Avogadros Number and Molar Mass of an Element

Copper, a metal known since ancient times, is used in electrical cables and pennies, among other things. The atomic masses of its two stable isotopes, 29CU (69.09 percent) and 29CU (30.91 percent), are 62.93 amu and 64.9278 amu, respectively. Calculate the average atomic mass of copper. The relative abundances are given in parentheses. 

Strategy Each isotope contributes to the average atomic mass based on its relative abundance. Multiplying the mass of an isotope by its fractional abundance (not percent) will give the contribution to the average atomic mass of that particular isotope. 

Solution First the percents are converted to fractions 69.09 percent to 69.09/100 or 0.6909 and 30.91 percent to 30.91/100 or 0.3091. We find the contribution to the average atomic mass for each isotope, then add the contributions together to obtain the average atomic mass. 

Check The average atomic mass should be between the two isotopic masses therefore, the answer is reasonable. Note that because there are more 29CU than 29CU isotopes, the average atomic mass is closer to 62.93 amu than to 64.9278 amu. 

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