Question

The isotopes of argon have the following relative abundances:
Ar-36 0.34%
Ar-38 0.06%
Ar-40 99.66%
Estimate the average atomic mass of argon.

How would I go about doing this?

Answer 1

You need the mass of the 36, 38, and 40 isotopes to calculate the atomic mass but since the problem says to estimate, I assume you are to use the mass numbers. The estimated atomic mass calculated this way gives a value that is too high from the actual value.
(36*0.0034) + (38*0.0006) + (40*0.9966) = ??

Answer 2

39.89amu