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Calculation
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 1Boron has two isotopes: Boron-10 and Boron-11. In nature 19% of Boron is Boron-10, and 81% is Boron-11.
Practice Problem 2A 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 Copper-63 and Copper-65. What is the isotopic mass of the most common isotope of Copper?
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 atomic 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