Determining an empirical formula from percent by mass data isn't too difficult. Generally, one begins by assuming 100 g of sample. Once the masses are determined, the number of moles is calculated for each element. The smallest whole-number ratio of moles will indicate the emirical formula.
| An Example | ||||||
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Problem: A compound containing only Al (26.98 g/mol) and Br (79.90 g/mol) is found to be 10.12% by mass Al. What is the empirical formula of the compound? |
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Solution: Assuming 100 g of sample, the masses present ofr the individual elements is
Now we can determine the number of moles of each element present in this theoretical sample of 100 g.
mol Al
10.12 g * ------- = 0.3751 mol Al
26.98 g
mol Br
89.88 g * ------- = 1.125 mol Br
79.90 g
Now, we find the mole ratio by dividing by the smaller number of moles.
1.125 mol Br
------------- = 2.9992 Br/Al
0.3751 mol Al
Clearly, this must be an integer ratio. And so, we can assum the exact ratio is 3 Br per Al Thus, the empirical formula is AlBr3 |
| Another Example | ||||||
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Problem: A compound containing only Fe (55.84 g/mol) and S (32.06 g/mol) is found to be 53.73% by mass Fe. What is the empirical formula of the compound? |
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Solution: Assuming 100 g of sample, the masses present ofr the individual elements is
Now we can determine the number of moles of each element present in this theoretical sample of 100 g.
mol Fe
53.73 g * ------- = 0.9622 mol Fe
55.84 g
mol S
46.27 g * ------- = 1.443 mol S
32.06 g
Now, we find the mole ratio by dividing by the smaller number of moles.
1.443 mol S
------------- = 1.4997 S/Fe
0.9622 mol Fe
Clearly, this must be an integer ratio. And so, we can assum the exact ratio is 1.5 S per Fe, or 3 S per 2 Fe. Thus, the empirical formula is Fe2S3 |