A sample of rubidium contains two different isotopes. 72.15% of the sample if Rubidium-85 while 27.85% is Rubidium-87. What is the average atomic
mass of the sample?

Average atomic mass  = 85.557 amu.

Explanation:

Given data:

Percent abundance of Rb-85 = 72.15%

Percent abundance of Rb-87 = 27.85%

Average atomic mass = ?

Solution:

Average atomic mass = (abundance of 1st isotope × its atomic mass) +(abundance of 2nd isotope × its atomic mass)  / 100

Average atomic mass = (72.15×85)+(27.85×87) /100

Average atomic mass =  6132.75 + 2422.95 / 100

Average atomic mass = 8555.7 / 100

Average atomic mass  = 85.557 amu.

it would be a gas formation