Question

3. Copper has 2 isotopes. Cu-63 has a mass 62.930 amu at 69.09 % abundance, while the remainder is Cu-65 which has a mass of 64.928 amu Calculate the atomic mass of copper​

Answer 1

As you know, the average atomic mass of an element is determined by taking the weighted average of the atomic masses of its naturally occurring isotopes.

Simply put, an element's naturally occurring isotopes will contribute to the average atomic mass of the element proportionally to their abundance.

avg. atomic mass avg. atomic mass = ∑ i ( isotope i × abundance x )

When it comes to the actual calculation, it's easier to use decimal abundances, which are simply percent abundances divided by

100

So, you know that copper has two naturally occurring isotopes, copper-63 and copper-65. This means that their respective decimal abundance must add up to give 1

.If you take x to be the decimal abundance of copper-63, you can say that the decimal abundance of copper-65 will be equal to 1 − x .

Therefore, you can say that

copper-63

x ⋅ 62.9296u + copper-65



(

1

−

x

)

⋅

64.9278

u

=

63.546

u

Solve this equation for

x

to get

62.9296

⋅

x

−

64.9278

⋅

x

=

63.546

−

64.9278

1.9982

⋅

x

=

1.3818

⇒

x

=

1.3818

1.9982

= 0.69152

This means that the percent abundances of the two isotopes will be

69.152 %

→ 63 Cu

30.848 %

→ 65 Cu

Step-by-step explanation: