Mole Concept
Counting Units
 A pair refers to how many shoes?
 A dozen refers to how many doughnuts or
eggs?
 How many pencils are in a gross?
 How many pieces of paper are in a ream?
In chemistry, a counting unit called a mole is
used to keep track of the number of atoms,
molecules, or formula units that are in a
sample of a substance.
Mole
 Unit used to measure the amount of a pure
substance (abbreviated mol)
 1 mol = 6.022 x1023 representative particles
 representative particles are the smallest piece of
any pure substance
 Element – atoms
 Covalent compounds – molecules
 Ionic compounds - formulas units
 6.022 x1023 is known as Avogadro’s number
 6.022 x1023 can be used as a conversion factor
to determine the number of particles in a
substance
Molar Mass
 mass (in grams) of one mole of any
substance
 units are grams per mole (g/mol)
 molar mass of any element = the AAM of
the element from the periodic table
Examples:
the molar mass for
 lithium (Li) =
 manganese (Mn) =
 silver (Ag) =
 molar mass of a molecule or
formula unit – number of atoms of
each element multiplied by the
mass of each element
Examples:
H2 O
C6H12O6
Using Molar Mass in Conversions
Example: Determine the mass of 0.0450
mol of chromium.
Example:
How many moles are in 3.25 g of water?
Example:
How many molecules of glucose, C6H12O6
are found in a 5.37 g sample?
Example:
Determine the percent composition of the
elements in copper (II) nitrate.
Empirical Formulas
Empirical formula – smallest whole number
mole ratio of elements in a compound
Solving Empirical Formula Problems
1. Grams to moles – use the molar mass to
convert the grams of each element to moles
2. Divide by small – divide the number of moles
of each element by the smallest number of
moles
example…
What is the formula of a compound
that is comprised of 56.4g K,
8.7g C and 34.9g O?
Example 2Determine the empirical formula for
methyl acetate, which is known to have
the following percent composition:
48.64% carbon, 8.16% hydrogen, and
43.20% oxygen.
Empirical and Molecular Formulas
Molecular formula – the actual number
of atoms in a molecular compound
Example: glucose
 molecular formula: C6H12O6
 empirical formula: C1H2O1
 ratio of the elements is the same in both
formulas, just reduced in the empirical
formula
To solve molecular formula
problems…
 Calculate the empirical formula
 divide empirical molar mass into the
given molecular molar mass, then
distribute whole number answer through
empirical formula
example problem…
Find the molecular formula for a
compound that is known to
contain 26.7% phosphorus,
12.1% nitrogen, and 61.2%
chlorine. Molecular mass of the
compound is known to be 695
g/mol.
Percent Composition
 is the percent by mass of any element in a
compound
 uses molar mass for elements and compounds
 is found by using the following formula:
% of element =
mass of element x 100
mass of compound