Question

The relative abundance of two rubidium isotopes of atomic weights 85 and 87 are 75% and 25% respectively. The average atomic weight of rubidium is:

• A. 75.5
• B. 85.5
• C. 86.5
• D. 87.5

Answer 1

Answer: (B)

85.5

$\text{Average atomic mass}={\sum }_{i=1}^{i=n}({\text{mass}}_{\left(i\right)}\times \text{fractional}{\text{abundance}}_{\left(i\right)})$

As for every $Rb$ atom having mass $87$, there are three atoms with mass $85$.

Hence, the average mass of $Rb$ atom is $=\frac{3\times 85+1\times 87}{4}=85.5$.

Hence, the correct option is $B$