1. Empirical & Molecular Formulas
Advanced Chem. Unit #2
Empirical & Molecular Formulas

2. Empirical Formula -
Formula giving the lowest whole number ratio of the atoms of the elements in a compound.
Empirical formulas may represent either atoms or moles.

3. Molecular Formula -
Shows the actual number of and type of atoms in a compound.

4. Examples: Molecular formula Empirical formula
H2O2 HO
N2H4
NH2
C6H6 (Benzene)(polystyrene) CH
C2H2 (Acetylene) CH
CO2
CO2

5. Examples: Molecular formula Empirical formula
CH2O (formaldhyde)
CH2O
C6H12O6 (glucose)
CH2O
CH2O
C2H4O2 (acetic acid)
CH2O
C5H10O5 (ribose)
CH2O
N2O5
N2O5

6. Formula Weight - Same as molecular weight, but of an ionic compound.
A.K.A. Chart mass

7. Calculating mass percentages (percent composition) from the formula.

8. Mass of “A” in the whole
Mass percent of “A” =
X 100
Mass of the whole

9. Formaldehyde (CH2O) is a toxic gas with a pungent odor. (Uses include:
Example 1:
Formaldehyde (CH2O) is a toxic gas with a pungent odor. (Uses include:
(1) large amounts to make plastics.
(2) a H2O solution of it is used to preserve biological specimens).
Calculate the percent of each element in the whole.

10. %C = g * 100 = 40% g
%H = g *2 *100 = 6.73% g
%O = g *100 = 53.3% g

11. Example 2: Calculate percent of each in:
KMnO4
g/mol
%K =
*100
= 24.7%
%Mn =
*100
= 34.8%
( *4)
= 40.5%
%O =
*100
100%

12. Example 3: Copper(II)nitrate
Cu(NO3)2
= g/mol
63.546
%Cu =
= 33.9%
*100
= 14.9%
( *2)
%N =
*100
%O =
( *6)
= 51.2%
*100
100%

13. Example 4: Halothane (an inhalation anesthetic)
CF3CHBrCl
= g/mol
%C = (12.011*2) * = 12.2%
%F = ( *3) * = 28.9%
%H = * = 0.5%
%Br = * = 40.5%
%Cl = * = 18.0% %

14. Calculating the mass of an Element in a given mass of a compound.
(% of Element “A”) x Mass of the compound = Grams of “A”

15. Example #1: Refer to previous example involving Formaldehyde (CH2O)
40%
%H =
6.73%
%O =
53.3%
Formula wt. =
g/mol

16. How many grams of each element are in 83.5 grams of Formaldehyde?
40%
x g
= g
%H =
6.73%
x g
= g
%O =
53.3%
x g
= g

17. How many grams of Nitrogen, Oxygen, and Hydrogen are in 48
How many grams of Nitrogen, Oxygen, and Hydrogen are in 48.5 grams of Ammonium nitrate ?
NH4NO3 =
g/mol
( x 2)
%N =
x 100
= 35.0%
( x 4)
%H =
x 100
= 5.04%
( x 3)
%O =
x 100
= 60.0%

18. How many grams of Nitrogen are in 48.5 grams of Ammonium nitrate ?
= 0.35
0.35 x 48.5 g =
17.0 g of Nitrogen

19. Urea is a fertilizer that is commonly purchased in a formula (46% Nitrogen by mass (in the form of Ammonium nitrate), the remainder are inert ingredients). How many pounds of Nitrogen are in 3 ton of this fertilizer ?

20. 3 ton =
6000 lb
that is 46% NH4NO3
6000 lb x 0.46 =
2760 lb of NH4NO3
2760 lb of NH4NO3 x 0.35 =
966 lb of Nitrogen

21. Procaine hydrochloride (Novocain) (C13H21ClN2O2) is a local anesthetic.
How many grams of each element are in a 0.23 cc dose of this substance ? (Assume Novocain = 1.0 g/mL)

22. 1st: Determine the mass of the sample.
Mass of sample = 0.23 grams

23. 2nd: Determine the molecular weight of the compound.
Molecular weight = g/mol

24. Which contains more carbon; 4. 71 grams of glucose, or 5
Which contains more carbon; grams of glucose, or grams of Ethyl alcohol (CH3CH2OH) ?
1st: Find %Carbon in each compound.
2nd: Calculate grams from percent in given masses.

25. Glucose Ethanol
40% %C
52.1%
Grams Of
Carbon
1.88 g
3.05 g

26. Which contains more Sulfur; 40. 8 grams of Calcium sulfate, or 35
Which contains more Sulfur; grams of Calcium sulfate, or grams of Sodium sulfite (Na2SO3)?
1st: Find %Sulfur in each compound.
2nd: Calculate grams from percent in given masses.

27. CaSO4 Na2SO3
23.6% %S
25.4%
Grams Of
Sulfur
9.63 g
8.94 g

28. Calculating the percent of C and H by combustion.
Suppose you have discovered a new compound composed of only C, H, and O, whose formula you wish to determine.
What are the products of combustion if the reactants consist of only C, H, and O ?
Answer:
CO2 and H2O

29. Every 1 mole of Carbon combusted will yield 1 mole of CO2.
Every 1 mole of Hydrogen combusted will yield 0.5 mole of H2O.

30. Calculate the percent of Carbon, Hydrogen, and Oxygen by combustion.
Acetic acid contains only C, H, & O. A 4.24mg sample is completely burned. It results in 6.21mg of CO2, and 2.54mg of H2O. What is mass percentage of each element in acetic acid?

31. Step 1: Convert mass of CO2 to moles of CO2.
6.21 mg =
g of CO2
Use DIMO to convert g to moles.
g =
mol CO2

32. Step 2: Relate moles of CO2 to moles of C.
Since 1 mol C produces 1 mol CO2
Then:
mol CO2 was produced by
mol of C.

33. NOW DO THESE SAME THREE STEPS FOR HYDROGEN
Step 3: Convert moles of C to grams of C
(use DIMO)
mol C =
g of C
NOW DO THESE SAME THREE STEPS FOR HYDROGEN

34. Step 1: Convert mass of H2O to moles of H2O.
2.54 mg =
g of H2O
Use DIMO to convert g to moles.
g =
mol H2O

35. Step 2: Relate moles of H2O to moles of H.
Since 1 mol H produces 0.5 mol H2O
Then:
mol H2O was produced by
mol of H.

36. CALCULATE MASS PERCENTAGES OF C & H
Step 3: Convert moles of H to grams of H
(use DIMO)
mol H =
g of H
TO FINISH:
CALCULATE MASS PERCENTAGES OF C & H
IN ACETIC ACID

37. g of C
%C =
x 100
g of Acetic acid
= 39.98% Carbon in Acetic acid

38. SUBTRACT %C and %H FROM 100%
g of H
%H =
x 100
g of Acetic acid
= 6.698% Hydrogen in Acetic acid
TO FIND %OXYGEN:
SUBTRACT %C and %H FROM 100%

39. 100% - (39.98% C % H) =
53.32% O
TOTAL = %

40. (contains only C, H, and O)
A 3.87mg sample of Ascorbic acid (Vitamin C) gives 5.80mg CO2 and 1.58mg of H2O when combusted. What is the percent composition of this substance ?
(contains only C, H, and O)

41. Step 1: Convert mass of CO2 to moles of CO2.
5.80 mg =
g of CO2
Use DIMO to convert g to moles.
g =
mol CO2

42. Step 2: Relate moles of CO2 to moles of C.
Since 1 mol C produces 1 mol CO2
Then:
mol CO2 was produced by
mol of C.

43. NOW DO THESE SAME THREE STEPS FOR HYDROGEN
Step 3: Convert moles of C to grams of C
(use DIMO)
mol C =
g of C
NOW DO THESE SAME THREE STEPS FOR HYDROGEN

44. Step 1: Convert mass of H2O to moles of H2O.
1.58 mg =
g of H2O
Use DIMO to convert g to moles.
g =
mol H2O

45. Step 2: Relate moles of H2O to moles of H.
Since 1 mol H produces 0.5 mol H2O
Then:
mol H2O was produced by
mol of H.

46. CALCULATE MASS PERCENTAGES OF C & H
Step 3: Convert moles of H to grams of H
(use DIMO)
mol H =
g of H
TO FINISH:
CALCULATE MASS PERCENTAGES OF C & H
IN ACETIC ACID

47. g of C
%C =
x 100
g of Ascorbic acid
= 40.8% Carbon in Ascorbic acid

48. SUBTRACT %C and %H FROM 100%
g of H
%H =
x 100
g of Ascorbic acid
= 4.57% Hydrogen in Ascorbic acid
TO FIND %OXYGEN:
SUBTRACT %C and %H FROM 100%

49. 100% - (40.8% C % H) =
54.63% O
TOTAL = 100%

50. Determining Empirical Formula
From
Mass Percent
Remember:
Empirical Formula -
“Formula in lowest terms”

51. The formula of the molecule.
Molecular formula -
The formula of the molecule.
Recall:
Compounds with different molecular formulas can have the same Empirical Formula.
CH2O (formaldhyde)
C6H12O6 (glucose)
CH2O
C2H4O2 (acetic acid)
C5H10O5 (ribose)