Question

Question #fcfcd

Answer 1

The 86amu isotope.

The relative atomic mass is the weighted mean of the masses of the isotopes, and the weighting is how much of each isotope is present.

In your example, you have an 86amu isotope and a 90amu isotope, so the mean if there were no weighting (I.e. equal abundance of each) would be 88amu.

The actual relative atomic mass is 87.08 so the average is nearer to 86 than to 90. This means the weighting of the 86amu isotope must be greater than the weighting of the 90amu isotope - in other words there is more of the 86amu than the 90amu isotope.

If we wanted we could calculate the relative abundance of each isotope. If we call `a` the abundance of the 86amu isotope, then the abundance of the 90amu isotope is `1 - a` because the total abundance must be 1 (there are no other isotopes).

We calculate the Relative atomic mass, `A_r` = (a x 86) + ((1-a) x 90) = 87.08

So `86a + 90 - 90a = 87.08` which means `-4a = -2.92` so `a = 0.73`

So the abundance of the 86amu isotope is 73% and the 90amu isotope has an abundance of 100 - 73 = 27%