Question

Copper is listed on the periodic table as having a relative atomic mass of 63.55. Reference books indicate two isotopes of copper, with relative masses of 62.93 and 64.93. What is the percent abundance of each isotope?

Answer 1

`69%` for the isotope that weighs 62.93 u and `31%` for the isotope that weighs 64.93 u.

You can approach this problem by using a single equation; let's say the first isotope contributes to the relative atomic mass by a fraction `x` (`x<1`). SInce there are only 2 isotopes to consider, the fraction the other isotope contributes with will automatically be `1-x`.

Let's set up the equation

`x * 62.93u + (1-x) * 64.93u = 63.55u`

Solving this for `x` will produce

`64.93u - 63.55u = 2x -> x = 1.38/2 = 0.69`

Multiplying these fractions (0.69 and 1 - 0.69 = 0.31) by 100% percent to get the results as a percentage will give

`69%` for the isotpe that weighs 62.93u and
`31%` for the one that weighs 64.93u.