Considering the definition of atomic mass, isotopes and atomic mass of an element, the correct answer is option (d): the average mass of the hypothetical element is 65 amu.
First of all, the atomic mass (A) is obtained by adding the number of protons and neutrons in a given nucleus of a chemical element.
The same chemical element can be made up of different atoms, that is, their atomic numbers are the same, but the number of neutrons is different. These atoms are called isotopes of the element.
On the other hand, the atomic mass of an element is the weighted average mass of its natural isotopes.
In other words, the atomic masses of chemical elements are usually calculated as the weighted average of the masses of the different isotopes of each element, taking into account the relative abundance of each of them.
In this case, one isotope has a mass of 64.23 amu and it is 26% abundant (the atomic mass is 6) and the other isotope has a mass of 65.32 amu and it is 74% abundant (100% -26%). Then, the average mass of the hypothetical element can be calculated as:
64.23 amu×0.26 + 65.32 amu×0.74= 65.0366 amu ≅65 amu
Finally, the correct answer is option (d): the average mass of the hypothetical element is 65 amu.
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