An element has two common isotopes. 98.89% of its atoms have an atomic mass of 12.09 amu, whereas the other 1.11% have an atomic mass of 13.10 amu. Using the isotopic composition provided, calculate the average atomic mass of the element.

Explanation:

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In this case, for the given natural occurring isotopes, which are those found for that element in nature, in order to compute its average atomic mass we must consider the atomic mass of each isotope and their abundance as shown below:

The result is:

Notice that for carbon, the two stable natural occurring isotopes are C-12 and C-13 in which the predominant one is the C-12, that is why the average atomic mass of carbon is 12.01 amu which is found in the periodic table.

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