1. From percentage to formula
Empirical Formula
From percentage to formula

2. Types of Formulas Empirical Molecular (true) Name CH C2H2 acetylene
The formulas for compounds can be expressed as an empirical formula and as a molecular(true) formula.
Empirical Molecular (true) Name
CH C2H2 acetylene
CH C6H6 benzene
CO2 CO2 carbon dioxide
CH2O C5H10O5 ribose

3. An empirical formula represents the simplest whole number ratio of the atoms in a compound.

4. It is not just the ratio of atoms, it is also the ratio of moles of atoms
In 1 mole of CO2 there is 1 mole of carbon and 2 moles of oxygen
In one molecule of CO2 there is 1 atom of C and 2 atoms of O

5. The molecular formula is the true or actual ratio of the atoms in a compound.

6. Learning Check EF-1 1) CH2O 2) C2H4O2 3) C3H6O3
A. What is the empirical formula for C4H8?
1) C2H ) CH ) CH
B. What is the empirical formula for C8H14?
1) C4H7 2) C6H ) C8H14
C. What is a molecular formula for CH2O?
1) CH2O 2) C2H4O ) C3H6O3

7. Learning Check EF-2
If the molecular formula has 4 atoms of N, what is the molecular formula if SN is the empirical formula? Explain.
1) SN
2) SN4
3) S4N4

8. Empirical and Molecular Formulas
molar mass = a whole number = n
simplest mass
n = 1 molar mass = empirical mass
molecular formula = empirical formula
n = 2 molar mass = 2 x empirical mass
molecular formula =
2 x empirical formula
molecular formula = or > empirical formula

9. Learning Check EF-3 2) C6H8O6 3) C9H12O9
A compound has a formula mass of and an empirical formula of C3H4O3. What is the molecular formula?
1) C3H4O3
2) C6H8O6
3) C9H12O9

10. Learning Check EF-4 1) C7H6O4 2) C14H12O8 3) C21H18O12
If there are g of O in the molecular formula, what is the true formula if the EF is C7H6O4?
1) C7H6O4
2) C14H12O8
3) C21H18O12

11. Calculating Empirical Formula
Pretend that you have a 100 gram sample of the compound.
That is, change the % to grams.
Convert the grams to mols for each element.
Write the number of mols as a subscript in a chemical formula.
Divide each number by the least number.
Multiply the result to get rid of any fractions.

12. Example
Calculate the empirical formula of a compound composed of % C, % H, and %N.
Assume 100 g so
38.67 g C x 1mol C = mole C gC
16.22 g H x 1mol H = mole H gH
45.11 g N x 1mol N = mole N gN

13. If we divide all of these by the smallest
one it will give us the subscripts for the empirical formula
3.220 mol C = mol N
16.09 mol H = mol N
3.219 mole N = 1
3.219 mol N
Empirical formula: CH5N

14. Learning Check EF-5
Aspirin is 60.0% C, 4.5 % H and 35.5 O. Calculate its simplest formula. In 100 g of aspirin, there are g C, 4.5 g H, and g O.

15. Solution EF-5 60.0 g C x ___________= ______ mol C
4.5 g H x ___________ = _______mol H
35.5 g O x ___________ = _______mol O

16. 5.00 mol C = ________________ ______ mol O
Divide by the smallest # of moles.
5.00 mol C = ________________
______ mol O
4.5 mol H = ________________
2.22 mol O = ________________
Are are the results whole numbers?_____

17. Finding Subscripts
A fraction between 0.1 and 0.9 must not be rounded. Multiply all results by an integer to give whole numbers for subscripts.
(1/2) x 2 = 1
(1/3) x 3 = 1
(1/4) x 4 = 1
(3/4) x 4 = 3
(1/5) x 5 =

18. Homework
Worksheet C: #1-7