Isotope chemistry / unit conversion help?

I'm having some problems figuring out a question right. I m not sure I m on the right track. It says: the concentration of the element Rb in a rock is 5ppm by mass. What is the concentration of the isotope 87Rb in the rock in units of moles/g? in moles/cm^3? In atoms/cm^3.

So far I thought that 87Rb occurs 27.83% (vs. 72.12% for 85Rb). So I got a total of 1.3915ppm (which I think = 1.3915*10^-6 g/g?). The mass of 87Rb is 86.909 amu so = 1.4426*10^-22g/mol (I think?). So I get a total of 2.00376*10^16 mol/g. If I ve done that correctly I m now not sure how to convert to moles/cm^3?
Thank you
Asked Feb 26, 2017

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