Argon has three common isotopes, with the masses and isotopic abundances shown below:

Question

Argon-36 ---> 35.97 u and 3.353 %

Argon-38 ---> 37.96 u and 0.868 %

Argon-40 ---> 39.96 u and ____ %

Determine the average atomic mass of argon to two decimal places.

Answer 1

Find % Argon-40 by adding 3.353% and 0.868% and subtract that from 100% to get 95.779%

Then proceed as follows:

35.97 x 0.03353 = 1.20607

37.96 x 0.00868 = 0.32949

39.96 x 0.95779 = 38.2733

Add these up to get the average atomic mass to 2 decimal places...

1.20607 + 0.32949 + 38.2733 = 39.81 (check the arithmetic because actual value is 39.95)