## Chemistry: The Molecular Science (5th Edition)

Estimate: The vast majority of argon is $^{40}$Ar, with trace amounts of its lighter isotopes, so we can expect the average atomic mass to be slightly less than 40. Calculate: The weighted average of isotopes is: (.003336)(35.968 u) + (0.00063)(37.963 u) + (0.9960)(39.962 u) = 39.95 u