Question

Rubidium has 2 common isotopes, 85rb and another isotope. If the abundance of 85trb is 72.2%, the abundance of the other isotope is 27.8%, and the average atomic mass of the 2 isotopes is 85.556 amu what is the mass of the other isotope?

Answer 1

What you do here is
You take the average atomic mass 85.556 - (85*72.2%)
then divide by 27.8% for the mass of the other isotope.
Essentially a ratio problem

Answer 2

Should the 72.2 and 27.8 be decimals though?

Answer 3

yes % = something/100, so 72.2% = 0.722

Answer 4

Thanks for the help!