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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

Verified Answer :

Edufirst

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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