Boron has an average atomic mass of 10.81. one isotope of boron has a mass of 10.012938 and a relative abundance of 19.80 percent. the other isotope has a relative abundance of 80.20 percent. what is the mass of that isotope? report to two decimal places

The average atomic mass is calculated by the expression:

Average atomic mass = Σzₓ(Aₓ)

where zₓ is the relative abundance of an isotope and Aₓ is the atomic mass of the isotope. 

We calculate the mass of the isotope as follows:

10.81 = .1980 (10.012938) + .8020 (x)
x = 11.00 > mass of the isotope