Rubidium has two common isotopes, 85Rb and 87Rb. If the abundance of 85Rb is 72.2% and the abundance of 87Rb is 27.8%, what is the average atomic mass of rubidium

85.556 amu

Explanation:

From the question given above, the following data were:

Isotope A (85–Rb):

Mass of A = 85 amu

Abundance (A%) = 72.2%

Isotope B (87–Rb):

Mass of B = 87 amu

Abundance (B%) = 27.8%

Average Atomic mass of rubidium =?

The average atomic mass of rubidium can be obtained as follow:

AVERAGE = [(Mass of A × A%)/100] + [(Mass of B × B%)/100)]

AVERAGE = [(85 × 72.2)/100] + [(87 × 27.8)/100]

= 61.37 + 24.186

= 85.556 amu

Therefore, the average atomic mass of rubidium is 85.555 amu