Three isotopes of Argon occur naturally:

36Ar (mass = 35.9675 amu)
38Ar (mass = 37.9627 amu)
40Ar (mass = 39.9624 amu)

If the percent abundance of 36Ar is 0.3400, what is the percent abundance of 40Ar?

1 answer

look up the atomic mass of Ar.

atomicmassAr=.34*35.9675+x*39.9624+(1-x-.34)35.9675) solve for x, you know the avg atomic mass of Ar, from the periodic table.