Rubidium has two naturally occurring isotopes, rubidium -85 ( atomic mass = 84.9118 amu; abundance = 72.15%) and rubidium -87 (atomic mass = 86.9092; abundance = 27.85%). calculate the atomic weight of rubidium

The answer to your question is: 85.458 amu

Explanation:

data

Rubidium-85   A = 84.9118 amu   abundance = 72.15%  

Rubidium - 87  A = 86.9092 amu abundance = 27.85%

Atomic weight = ?

Atomic weight = 84.9118(0.7215) + 86.9092(0.2785)

Atomic weight = 61.2538 + 24.2042

Atomic weight = 85.458 amu

the answer is d. 74.02