Pretest 2.2

Part 2

8.         Two isotopes of boron, B, are found in nature. The relative abundance of Boron 11 is 80.22 %. The second isotope of boron has one less neutron than Boron 11.

Calculate the average atomic mass of boron.

9.         Richard recorded data on the three stable isotopes of a newly discovered element. Unfortunately, a coffee spill in the late hours of the night resulted in smudged data. Richard was able to recover data on the average atomic mass (24.72 a.m.u.), as well as data on the first two isotopes in the chart below.

 Isotope Natural abundance (%) 24W 78.99 26W 11.01 ?W ?

What is the atomic mass of the missing isotope?

Solutions

8.             Determine atomic mass of second isotope of boron

11 m – 1 neutron = 10 m

Determine relative abundance of boron 10

100 % – 80.22 % = 19.78 %

Calculate average atomic mass of boron

Average atomic mass =

=

= 10.80 m

9.             let x = is the atomic mass of the missing isotope

0.7899(24) + 0.1101(26) + (1- 0.7899 - 0.1101)x = 24.72

X = 29 atomic mass units