# Thread: Isotope question

1. 7) Magnesium consists of three naturally occurring isotopes. The percent abundance of these isotopes is as follows: 24Mg (78.70%), 25Mg (10.13%), and 26Mg (11.7%). The average atomic mass of the three isotopes is 24.3050 amu. If the atomic mass of 25Mg is 24.98584 amu, and 26Mg is 25.98259 amu, calculate the actual atomic mass of 24Mg.
Correct answer is 23.98504, I keep getting 23.8041. Can anyone see where I am going wrong?

Isotope. AMU. %abun. AMUx%abun. Adv AMU of Mg 24.3050.

24mg ? .7870. 25Mg. 24.98584. .1013. 2.531065592. 26Mg. 25.98259. .1170. 3.03996303 ----------------- 5.571028622Adv AMU. 24.3050 -AMUx%. 5.5710 ----------- 18.7339/.7870= 23.8041

2.

3. Have a look at Wikipedia. I think the abundance of as 10.13% is a little too high.

4. Those three percentages add up to >100%.

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