Transcript Empirical Formulas

Empirical Formulas
• The empirical formula is simply the simplest
whole number ratio of the elements in a
compound.
• The molecular formula is the actual number of
atoms that makes up the molecule
• Example:
the empirical formula of ethene (C2H4) is CH2
the empirical formula of butene (C4H8) is CH2
Comparing
Name
Molecular Empirical
Hydrogen peroxide
H2O2
HO
Low
ratio
1:1
Glucose
C6H12O6
CH2O
1:2:1
Benzene
C6H6
CH
1:1
Ethene
C2H4
CH2
1:2
Butene
C4H8
CH2
1:2
Aniline
C6H7N
C6H7N
6:7:1
Water
H2O
H2O
2:1
Steps to find Empirical Formulas:
1. Always start by assuming that you
have 100g of the substance (this
works nicely with %)
2. Use the percents given to assign how
much of the 100 g is for each element
3. Convert grams to moles using molar
mass for each element
4. Find the lowest whole ratio between
the elements (divide each by the
smallest number of moles)
Example:
•
Calculate empirical formula of a
compound that is 85.6% carbon and
14.4% hydrogen.
1)
85.6g C and 14.4g H
Example:
2) x mol C = 85.6g x 1mol = 7.13 mol C
12.01g
x mol H = 14.4g x 1 mol = 14.3 mol H
1.01g
Example:
3) Lowest mole ratio:
C = 7.13 /7.13 = 1
H = 14.3/ 7.13 = 2.01 (so 2)
ANSWER: CH2
What if I don’t get whole numbers?
Example:
mol Cr = 1.31/1.31 = 1
Mol O = 1.97/1.31 = 1.5
• You need to multiply everything by something to
make whole numbers!
• Try multiplying by 2 or 3
1x2=2
1.5 x 2 = 3
Answer = Cr2O3
Practice:
Find the empirical formula for the following:
1. 17.5% hydrogen and 82.4% nitrogen
2. 46.3% lithium and 53.7% oxygen
Try it:
Pg. 91 #13-15
Pg. 94 #2,4, 6,7
Pg. 97 #17-19