Chemical Formulas and Compounds
Using Chemical Formulas
Using Chemical Formulas
 Objective
• Calculate the formula mass or molar mass of any given compound
• Use molar mass to convert between mass in grams and amount in moles of
a chemical compound
• Calculate the number of molecules, formula units, or ions in a given molar
amount of a chemical compound
• Calculate the percentage composition of a given chemical compound
Using Chemical Formulas
 Chemical Formulas
• A chemical formula indicates
• the elements present in a compound
• the relative number of atoms or ions of each element present in a
compound
• Chemical formulas also allow chemists to calculate a number of other
characteristic values for a compound:
• formula mass
• molar mass
• percentage composition
Using Chemical Formulas
 Formula Masses
• The formula mass of any molecule, formula unit, or ion is the sum of the
average atomic masses of all atoms represented in its formula
formula mass of water, H2O
average atomic mass of H: 1.01 amu
average atomic mass of O: 16.00 amu
1.01 amu
2 H atoms 
 2.02 amu
H atom
16.00 amu
1 O atom 
 16.00 amu
O atom
average mass of H2O molecule = 18.02 amu
Using Chemical Formulas
 Formula Masses
• The mass of a water molecule can be referred to as a molecular mass
• The mass of one formula unit of an ionic compound, such as NaCl, is not a
molecular mass
• The mass of any unit represented by a chemical formula (H2O, NaCl) can
be referred to as the formula mass
Using Chemical Formulas
 Formula Masses
• Sample Problem
• Find the formula mass of potassium chlorate, KClO3
Using Chemical Formulas
 Formula Masses
• Sample Problem Solution
• The mass of a formula unit of KClO3 is found by adding the masses
of one K atom, one Cl atom, and three O atoms
39.10 amu
 39.10 amu
K atom
35.45 amu
1 Cl atom 
 35.45 amu
Cl atom
1 K atom 
3 O atoms 
16.00 amu
 48.00 amu
O atom
formula mass of KClO3 = 122.55 amu
Using Chemical Formulas
 Molar Masses
• The molar mass of a substance is equal to the mass in grams of one mole,
or approximately 6.022 × 1023 particles, of the substance
• The molar mass of a compound is calculated by adding the masses of the
elements present in a mole of the molecules or formula units that make up
the compound
Using Chemical Formulas
 Molar Masses
• One mole of water molecules contains exactly two moles of H atoms and
one mole of O atoms. The molar mass of water is calculated as follows
2 mol H 
1 mol O 
1.01 g H
 2.02 g H
mol H
16.00 g O
 16.00 g O
mol O
molar mass of H2O molecule = 18.02 g/mol
• A compound’s molar mass is numerically equal to its formula mass
Using Chemical Formulas
 Formula Masses
• Sample Problem
• What is the molar mass of barium nitrate, Ba(NO3)2
Using Chemical Formulas
 Formula Masses
• Sample Problem Solution
• One mole of barium nitrate, contains one mole of Ba, two moles of N
(1 × 2), and six moles of O (3 × 2)
137.33 g H
 137.33 g Ba
mol Ba
14.01 g
2 mol N 
 28.02 g N
mol N
1 mol Ba 
16.00 g O
6 mol O 
 96.00 g O
mol O
molar mass of Ba(NO3)2 = 261.35 g/mol
Using Chemical Formulas
 Molar Mass Conversions
Using Chemical Formulas
 Molar Mass Conversions
• Sample Problem
• What is the mass in grams of 2.50 mol of oxygen gas (O2)
Using Chemical Formulas
 Molar Mass Conversions
• Sample Problem Solution
• Given: 2.50 mol O2
• Unknown: mass of O2 in grams
moles O2
grams O2
amount of O2 (mol) × molar mass of O2 (g/mol) = mass of O2 (g)
2 mol O 
16.00 g O
 32.00 g
mol O
32.00 g O2
2.50 mol O2 
 80.0 g O2
mol O2
Using Chemical Formulas
 Molar Mass Conversions
• Sample Problem
• Ibuprofen, C13H18O2, is the active ingredient in many nonprescription
pain relievers. Its molar mass is 206.31 g/mol
• If the tablets in a bottle contain a total of 33g of ibuprofen, how
many moles of ibuprofen are in the bottle?
• How many molecules of ibuprofen are in the bottle?
• What is the total mass in grams of carbon in 33g of ibuprofen?
Using Chemical Formulas
 Molar Mass Conversions
• Sample Problem Solution
• Given: 33g of C13H18O2, molar mass 206.31 g/mol
• Unknown: moles C13H18O2
molecules C13H18O2
total mass of C
Using Chemical Formulas
 Molar Mass Conversions
• Sample Problem Solution
grams
moles
1 mol C13H18O2
g C13H18O2 
 mol C13H18O2
206.31 g C13H18O2
moles
molecules
6.022  1023 molecules
mol C13H18O2 
 molecules C13H18O2
mol
moles C13H18O2
moles C
grams C
13 mol C
12.01 g C
mol C13H18O2 

 gC
mol C13H18O2
mol C
Using Chemical Formulas
 Molar Mass Conversions
• Sample Problem Solution
33 g C13H18O2 
1 mol C13H18O2
 0.16 mol C13H18O2
206.31 g C13H18O2
6.022  10 23 molecules
0.16mol C13H18O2 

mol
9.6  10 22 molecules C13H18O2
0.16 mol C13H18O2 
13 mol C
12.01 g C

 25 g C
mol C13H18O2
mol C
Using Chemical Formulas
 Percentage Composition
• It is often useful to know the percentage by mass of a particular element in
a chemical compound
• To find the mass percentage of an element in a compound, the following
equation can be used
mass of element in sample of compound
 100 
mass of sample of compound
% element in compound
• The mass percentage of an element in a compound is the same regardless
of the sample’s size
Using Chemical Formulas
 Percentage Composition
• The percentage of an element in a compound can be calculated by
determining how many grams of the element are present in one mole of
the compound
mass of element in 1 mol of compound
 100 
molar mass of compound
% element in compound
• The percentage by mass of each element in a compound is known as the
percentage composition of the compound
Using Chemical Formulas
 Percentage Composition
Using Chemical Formulas
 Molar Mass Conversions
• Sample Problem
• Find the percentage composition of copper(I) sulfide, Cu2S
Using Chemical Formulas
 Molar Mass Conversions
• Sample Problem Solution
• Given: formula, Cu2S
• Unknown: percentage composition of Cu2S
formula
molar mass
mass percentage of each element
Using Chemical Formulas
 Molar Mass Conversions
• Sample Problem Solution
2 mol Cu 
1 mol S 
63.55 g Cu
 127.1 g Cu
mol Cu
32.07 g S
 32.07 g S
mol S
Molar mass of Cu2S = 159.2 g
127.1 g Cu
 100  79.85% Cu
159.2 g Cu2S
32.07 g S
 100  20.15% S
159.2 g Cu2S
Using Chemical Formulas
 Molar Mass Conversions
• Sample Problem
• As some salts crystallize from a water solution, they bind water
molecules in their crystal structure. Sodium carbonate forms such a
hydrate, in which 10 water molecules are present for every formula unit
of sodium carbonate. Find the mass percentage of water in sodium
carbonate decahydrate, Na2CO3•10H2O, which has a molar mass of
286.19 g/mol.