Question

Sodium only has one naturally occuring isotope, Na23
, with a relative atomic mass of 22.9898 u.
A synthetic, radioactive isotope of sodium, Na22
, is used in positron emission tomography. Na22
has a relative atomic mass of 21.9944 u.


A 1.6421 g
sample of sodium containing a mixture of Na23
and Na22
has an apparent "atomic mass" of 22.9441 u
. Find the mass of Na22
contained in this sample.

Answer 1

Let X = fraction of Na atoms with a mass of 22
Then 1 - X = fraction of Na with a mass of 23
===========================
21.9944X + 22.9898(1-X) = 22.9441
Solve for X and 1- X
21.9944 X + 22.9898 - 22.9898X = 22.9441
Finish for X
Then Na22 = 1.6421 g sample x fraction of Na22 = g Na22.
Post your work if you get stuck.