Atomic Mass of First Isotope of Argon = 35.96755g

Natural Abundance of First Isotope of Argon, P_a1= 0.337% = 0.00337

Total Atomic Mass of 1^st Isotope, M_a1 = 35.96755×0.00337

= 0.12121g

Atomic Mass of Second Isotope of Argon = 37.96272g

Natural Abundance of Second Isotope of Argon, P_a2 = 0.063%

=0.00063

Total Atomic Mass of 2^nd Isotope, M_a2 = 37.96272×0.00063

= 0.02392g

Atomic Mass of Third Isotope of Argon = 39.9624g

Natural Abundance of Third Isotope of Argon, P_a3 = 99.6%

Total Atomic Mass of 3^rd Isotope, M_a3 = 39.9624×0.996

= 39.8026g

So Average Atomic Mass of Argon =

→

→ 39.9477g

Therefore, the average atomic mass of Argon is 39.95g