 Site Navigation                          Chemistry: Average Atomic Mass

An element can exist in a number of forms, called isotopes. Isotopes are forms of the same atom that vary in mass.

For example, there are two different types (isotopes) of copper atoms. One type of copper atoms weighs in at 62.93 amu, the other has a mass of 64.94 amu. The lighter isotope is more common with  69.09% of the naturally occurring copper having a mass of 62.93 amu per atom. The remainder of the atoms, 30.91 %, have a mass of 64.94 amu.

To find the AVERAGE ATOMIC MASS of an atom, we take into account all of the isotopes that exist and the percentage of each type. The calculation of the average atomic mass is a WEIGHTED AVERAGE.

Average atomic mass = Σ (mass of isotope × relative abundance)

The bottom line is that to find the average atomic mass of copper, we insert the information about copper’s isotopes into the formula and solve. There are two isotopes, so we will be adding the contributions of 2 isotopes. (That’s where the Σ sign comes in. ) The relative abundance is simply the percentage of the isotope, but in decimal format.  0.90% corresponds to a relative abundance of 0.6090.

Average atomic mass of copper = (62.93 amu × 0.6909) + (64.94 amu × 0.3091)= 63.55

From the calculation, we know that an AVERAGE atom of copper has a mass of 63.55 amu. Notice that in this problem, we would predict that the average is closer to the weight of the lighter isotope. This is because the lighter form of copper is more abundant.

Question Group #1
Directions and/or Common Information:

 Isotope name Isotope mass (amu) percentage Silver-107 106.90509 51.86 Silver-109 108.90470 remainder
 Find the missing percentage.
 1

 Find the average atomic mass of an atom of silver.
 2

Question Group #2
Directions and/or Common Information: Silicon has three naturally occurring isotopes.

 Isotope name Isotope mass (amu) Relative Abundance Silicon-28 27.98 92.21 Silicon-29 28.98 4.70 Silicon-30 29.97 3.09
 Look over the data before you begin the problem. Estimate the value of the answer before you begin the calculation. Will the weighted average be closer to 28,  29, or 30?
 1

 Now, find the average atomic mass of an atom of silicon
 2

Question Group #3
Directions and/or Common Information: Iron has four isotopes.

 Isotope name Isotope abundance Isotope mass (amu) Iron-54 5.90% 53.94 Iron-56 91.72% 55.93 Iron-57 2.10% 56.94 Iron-58 0.280% 57.93
 Estimate the average mass.
 1

 Calculate the average atomic mass of iron.
 2

Question Group #4
Directions and/or Common Information: A chemistry student’s grade is weighted. Tests are worth 50.%, labs are 25%, and homework is worth 25%. A student's test average is 85.0%, lab average is 77.0%, and homework is 91.0%.
 What is the student’s average?
 1

E Saylor

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