Question

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

Answer 1

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