The element rubidium (atomic mass 85.5) consists of two isotopes, one mass number 85 (Rb-85) and the other mass number 87 (Rb-87). Which one of the following statements about the isotopes of rubidium is correct?

Alternative approach.

Here's another approach to use in order to find the abundances of the two .

As you know, each isotope will contribute to the average of rubidium in proportion to their abundance.

##color(blue)(|bar(ul(color(white)(a/a)"avg. atomic mass" = sum_i i xx "abundance"_icolor(white)(a/a)|)))##

Here ##i## represents the atomic mass of an isotope ##i##. This equation uses decimal abundance, which is simply percent abundance divided by ##100##

##color(blue)(|bar(ul(color(white)(a/a)"decimal abundance" = "percent abundance"/100color(white)(a/a)|)))##

For example, if an isotopes has a ##13%## percent abundance, it will have a

##"decimal abundance" = 13/100 = 0.13##

So, you know that your element has two stable isotopes, ##""^85"Rb"## and ##""^87"Rb"##. If you take ##x## to be the decimal abundance of ##""^85"Rb"##, you can say that the decimal abundance of ##""^87"Rb"## will be ##1-x##.

This is the case because the abundances of the two must add up to give ##100%##, or ##1## as a decimal abundance.

You know that the average atomic mass of rubidium is ##"85.5 u"##, and that the two isotopes have atomic masses equal to ##"85 u"## and ##"87 u"##, respectively.

The equation will thus take the form

##85.5 color(red)(cancel(color(black)("u"))) = 85color(red)(cancel(color(black)("u"))) xx x + 87color(red)(cancel(color(black)("u"))) xx (1-x)##

##85.5 = 85x + 87 - 87x##

Rearrange to solve for ##x##

##87x - 85x = 87 - 85.5##

##2x = 1.5 implies x = 0.75##

So, the abundances of the two isotopes will be

##"For """^85"Rb: " overbrace(0.75)^(color(purple)("decimal abundance")) = overbrace(75%)^(color(green)("percent abundance"))##

##"For """^87"Rb: " 1 - 0.75 = overbrace(0.25)^(color(purple)("decimal abundance")) = overbrace(25%)^(color(green)("percent abundance"))##

Once again, this shows that ##""^85"Rb"## is three times as abundant as ##""^87"Rb"##.