chemistry - how to calculate average atomic mass?

Question

how do you calculate the average atomic mass?

Answer 1

Do you have the percentage abundances of the isotopes?
E.g. for chlorine, it exists as 2 isotopes, namely Cl-37 and Cl-35.
Cl-37 (with 17 protons and 20 neutrons) has an abundance of 25% with Cl-35 (with 17 protons and 18 neutrons) has an abundance of 75%.
Thus average relative atomic mass of chlorine
= [(25/100) x 37] + [(75/100) x 35]
= 35.5

Hope this helps :)

Answer 2

Average atomic mass of what?

Atomic mass is simply the sum of the atomic weights of all involved atoms. The exact atomic mass depends on the specific number of neutrons and protons in the nucleus of each atom. Average means across the different common isotopes. This average atomic mass is given in most periodic tables.

For example
glucose = C6H12O6
average atomic mass of glucose = 6(12.01)+12(1.008)+6(16.00)

Answer 3

You make a sum of products of the mass of the individual isotope times the proportion that the isotope exists in the mixtures of isotopes that we call a pure element.

say, you had an element whose atomic weights were 51,52, and 53 whose proportions were 0.2, 0.01, and 0.79

0.2*51 + 0.01*52 + 0.79*53

Answer 4

you don't have to...the average atomic mass of all isotopes is what is presented on the periodic table