Transcript What is the empirical formula of the compound?

Empirical Formula
Chemistry
Chapter Ten
Empirical Formula
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When we cook rice, we commonly use
one cup of rice and one cup of water.
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However, we can double, triple, etc the
recipe if we need to.
+
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=
We do this in chemistry too!
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The formula for some compounds shows the
basic ratio of elements.
Empirical Formula
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What is an empirical formula?
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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
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For example: CH4
Different compounds can have the
same empirical formula!
Are these examples empirical?
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H2O
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NaCl
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C2H4
How do we calculate the
empirical formula?
From percent composition:
 Change percentage to mass in grams
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Convert mass of given to moles
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Even conversion (always assume 100 g sample)
Use conversion factor fraction (mass to moles)
Divide each by the lowest number of moles
Multiply all by the lowest number possible (if
necessary) to get all whole numbers
Plug subscripts into formula
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?
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Step One:
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25.9% Nitrogen = 25.9 grams N
74.1 % Oxygen = 74.1 grams O
Sample Problem #1 solution
A compound is analyzed and found to contain 25.9%
nitrogen and 74.1% oxygen. What is the empirical
formula of the compound?
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Step Two:
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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 solution
A compound is analyzed and found to contain 25.9%
nitrogen and 74.1% oxygen. What is the empirical
formula of the compound?
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Step Three:
1.85 mol N/ 1.85 = 1
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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 solution
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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:
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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! 
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Calculate the empirical formula of
each compound.
 94.1% O, 5.9% H
 67.6% Hg, 10.8% S, 21.6% O
 42.9% C, 57.1% O