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10 October, 23:42

A hypothetical element consists of 3 isotopes, and the atomic weight is 39.2 amu. Isotope1 is 78.8% with mass number of 39; isotope 2 is 3.1% with mass number of 41. What is the mass of isotope 3?

1. 38

2. 43

3. 40

4. 42

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Answers (1)
  1. 10 October, 23:53
    0
    The atomic mass of the elements is the weighted average of the masses of the isotopes.

    This is, atomic mass = % of isotope 1 * mass number of isotope 1 + % of isotope 2 * mass number of isotope 2 + ... + % of isotpe 3 * mass number of isotope n.

    Applied to our hypotethical element with 3 isotopes:

    aomic mass of the element = 39.2 = 78.8% * 39 + 3.1% * 41 + 18.1% * x

    Solve for x, x = [39.2 - 30.732 - 1.271] /.181 = 39.76 = 40

    Answer: 40
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