How do you calculate the average atomic mass of magnesium, given the following percent abundances and isotopic masses: #78.99% .^24Mg (23.98504 u)#, #10.00% .^25Mg (24.98584 u)#, and #11.01% .^20Mg (25.98259 u)#?

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Answer 1 · 2016-09-06T17:18:37

24.30505 u

If the abundance are as stated:

78.99% of 23.98504 u
10.00% of 24.98584 u
11.01% of 25.98259 u

I would check that the percentage values equal 100:

78.99 + 10.00 + 11.01 = 100

Which they do, and then just use "ratio maths":

(0.7899 x 23.98504) + (0.100 x 24.98584) + (0.1101 x 25.98259) = 24.30505 u

A quick check on Google shows: 24.305 u ± 0.0006 u

And so the answer looks right.

Here is an interesting question: Why does Google give 24.305 u ± 0.0006 u?