We’re being asked to calculate the mass of the third isotope of magnesium. We can use the following equation:
where atomic mass = average atomic mass of the element and f.a. = fractional abundance of the isotope. To get f.a., we simply need to divide the given percent abundance by 100.
Magnesium has three naturally occurring isotopes: 24 Mg (23.985 amu) with 78.99% abundance, 25 Mg (24.986 amu) with 10.00% abundance, and a third with 11.01% abundance.
Look up the atomic weight of magnesium, and then calculate the mass of the third isotope.
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