Expressing the composition of a compound by it's mass-percent of each element is oftentimes useful. In fact, elemental analysis methods (such as combustion analysis) are generally only able to produce this kind of data.
A simple procedute to find the mass-percent ofr a given element in a compound is to begin by determining the molar mass. This is accomplished by simply adding up the products of the number of each type of atom times its molar mass, which can be found in a peridoic table.
Once the molar mass is determined, the percentage by mass for each elements is fairly straight-forward to calculate.
| An Example | ||||||||||||||||||||
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Problem: What is the molar mass of and mass percentage for each element in Co(NO3)2? |
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Solution: Using the number of atoms of each element in a formula unit of the compound, we can construct the following table:
Now we can determine the percentage of that total that is due to each element.
Co: 58.933 g/mol
------------- x 100% = 32.214 %
182.941 g/mol
N: 28.014 g/mol
------------- x 100% = 15.313 %
182.941 g/mol
O: 95.994 g/mol
------------- x 100% = 52.473 %
182.941 g/mol
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| An Example | ||||||||||||||||||||
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Problem: What is the molar mass of and mass percentage for each element in C7H6O2? |
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Solution: Using the number of atoms of each element in a formula unit of the compound, we can construct the following table:
Now we can determine the percentage of that total that is due to each element.
C: 84.077 g/mol
------------- x 100% = 68.846 %
122.123 g/mol
H: 6.048 g/mol
------------- x 100% = 4.952 %
122.123 g/mol
O: 31.998 g/mol
------------- x 100% = 26.201 %
122.123 g/mol
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