How to find the isotopes?

Question

Iron has two isotopes.

57

₂₆

 which has a mass of 57.46 amu and an abundance of 85.93%.

58

₂₆

 which has a mass of 58.47 amu and an abundance of 14.07%.

Calculate the average atomic mass of Iron according to these values.

 amu

Answer 1

(0.8593 x 57.46) + (0.1407 x 58.47) = 49.38 + 8.23 = 57.61 amu = average atomic mass

Answer 2

So Janet J.,

This is just a question of finding an average. Now, the only possible issue you might be having (other than not knowing what an average is??), is, are the % data given for #'s of atoms by count, or are they given for proportions of atoms by mass? As far as I know, these data are always given as % by count. So you are safe to assume that. So, average the masses: atomic_mass1*countfraction1 + atomic_mass2*countfraction2. Not such a big deal to calculate? (By countfraction, I mean the % expressed as a decimal fraction). Don't forget to round the result appropriately!

-- Cheers, --Mr. d.

P.S. Just for an experiment, why don't you figure out how to calculate the average mass if the %'s had been as mass, by separated out masses, and see how far different that result would have been. You may have to really think how to do that. It is a bit analogous to calculating average speed for a trip, where speeds and % of the time travelled at each are given. You don't just average the (speed * the time travelled at that speed)/total time, do you!

-- Cheers, --Mr. d.