Part A

Rubidium has two naturally occurring isotopes: $Rb-85$ with a mass of $84\mathrm{.}9118a$ and a natural abundance of $72\mathrm{.}17\%,$ and $Rb-87.$ Use the atomic mass of rubidium listed in the periodic table to determine the mass of $Rb-87.$

Express your answer to four significant figures and include the appropriate units.

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