Question

What is the average atomic mass of gold if half of the gold found in nature has a mass of 197 amu and half has a mass of 198 amu?

Answer 1

The average atomic mass is `197.5`.

Explanation:

Multiply the mass of each isotope times its percentage as a decimal number and add the results. `(50%)/100=0.5`

`"average atomic mass"=(0.5xx197)+(0.5xx198)=197.5`

Answer 2

The average atomic mass is 198.5 u.

Explanation:

Assume that you have 2 atoms of gold.

Then you have 1 atom of each isotope.

`"Mass of 1 Au-197 atom = 197 u"`
`"Mass of 1 Au-198 atom = 198 u"`
`stackrel(———————————————)("Mass of 2 Au atoms"color(white)(Xl) = "395 u")`

`"Average mass" = "395 u"/2 = "197.5 u"`