Question

Copper has two naturally occurring isotopes, 63 Cu (isotopic mass 62.9396 amu) and 65Cu (isotopic mass 64.9278 amu. What is the percent abundance of 6% Cu in a naturally occurring sample of copper? a) 15.3% b) 30.7% c) 61.0% d) 69.3% a

Answer 1

Copper has two isotopes.

Isotopic or Atomic mass of ⁶³Cu = 62.9396 amu

Isotopic or Atomic mass of ⁶⁵Cu = 64.9278 amu

We know that Average atomic mass of Copper = 63.55 amu

Let the Isotopic abundance of ⁶⁵Cu = x

Isotopic abundance of ⁶³Cu = 100 - x "

Suppose an element has two isotopes X,Y. Let m, n are the isoptic abundance of X, Y respectively then the average atomic mass of element is given by

A =( m*X + n*Y)/100

Here the two isotopes of copper are Cu-63 and Cu-65.

Let Isotopic abundance of Cu-65 = x

Then the Isotopic abundance of Cu-63 = 100-x

63.55 = 64.9278x + 62.9396(100-x)/100

63.55*100 = 64.9278x + 6293.96 - 62.9396x

6355 = 1.9882x + 6293.96

1.9882x = 61.04

x = 61.04/1.9882 = 30.7%

Option B is the correct answer.