1. Here’s a general strategy for determining empirical formulas.
QUESTION:
A sample of a compound contains mol P and mol O. What is the empirical formula of the compound? A. PO, B. PO4, C. P2O5, D. P3O4
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 moles of phosphorus and moles of oxygen. What is the empirical formula of the compound? A. PO, B. PO4, C. P2O5, D. P3O4
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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.
Here’s a table that shows how this strategy is implemented.
From the question, we note that the compound contains moles of phosphorus
HIGHLIGHT number in question and in table
and also contains moles of oxygen
So, the phosphorus-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 2.502
Note that we’re keeping four significant figures in the result because our original numbers have four significant figures.
We can see here that is almsot halfway between two whole numbers. When the decimal part is at least 0.1 away from
a whole number, you should not just round it off.
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
and
2.502 times 2 equals 5.004
Now, we have two numbers that are very close to whole numbers.
Therefore, the phosphorus-to-oxygen ratio is “2 is to 5”
The empirical formula is P2O5.
The correct answer is choice C.
CLICK PAUSE END RECORDING
Phosphorus
Oxygen
0.5860
1.466
1.000
2.502
2.000
5.004

2. 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