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:


percentage

Silver107

106.90509

51.86

Silver109

108.90470

remainder



Find the missing percentage. 

Question Group #2 

Directions and/or Common Information: Silicon has three naturally occurring isotopes.   Relative Abundance  Silicon28  27.98  92.21  Silicon29  28.98  4.70  Silicon30  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? 

Question Group #3 

Directions and/or Common Information: Iron has four isotopes.    Iron54  5.90%  53.94  Iron56  91.72%  55.93  Iron57  2.10%  56.94  Iron58  0.280%  57.93 


Estimate the average mass. 

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? 




E Saylor


Show Related AlgebraLab Documents

