1. Percent Composition, Empirical Formulas, Molecular Formulas

2. Formula Mass The sum of atomic masses of all atoms in its formula
Important role in nearly all chemical calculations
Can be calculated for compounds and diatomic elements
The atoms present in one formula unit of a substance
Can be calculated for both ionic and molecular substances

3. Calculating Formula Mass
Calculate the formula mass of calcium chloride
Write the formula from the name given
Ca2+ (from group II) and Cl- (from group VII)
Formula is CaCl2 due to charge balance
Formula mass: Sum of the atomic masses of atoms in the formula (1 Ca atom + 2 Cl atoms)
Write the name of the compound by combining Ca2+ and Cl-
1 amu Ca + 2 amu Cl is the formula mass of CaCl2
= amu
= amu
Formula mass of CaCl2

4. Practice
Complete # 1-3, 10-12

5. Percent Composition 1 pizza 2 slices pineapple 6 slices pepperoni
4 slices cheese
What % of the pizza slices are
pineapple?
pepperoni?
just cheese?
(2/12) x 100 = 17% pineapple, (6/12) x 100 = 50% pepperoni, (4/12) x 100 = 33% cheese

6. So… Percent Composition Part _______ Percent = x 100% Whole
Percent Composition – the percentage by mass of each element in a compound
Part
_______
Percent =
x 100%
Whole
So…

7. Percent Composition Molar Mass of KMnO4 K = 1(39.1) = 39.1
Example: What is the percent composition of Potassium Permanganate (KMnO4)?
Molar Mass of KMnO4
K = 1(39.1) = 39.1
Mn = 1(54.9) = 54.9
O = 4(16.0) = 64.0
MM = 158 g

8. Percent Composition Molar Mass of KMnO4 = 158 g 39.1 g K % K x 100 =
Example: What is the percent composition of Potassium Permanganate (KMnO4)?
Molar Mass of KMnO4
= 158 g
39.1 g K
% K
x 100 =
24.7 %
158 g
54.9 g Mn
34.8 %
x 100 =
% Mn
158 g
K = 1(39.10) = 39.1
64.0 g O
x 100 =
40.5 %
Mn = 1(54.94) = 54.9
% O
158 g
O = 4(16.00) = 64.0
MM = 158

9. Percent Composition
Determine the percentage composition of sodium carbonate (Na2CO3)?
Molar Mass
Percent Composition
46.0 g
x 100% =
43.4 %
Na = 2(23.00) = 46.0
C = 1(12.01) = 12.0
O = 3(16.00) = 48.0
MM= g
% Na =
106 g
12.0 g
x 100% =
11.3 %
% C =
106 g
48.0 g
x 100% =
45.3 %
% O =
106 g

10. Percent Composition
Determine the percentage composition of ethanol (C2H5OH)?
% C = 52.13%, % H = 13.15%, % O = 34.72%
_______________________________________________
Determine the percentage composition of sodium oxalate
(Na2C2O4)?
% Na = 34.31%, % C = 17.93%, % O = 47.76%

11. Percent Composition
Calculate the mass of bromine in 50.0 g of Potassium bromide.
1. Molar Mass of KBr
K = 1(39.10) = 39.10
Br =1(79.90) =79.90
MM = 119.0 2.
79.90 g
___________ =
119.0 g
x 50.0g = 33.6 g Br

12. Percent Composition
Calculate the mass of nitrogen in 85.0 mg of the amino acid lysine, C6H14N2O2.
1. Molar Mass of C6H14N2O2
C = 6(12.01) = 72.06
H =14(1.01) = 14.14
N = 2(14.01) = 28.02
O = 2(16.00) = 32.00
MM = 146.2 2.
28.02 g
___________
= 0.192
146.2 g
x 85.0 mg = 16.3 mg N

13. Formulas
Percent composition allow you to calculate the simplest ratio among the atoms found in compound.
Empirical Formula – formula of a compound that expresses lowest whole number ratio of atoms.
Molecular Formula – actual formula of a compound showing the number of atoms present
Examples:
C6H12O6
- molecular
C4H10
- molecular
C2H5
- empirical
CH2O
- empirical

14. Formulas Is H2O2 an empirical or molecular formula?
Molecular, it can be reduced to HO
HO = empirical formula

15. Relating Empirical and Molecular Formulas
n represents a whole number multiplier from 1 to as large as necessary
Calculate the mass of the empirical formula
Divide the given molecular mass by the calculated empirical mass
Answer is a whole number multiplier

16. Relating Empirical and Molecular Formulas
Multiply each subscript in the empirical formula by the whole number multiplier to get the molecular formula

17. Calculate Empirical Formula from Percent Composition
Lactic acid has a molar mass of g. It has an empirical formula of CH2O.
What is the molecular formula of lactic acid?
Lactic acid builds up in muscles after aerobic exercise
Since percent represents parts of one element per 100 parts of the total compound

18. Determination of the Molecular Formula
Obtain the value of n (whole number multiplier)
Multiply the empirical formula by the multiplier
Molecular formula = n х empirical formula
Molecular formula = 3 (CH2O)
C3H6O3