Lithium has 2 isotopos. one has a mass of
16.015 am and the other 7.016 anu. the on
average atomic mass is 6.941 amu. what are
the lo abundances of each (to 2 decimal
places)?

The isotope with mass 6.015amu =7.5% abundance

The isotope with mass 7.016amu = 92.5%

Explanation:

Step 1: Given data

Lithium has an average atomic mass of 6.941 amu

Lithium has 2 isotopos with a mass, respectively 6.015amu and 7.016amu

Step 2: Calculate the abundances

Isotope with mass 6.015 amu has an abundance of X

Isotope with mass 7.016 amu has an abundance of Y

The average atomic mass is the average mass of the isotopes with its respectively abundance.

X+Y = 1 or X = 1 - Y

6.941 = 6.015X +7.016Y

This gives us:

6.941 = 6.015(1-Y) +7.016Y

6.941 = 6.015 - 6.015Y + 7.016Y

6.941 - 6.015 = -6.015Y +7.016Y

0.926 = 1.001Y

Y = 0.925

Y = 92.5%

X = 1- 92.5 = 7.5%  

To control we can do the following equation:

6.015*0.075 + 7.016*0.925 = 6.941

This means the abundances of the 2 isotopes are:

The isotope with mass 6.015amu =7.5% abundance

The isotope with mass 7.016amu = 92.5%

the answer for sodium is 0.186

the answer for rubidium is 0.251