Question

A student looked up the naturally occurring isotopes of bromine and found the following information: 50.54% of the naturally occurring isotopes of bromine have an atomic mass of 78.92 while 49.46% of the naturally occurring isotopes of bromine have an atomic mass of 80.92 . Calculate the relative atomic mass of bromine.

Answer 1

Answer:

Method 1

Let's assume we have 10 000 atoms of Br. Then 5069 atoms will have mass 78.92 u, and 4931 atoms will have mass 80.92 u.

Mass of the 5069 atoms = 5069 × 78.92 u = 400 000 u

Mass of the 4931 atoms = 4931 × 80.92 u = 399 000 u

Mass of 10 000 atoms = 799 100 u

Average mass of an atom =

799

100

u

10

000

atoms

= 79.91 u/atom

Method 2

The average mass is the mass of each isotope multiplied by its percentage. Thus,

Average mass = 50.69 % × 78.92 u + 49.31 % × 80.92 u = 40.00 u + 39.90 u = 79.91 u

The second method is easier, but it doesn't explain intuitively why the answer is the average atomic mass.