Naturally occuring boron is 80.20% boron-11 (atomic mass 11.01 amu) and 19.80% of some other isotope. What must the atomic mass of this second isotope be in order to account for the 10.81 amu average atomic mass of boron?

***NOTE: I will be more than happy if somebody just clarifies the question, I'm not specifically looking for the answer. :)

Question

Naturally occuring boron is 80.20% boron-11 (atomic mass 11.01 amu) and 19.80% of some other isotope. What must the atomic mass of this second isotope be in order to account for the 10.81 amu average atomic mass of boron?

***NOTE: I will be more than happy if somebody just clarifies the question, I'm not specifically looking for the answer. :)

xishem · Answer

The average atomic mass of an element is found by taking the weighted average of all naturally occurring isotopes. The algorithm will be something like this:$$Avg=(Percent_{} Occurrence)*(Atomic_{}Weight)+(PercentOccurrence)*(AtomicWeight)+...$$Do this for each isotope of the element.

anonymous · Answer

Okay, thanks for the help, and equation. :)