 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
 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)
40.08 amu
= 40.08 amu
1 Ca atom
35.45 amu
2 Cl atom 
= 70.90 amu
1 Cl atom
1 Ca atom 
110.98 amu
Formula mass of CaCl2
Complete # 1-3, 10-12
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
Percent Composition
• Percent Composition – the
percentage by mass of
each element in a
compound
Part
_______
Percent =
x 100%
Whole
mass of each element
mass percent of each 
100%
element in a compound molar mass of the compound
Percent Composition
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
Percent Composition
Example: What is the percent composition of
Potassium Permanganate (KMnO4)?
Molar Mass of KMnO4
% K
39.1 g K
158 g
= 158 g
x 100 = 24.7 %
54.9 g Mn x 100 = 34.8 %
% Mn 158 g
K=
1(39.10) = 39.1
Mn = 1(54.94) = 54.9
O = 4(16.00) = 64.0
MM = 158
64.0 g O x 100 = 40.5 %
% O
158 g
Percent Composition
Determine the percentage composition of sodium carbonate
(Na2CO3)?
Molar Mass
Na = 2(23.00) = 46.0
C = 1(12.01) = 12.0
O = 3(16.00) = 48.0
MM= 106 g
Percent Composition
46.0 g
% Na =106 g
12.0 g
% C = 106 g
48.0 g
% O = 106 g
x 100% = 43.4 %
x 100% = 11.3 %
x 100% = 45.3 %
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%
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.
3.
79.90 g
___________
= 0.6714
119.0 g
0.6714 x 50.0g = 33.6 g Br
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.
3.
28.02 g
___________
= 0.192
146.2 g
0.192 x 85.0 mg = 16.3 mg N
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:
C4H10 - molecular
C2H5 - empirical
C6H12O6 - molecular
CH2O
- empirical
Formulas
Is H2O2 an empirical or molecular formula?
Molecular, it can be reduced to HO
HO = empirical formula

n represents a whole number multiplier from
1 to as large as necessary
molar mass
n
empirical formula mass


Calculate the mass of the empirical formula
Divide the given molecular mass by the
calculated empirical mass
› Answer is a whole number multiplier

Multiply each subscript in the empirical
formula by the whole number multiplier to
get the molecular formula
Lactic acid has a molar mass of 90.08 g.
It has an empirical formula of CH2O.
 What is the molecular formula of lactic
acid?



Obtain the value of n (whole number multiplier)
Multiply the empirical formula by the multiplier
molar mass
n
empirical formula mass
90.08 g / mol
3
30.03 g / mol
Molecular formula = n х empirical formula
Molecular formula = 3 (CH2O)
C3H6O3