We’re being asked to determine the atomic weight of rubidium-87.
We can use the following equation:
atomic mass = average atomic mass of the element
f.a. = fractional abundance of the isotope.
To get f.a., we simply need to divide the given percent abundance by 100.
We’re given the following values:
Average Atomic Mass = 85.4678 amu
Rubidium -85: Mass = 84.9118 amu
f.a. = 0.7215
Rubidium -87: Mass = ????
We don’t know the percent abundance of 1 isotope but we can use the fact that the f.a. of all isotopes of an element add up to 1.
f.a. = (1 - 0.7215) = 0.2785
Rubidium has two isotopes, rubidium-85 and rubidium-87. The atomic weight of rubidium is 85.4678 amu. If the atomic weight of rubidium-85 is 84.9118 amu which is present as 72.15% of all rubidium isotopes, what is the atomic weight of rubidium-87?
A) 86.9082 amu
B) 87.0054 amu
C) 86.0000 amu
D) 86.9563 amu
E) 87.1123 amu
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