Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 107	Fundamentals of Chemistry	Fall 2009
Lecture Notes:: Isotopes	© R. Paselk 2000
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Symbol Z A p n e- 14C  6 14 6 8 6 238U6+ 92 238 92 146 86 35Cl- 17 35 17 18 18  18O2-  8 18 8  10 10

 

 

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 100 atoms for convenience, then

(69.09 atoms)(62.93 amu/atom) + (30.91 atoms)(64.93 amu/atoms) =

4347.8 amu + 2007.0 amu = 6355.8 amu for 100 atoms

= 63. 558 amu/atom

How about sig figs? 100 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 for sig figs the one'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:

= 63. 56 amu/atom

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Last modified 27 October 2004