The Element rhenium has two naturally occurring isotopes, 185 Re and 187 Re, with an average atomic mass of 186.207 amu. Rhenium is 62.60% 187 Re, and the atomic mass of 187 Re is 186.956 amu. Calculate the mass of 185 Re. How do you do this?

Question

aaronq · Answer

write the expression as if you were to calculate the average atomic mass,
 i.e. $\Sigma (mass_{isotope\;A}*\%abundace_{isotope\;A})$

since: (abundance % of isotope 185) + (abundance % of isotope 187) = 1

you can substitute in: (abundance % of isotope 187)= 1-(abundance % of isotope 185)

anonymous · Answer

I don't get it, where did the one come from?

aaronq · Answer

they have to equal to 100%,  1 is the same as 100% without the percent.

anonymous · Answer

So is the equation 62.60=1-x?

aaronq · Answer

it would be  0.6260=1-x   or    62.60=100-x  both are equally valid.

but that's what you have to substitute into the first equation

anonymous · Answer

Thank you, I understand now!

aaronq · Answer

good stuff. no problem, dude