Question

A new element was recently discovered and found to have an atomic weight of 342.38 amu. this element has two isotopes, the lighter of which has a mass of 340.91 amu and an abundance of 68.322%. what is the mass of the heavier isotope?

Answer 1

Atomic weight of new element is 342.38 amu. The mass of lighter isotope is 340.91 amu and abundance 68.322 %.

Since, the total abundance is 100% , the abundance of heavier isotope will be (100-68.322) %=31.678%

The formula for atomic weight or average atomic weight of element is as follows:

Average atomic weight= Mass of lighter isotope×% abundance of lighter isotope+Mass of heavier isotope×% abundance of heavier isotope.

Putting the values,

Average atomic weight= 340.91×0.68322+Mass of heavier isotope×0.31678

342.38 amu=232.91+Mass of heavier isotope×0.31678

On rearranging,

`Mass of heavier isotope=(342.38-232.91)/(0.31678)=345.55 amu`

Thus, mass of heavier isotope will be 345.55 amu.