Question

Element M has two isotopes,^104M and ^106M.

The relative atomic mass of M is 104.4.What is the relative abundance of ^106M?
A:10%
B:20%
C:30%
D:40%

Answer 1

To calculate atomic mass, you have to take to weighted average of the isotopes' masses. What that means is M = RA*106 + (1 – RA)*104, where RA is relative abundance expressed in decimal form. If you simplify the right side of that equation, you get M = 2*RA + 104. Doing a little more algebra yields RA = (M –104)/2 = (104.4 – 104)/2 = 0.4 / 2 = 0.2, which is 20%. So the answer is B.