What is the average atomic mass of an element if, out of 100 atoms, 5 have a mass of 176 amu, 19 have a mass of 177 amu, 27 have a mass of 178 amu, 14 have a mass of 179 amu, and 35 have a mass of 180 amu?

1 Answer
Jul 3, 2018


#"Average Atomic Mass" = 178.55 color(white)(l) "amu"#


The #"Average Atomic Mass"# of an element is defined as "the weighted average mass of all naturally-occurring (occasionally radioactive) isotopes of the element." (and hence the name "average") [1]

Dividing the sum of the product of the #color(purple)("relative atomic mass of each of the isotope")# and #color(grey)("the occasion of their presence in the sample")# by #"the sum of atoms in the sample"# gives the weighted average of the relative atomic mass.

#"Average Atomic Weight" #
#= (color(grey)(5) xx color(purple)(176) + color(grey)(19) xx color(purple)(177) + color(grey)(27) xx color(purple)(178) + color(grey)(14) xx color(purple)(179) + color(grey)(35) xx color(purple)(180)) / (color(black)(100))#
#= 178.55 color(white)(l) "amu"#

[1] Vitz, Ed, et al. “4.13: Average Atomic Weights.” Chemistry LibreTexts, Libretexts, 5 Sept. 2017,