Rubidium has two naturally occurring isotopes: 85Rb and 87Rb. The natural abundance of 85Rb is 72.165% and it has an isotopic mass of 84.9118 u.

Question

Calculate the isotopic mass of 87Rb

Answer 1

abundance Rb 85 = 72.165%. Then Rb 87 = 100% - 72.165% = 27.835%
(72.165*84.9118) + (X*27.835) = atomic mass Rb.
Look up the atomic mass of Rb on the periodic table (it should have been given in the problem---perhaps you just didn't type it in). Anyway, look up the atomic mass of Rb from the periodic table and solve for X in the equation i wrote above. Post your work if you get stuck.

Answer 2

so the answer will be 85.06912663u?