Question

A hypothetical element, E, has two stable isotopes. One isotope has a natural abundance of 68.037% and has an atomic mass of 46.449 u. If the atomic weight of E is 47.574 u, what is atomic mass (in units of u) of the second isotope.

Answer 1

• If the abundance of the first isotope is 68.037%, then the abundance of the second isotope is 100%-68.037%.

Substituting into the atomic mass formula,

$47.574=(46.449)(0.68037)+x(1-0.68037)\\\\ 47.574=31.60250613+0.31963x\\\\15.97149387=0.31963x\\\\x \approx \boxed{49.969 \text{ u}}$