural abundance of 27.83%. Calculate the atomic mass of rubidium.

Solution 101P

Here, we have to calculate the atomic mass of rubidium.

Atomic mass of an element is defined the average mass of the isotopes that compose that element. Atomic mass of each element is based on the natural abundance of each isotope.

Atomic mass of an element can be possible to calculate by using the following equation.

Atomic mass = fraction of isotope n) x (mass of isotope n) ----(1)

Given that,

Isotope 1 , Rb-85 = 72.17% = 72.17/100 = 0.7217

Mass of isotope 1, Rb-85 = 84.9118 amu

Isotope 2, Rb-87 = 27.83% = 27.83/100 = 0.2783

Mass of isotope 2, Rb-87 = 86.9092 amu

From equation (1)

Atomic mass = fraction of isotope n) x (mass of isotope n)

= (fraction of Ru-85 x mass of Ru-85) +(fraction of Ru-87 x mass of Ru-87)

= (0.7217 x 84.9118) + (0.2783 x 86.9092)

= 61.2808 + 24.1868 = 85.4676 amu

Therefore, the atomic mass of Rubidium is 85.48 amu