# chemistry

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magnesium has 3 isotopes. Magnesium-24 has a percent abundance of 78.99%. Magnesium-26 has a percent of 11.01%. what is the percent abundance of magnesium-25? assume that there are no other magnesium isotopes.

• chemistry -

Don't all of the percentages add up to 100%? And you have two of the three?

• chemistry -

thank you

• chemistry -

There is a fairly simple answer to this:
The question states that you assume there are three magnesium isotopes. If Magnesium 24 has an abundance of 78.99%, and Magnesium 26 has a percent abundance of 11.01%, the percent abundance of Magnesium 25 is simple to calculate.
The abundances must add up to 100%, and seeing as we are assuming that there is only one more isotope of Magnesium, we just subtract the sum of the first two abundances from 100%, as shown below:
100% - (78.99% + 11.01%)
100% - 90%
10%
So the abundance of Magnesium 25 would be 10%.

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