Question

An unknown element Q has two known isotopes: ⁶¹Q and ⁶³Q. If the average atomic mass is 61.2 amu, what are the relative percentages of the isotopes?

Answer 1

The average atomic mass of the element is the sum of the individual masses of its isoptopes , each multiplied by its relative abundance .

Let the relative abundance of 61Q isotope be x .

So the relative abundance of 63Q isotope will be 100-x.

putting the values in the formula , we get ,

61.2 = [61.x + 63(100-x)] / 100

61.2 = [61x + 6300 - 63x ] / 100

6120 = 6300 - 2x

x = 90%

And 100 - x = 100 - 90 = 10%

So the realtive percentage of 61Q is 90% and 63Q is 10%.