# Question #2e766

##### 1 Answer

#### Explanation:

So, you know that you're dealing with two isotopes, let's say

The relative atomic mass of the element will be determined by the atomic masses of the two isotopes **in proportion** to their respective abundances.

#"relativ atomic mass" = sum_i ("isotope"""_i xx "abundance"""_i)#

SInce you're only dealing with two isotopes, you can say that their abundances must **add to give 100%**.

If you take *decimal abundance*, which is simply the *percent abundance* divided by

This means that you can write

#"20 u" * x + "22 u" * (1-x) = "20.2 u"#

#20x + 22 - 22x = 20.2#

#2x = 1.8 implies x = 1.8/2 = 0.9#

The decimal abundance of

The percent abundances of the two isotopes are

#""^20"X: " color(green)(90%)#

#""^22"X: " 10%#

The result makes sense because the relative atomic mass of the element is **much closer** to the atomic mass of *significantly larger* percent abundance that