Question

1. Calculate the average atomic mass of rubidium. Rubidium has two isotopes, 85Rb and 87Rb. 85Rb has an atomic mass of 84.912 amu and occurs at an abundance of 72.17%. 87Rb has an atomic mass of 86.909 amu and occurs at an abundance of 27.83%. Show your work.

Answer 1

you should be asking this in the chem section, but use this formula:

average atomic mass=\(\Sigma(mass_{Isotope A}*\%abundance_{Isotope A}) \)

Answer 2

Is it possible that you write that more clearly?

Answer 3

"\(\Sigma\)" means summation (addition)

you're basically taking the weighted average of the isotopes.

so multiply each isotope's mass by it's abundance and add the values that you get.

Answer 4

72.1752 + 75.61083 = 150.78603

So would 150.78603 be the average atomic mass of rubidium?

Answer 5

hmm that doesn't seem right, are you sure you multiplied each mass by it's own abundance?

Answer 6

Yes

Answer 7

i don't think you did it right, it should be:

0.7217*84.912+86.909*0.2783=85.4677651

Answer 8

Do you do anything with 85Rb, and 87Rb?

Answer 9

those are the names of the isotopes (e.g. \(^{85}Rb\) for isotope with 84.912 amu mass), their masses and their abundance is what you need to use.

Answer 10

How does this look?


Average Atomic Mass = (Mass of isotope A ∗ % Abundance of isotope A) + (Mass of isotope B ∗ % Abundance of isotope B)

Average Atomic Mass = (84.912 ∗ 0.7217) + (86.909 ∗ 0.2783)

Average Atomic Mass = 61.2809904 + 24.1867747

85.4677651 = 61.2809904 + 24.1867747

Average Atomic Mass = 85.4677651

Answer 11

thats right, dude. \(\checkmark\)

Answer 12

Thanks soo much for your help!

Answer 13

no problem !