Question

an element has two isotopes. calculate the (weighted) average atomic mass using the following data.
% abundance mass
72.172% 84.9117897 amu mass contribution is 1 amu
27.832% 86.9091805 amu mass contribution is 2 amu
the average atomic mass is 3 amu.
a. 84.9117897 b. 86.9091805 c. 85.471 d. 61.282537
e. 24.188563 f. 6128.2537 g. 2418.8563 h. 8547.1

Explanation:

Step1: Calculate first mass - contribution

Multiply the first isotope's percentage abundance (in decimal form) by its mass. The first isotope has an abundance of 72.172% or 0.72172 in decimal form and a mass of 84.9117897 amu. So, the mass - contribution is $0.72172\times84.9117897\approx61.282537$ amu.

Step2: Calculate second mass - contribution

Multiply the second isotope's percentage abundance (in decimal form) by its mass. The second isotope has an abundance of 27.832% or 0.27832 in decimal form and a mass of 86.9091805 amu. So, the mass - contribution is $0.27832\times86.9091805\approx24.188563$ amu.

Step3: Calculate average atomic mass

Add the two mass - contributions together. $61.282537 + 24.188563=85.471$ amu.

Answer:

1. D. 61.282537
2. E. 24.188563
3. C. 85.471