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Empirical Formula
Chemistry
Chapter Ten
Mrs Luckett
Empirical Formula

When we cook rice, we commonly use
one cup of rice and one cup of water.

However, we can double, triple, etc the
recipe if we need to.
+

=
We do this in chemistry too!

The formula for some compounds shows the
basic ratio of elements.
Empirical Formula

What is an empirical formula?


Gives us the smallest whole-number ratio
of subscripts for each element found in a
compound
You have an empirical formula when you
can’t divide the subscripts by the same
number to get a whole number


For example: CH4
Different compounds can have the
same empirical formula!
Are these examples empirical?

H2O
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NaCl
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C2 H4
Empirical and Molecular formula
foldable
Flaps:
1. Change percent to mass in grams
2. Convert mass to moles
3. Divide each lowest moles
4. Multiply and Plug in subscripts
Sample Problem #1
A compound is analyzed and found to
contain 25.9% nitrogen and 74.1%
oxygen. What is the empirical formula of
the compound?

Step One: Even conversion (change % to
grams by assuming 100 gram sample)
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
25.9% Nitrogen = 25.9 grams N
74.1 % Oxygen = 74.1 grams O
Sample Problem #1
A compound is analyzed and found to contain 25.9%
nitrogen and 74.1% oxygen. What is the empirical
formula of the compound?

Step Two: convert grams to moles

25.9 g N x
1 mole
=
14.007 g N
74.1 grams O x
1 mole
=
15.999 g O
1.85 mol N
4.63 mol O
Sample Problem #1
A compound is analyzed and found to contain 25.9%
nitrogen and 74.1% oxygen. What is the empirical
formula of the compound?

Step Three: divide by the lowest number of
moles
1.85 mol N/ 1.85 = 1

4.63 mol O / 1.85 = 2.5
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Are these both whole numbers?
No! Now we need to do one more step
Sample Problem #1


A compound is analyzed and found to contain
25.9% nitrogen and 74.1% oxygen. What is the
empirical formula of the compound?
Step Four: multiply by lowest whole number
possible (if necessary)
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Can I multiply by one to get whole numbers? NO
Can I multiply by two to get whole numbers? YES
N: 1 x 2 = 2
O 2.5 x 2 = 5
Now plug it into the formula: N2O5
Now you try! 

Calculate the empirical formula of
each compound.
 94.1% O, 5.9% H
 67.6% Hg, 10.8% S, 21.6% O