Transcript Empirical and Molecular Formula Notes

Chemistry
Empirical and Molecular Formulas
Objectives
Students will be able to:
 Determine the empirical formula of a
compound
 Determine the molecular formula of a
compound

Two types of formulas
Empirical Formula

Gives the formula of a
compound in the simplest
whole number ratio of
atoms/moles in a
compound
Molecular Formula

“True Formula”—gives
the actual ratio of
atoms/moles in a
compound
B. Empirical Formula
 Smallest
whole number ratio of
atoms in a compound
C 2H 6
reduce subscripts
CH3
Determine the empirical formulas
for each of the following molecular
formulas.
1. C8H18…..______________ • C2H9
2. H2O2……______________
3. Hg2Cl2…..______________
4. C3H6…….______________
5. Na2C2O4..._____________
6. H2O......________________
7. C4H8….._______________
8. C4H6…..________________
9. C7H12…._______________
10. C2H4O2….___________
•
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HO
HgCl
CH2
NaCO2
H2O
CH2
C2H3
C7H12
CH2O
Finding Empirical Formula
1. Use the % of each element as its
mass in the compound (ex 36.8% N, 63.2%O)
2. Convert mass of each element to
moles.
3. Divide moles by the smallest # of
moles to find subscripts.
4. If #’s are not whole #’s, multiply
subscripts by 2, 3, or 4 to get whole #’s.
B. Empirical Formula
 Find
the empirical formula for a
sample of 25.9% N and 74.1% O.
25.9 g 1 mol
= 1.85 mol N
=1N
1.85 mol
14.0 g
74.1 g 1 mol
= 4.63 mol O
= 2.5 O
16.0 g
1.85 mol
B. Empirical Formula
N1O2.5
Need to make the subscripts whole
numbers  multiply by 2
N2O5
Molecular formula: can be the same as the empirical formula or a
Whole # multiple of the empirical.
It is the true number of atoms of each element in a compound.
Steps for finding Molecular Formula
1.
2.
3.
4.
Determine the empirical formula(either given or you find it)
Find the molar mass of the empirical formula
Divide the Molecular Formula mass (given in problem)
by the EF mass to create a ratio (n).
Multiply the EF formula by the whole # ratio
(n).
n= MFmass
molecular formula = (EF)n
EFmass
Finding the Molecular Formula
The empirical formula for adipic acid is C3H5O2.
The molecular mass of adipic acid is 146 g/mol.
What is the molecular formula of adipic acid?
Step 1 (given in problem) Determine Empirical
Formula
Step 2. Find the Molar mass of C3H5O2
3(12.0 g) + 5(1.0) + 2(16.0) = 73.0 g
Finding the Molecular Formula
The empirical formula for adipic acid is C3H5O2.
The molecular mass of adipic acid is 146 g/mol.
What is the molecular formula of adipic acid?
3(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g
146
2
73
Finding the Molecular Formula
The empirical formula for adipic acid is C3H5O2.
The molecular mass of adipic acid is 146 g/mol.
What is the molecular formula of adipic acid?
Step 4. Multiply the empirical formula by this
number to get the molecular formula.
3(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g
146
2
73
(C3H5O2) x 2 =
C6H10O4