Question

Calculate the atomic weight of rubidium, which has two isotopes with the following properties: 85^Rb (84.91 amu, 72.71% natural occurrence) and 87^Rb (86.91 amu, 27.83% natural occurrence). Round your answer to 4 significant figures

Answer 1

The atomic weight of Rubidium (Rb) is 85.91 amu, using 4 significant figures.

Step-by-step explanation:

The average atomic mass of Rubidium (Rb) is composed by the mass of the two given isotopes 85^Rb and 87^Rb but each isotope has a different abundance in nature expressed by percentage (natural occurrence).

Therefore, we need to calculate how much of this mass with its abundance percentage contribute to the average atomic mass for each isotope.

First step: Calculate the contribution in mass using the natural occurrence percentage.

85^Rb = 84.91 ×
`(72.71)/(100)` = 61.73 amu

87^Rb = 86.91 ×
`(27.83)/(100)` = 24.18 amu

The answers for each isotope are given using a typical percentage formula.

Second step: Add the two mass values founded.

24.18 amu + 61.73 amu = 85.91 amu

Third Step: Check the significant figures

The answer is 85.91 amu and it has 4 significant figures.