An unknown element is a mixture of isotopes ¹²⁰X and ¹²²X. The average atomic mass of X is 120.26 amu. What is the percent abundance of ¹²²X?

Percent abundance of ¹²²X is 13%

Explanation:

The ¹²⁰X has a mass of 120 amu and ¹²²X has a mass of 122 amu. The average atomic mass is:

Average: 120.26amu: X*120amu + Y*122amu (1)

Where X is percent abundance of ¹²⁰X and Y percent abundance of ¹²²X

That means you can also write:

X + Y = 1 (2)

Because the sum of both percentages is 100%

Replacing (2) in (1):

120(1-Y) + 122Y = 120.26

120 - 120Y + 122Y = 120.26

2Y = 0.26

Y = 0.13 = 13%