1. Here’s a general strategy for determining empirical formulas.
QUESTION:
A sample of a compound contains g Fe and g O. What is the empirical formula of the compound? A. FeO, B. FeO2, C. Fe2O3, D. Fe3O4
General strategy: determine count-to-count (or mole-to-mole) ratio; reduce ratio to small whole numbers (first divide all counts by the smallest count; then, if necessary, find smallest multiplier to make all numbers very close to whole numbers)
A sample of a compound contains g Fe and g O. What is the empirical formula of the compound? A. FeO, B. FeO2, C. Fe2O3, D. Fe3O4
PAUSE
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Here’s a general strategy for determining empirical formulas.
You can determine empirical formulas if you have enough information to determine the relative numbers of the atoms of different elements
in the compound. In other words, if you can get a count-to-count ratio for the atoms of the elements in the compound. Remember that moles are
just group counts, so you can also use mole-to-mole ratio.
Once you have determined the mole-to-mole ratio, all you have to do is to reduce the ratio to a small whole number ratio.
Since we’re given masses in this question.... the first thing we need to do is convert them to moles
By now you should know how to do this. Here’s a quick review
We start with known masses of iron and oxygen
then multiply
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by a conversion factor based on the molar mass
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55.85 grams per mole is the molar mass of iron
and
16.00 grams per mole is the molar mass of oxygen
These calculations give us...
moles of iron
CLICK and moles of oxygen
CONTINUED ON NEXT SLIDE
1 mol Fe
19.27 g Fe x
= mol Fe
55.85 g Fe
1 mol O
7.360 g O x
= mol O
16.00 g O

2. So, the iron-to-oxygen mole ratio is 0.3450 to 0.4600.
QUESTION:
A sample of a compound contains g Fe and mol O. What is the empirical formula of the compound? A. FeO, B. FeO2, C. Fe2O3, D. Fe3O4
General strategy: determine count-to-count (or mole-to-mole) ratio; reduce ratio to small whole numbers (first divide all counts by the smallest count; then, if necessary, find smallest multiplier to make all numbers very close to whole numbers)
So far, we have determined that the sample contained moles of iron
HIGHLIGHT in table
and moles of oxygen
HIGHLIGHT in table
So, the iron-to-oxygen mole ratio is to
A good strategy for reducing this ratio to a whole number ratio is to first divide both numbers by the smaller number.
HIGHLIGHT “first divide all counts by the smallest count”
In this case, is the smaller number.
Dividing by gives us 1.000
HIGHLIGHT 1.000
and dividing by gives us
HIGHLIGHT 1.333
Note that we’re keeping four significant figures in the result because our original numbers have four significant figures.
When the decimal part is at least 0.1 away from a whole number, you should not just round it off.
We should not just round off to a whole number.
What we need to do at this point is find the smallest whole number multiplier that will make all the numbers close to a whole number.
HIGHLIGHT “if necessary, find smallest multiplier to make all numbers very close to whole number”
Try 2. If that doesn’t work. Try 3, and then 4, and so on....
OK. Let’s try multiplying both numbers by 2
1.000 times 2 equals 2.000
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and
1.333 times 2 equals 2.666
2.666 is still too far from a whole number.
So let’s try multiplying the numbers by 3.
1.000 times 3 equals 3.000
and times 3 equals 3.999
CLICK Now, we have two numbers that are very close to whole numbers is essentially equal to 4.
Therefore, the iron-to-oxygen ratio is “3 is to 4”
The empirical formula is Fe3O4.
The correct answer is choice D.
CLICK PAUSE END RECORDING
Iron
Oxygen
mol
mol
1.000
1.333
2.000
2.666
3.000
3.999

3. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08