Average Atomic Mass

Average Atomic Mass
Due Monday Oct 14, 2013
• Calculate the average atomic mass for an
Average Atomic Mass
The weighted average mass of all the
isotopes found in nature for that
Average Atomic Mass
• Measured in atomic mass units (amu)
Mass Spectrometer
Instrument that determines the
percentages and individual masses of
each isotope
The average atomic mass is calculated by
the percent of each isotope’s mass
[(mass of isotope) (%abundance)]
+ [(mass of isotope) (%abundance)]
+ [(mass of isotope) (%abundance)] …
Practice Problem 1
Boron has two isotopes: Boron-10 and
Boron-11. In nature 19% of Boron is Boron10, and 81% is Boron-11.
Practice Problem 2
A new element, Tyserium (Ty), has recently been
discovered on Mars and consists of two isotopes.
One isotope has a mass of 331 amu and is 35.0 %
abundant. The other isotope is 339 amu and is
65.0 % abundant. What is the mass of Ty as it
appears on the periodic table?
Logic Problem #1
Copper has an average atomic mass of
63.546 amu and only exists as Copper63 and Copper-65. What is the isotopic
mass of the most common isotope of
Logic Problem #2
The relative atomic mass of chlorine is 35.5
amu. What does this tell you about the
relative abundance of the two naturally
occurring isotopes of chlorine, Chlorine-35
and Chlorine-37.
Check Yourself!
• Make sure your calculated average
mass is within the range of
the given
mass numbers
– X-230, X-235, X-240
• Rationalize your results
– 90% of X-230, 5% of X-235, 5% of X-240
• Check your answers on the periodic table
If you have any questions, jot them down!