We want to be able to figure out the atomic mass of a sample with a particular isotopic composition.
Example: Cu occurs as an isotopic mixture of 69.09% 63Cu (mass = 62.93 amu) and 30.91% 65Cu (64.93 amu). What is the atomic mass of copper in this sample?
Assume the sample consists of 1 atom for convenience, then
(0.6909 atoms)(62.93 amu/atom) + (0.3091 atoms)(64.93 amu/atoms) =
43.478 amu + 20.070 amu = 63.558 amu for 1 atom
= 63. 558 amu/atom
How about sig figs? 1 is a count, therefore exact. The two multiplications each have 4 sig figs so the calculations each have 4 sig figs (note I keep one extra, that is 5 sig figs, in the calculations to avoid rounding errors.) . For the addition we use the add/subt. rule and look at decimal place, for our four sig figs the hundredth's place is then the sig fig (again, during calculation its best to keep one extra sig fig to avoid rounding errors). The final answer then has 4 sig figs:
An example of the reverse problem can be found on the posted Final, number II.3.
Most samples of matter consist of combinations of atoms or ions to give compounds characterized by molecules or formulae. We are thus interested in molecular masses, formula masses etc.
(Of course there are no atoms actually present, just ions. However, all the parts for the atoms are there, so we still ask the question in terms of atoms.)
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© R A Paselk
Last modified 6 February 2015