The Mole
Remembering Terms
Atomic number- # of _________
 Mass Number- # of particles in the
________ (_______ + ________)
 __________- average mass (in atomic
mass units, amu) of all naturally
occurring isotopes according
to their abundance


Given: one atom 56Fe
◦ What is the atomic #?
 __________
◦ What is the mass #?
 ___________
◦ What is the atomic mass?
 ____________

Given: a sample of iron
◦ What is the atomic #?
 _____________
◦ What is the mass #?
 _____________
◦ What is the atomic mass?
 ______________
Calculating Atomic Mass

Finding atomic mass of an element
◦ Look it up on the periodic table
◦ What is the atomic mass of Na? C? Ag?
 Na
 C
 Ag

________
________
________
Finding atomic mass of a compound
◦ Calculate it using each element’s atomic mass from the
periodic table
◦ Remember to take into account how many atoms of each
type are in the compound
◦ What is the atomic mass of NaCl? H2O? Na3PO4?
 NaCl
 H2O
 Na3PO4
________
________
________
The Mole

A name for a certain number of things
◦ Just like
 A dozen is the name for ___ things
 A baker’s dozen is the name for ___ things
 A gross is the name for ____ things (a dozen dozens)

A mole(mol) is the name for ________things,
also known as Avogadro’s number
◦ Atoms, molecules, formula units
Using Avogadro’s Number

Calculating number of moles of each element in
a compound
◦ How many moles of Na are in 1 mol of NaCl?
 _____________________________
◦ How many moles of Cl are in 3 moles of MgCl2?
 _____________________________

Calculating number of things
◦ How many molecules are in 4 moles of CO2?
◦ How many atoms are in 2.5 moles of CO2?
The Mole (cont)

Definition of the Mole
◦ A mole equals the number of atoms in 12 g of carbon-12

Because of this definition,
◦ Atomic mass in amu’s equals the mass in grams
(molecular mass or molar mass)
◦ What is the molecular mass of Na? C? Ag?
 Na – ____________
 C – _____________
 Ag – ____________
◦ What is the molecular mass of NaCl? H2O? Na3PO4?
 NaCl – ____________
 H2O – _____________
 Na3PO4 – ___________
Molar Calculations Using Mass

Calculating # of moles from mass or mass from
number of moles
◦ How many moles of NaNO3 are in 308.25g? How many
moles of O are in the sample?
◦ How many kilograms are in 28.34 moles of NaCl?
Molar Calculations Using Mass
(cont)

Calculating # of things from mass or mass from
number of things
◦ How many molecules of water are in 20g?
◦ How many atoms are in 0.37g of N2?
% Composition of Compounds
Gives the % of each element in a
compound by mass
 (Molecular mass of element in compound/
Molecular mass of compound) x 100
 What is the % composition of each
element in Na2SO3?

◦ Na _______________________
◦S
______________________
◦ O ________________________
Empirical Formulas
Simplest, reduced formula for a compound
 In (almost all) ionic compounds, the empirical
formula matches the molecular formula

◦ For example: Magnesium chloride
 Molecular formula- ______
 Empirical formula- _______

In covalent compound, they often don’t match
◦ For example: Glucose
 Molecular formula- ______
 Empirical formula- _______
Calculating Empirical Formulas

Known:
◦ % of compound that each element represents
(% composition)
or
◦ how many grams of each element are in the
compound

Steps
1.
Change each element’s amount in g to amount
in mol
 If given % comp, the number of grams is the %.
Calculating Empirical Formulas

Steps (cont)
2. Find the smallest amount of moles among the
elements and divide each number of moles by this
number
3. Determine the empirical formula


If the divided numbers are whole integers (or very close), then
these are the subscripts for each of the corresponding elements.
If some of the divided numbers are not whole integers, multiply all
of them by a # that will make all of them whole integers
Calculating Empirical Formulas
What is the empirical formula of a compound
that is 76.57% C, 6.43% H, and 17% O by
mass?
Step 1
Step 2
Step 3

Calculating Empirical Formulas
What is the empirical formula for a
compound that contains C, H, and O if a 20g
sample has 9.054g C and 1.900g H?
Step 1
Step 2
Step 3

Calculating Empirical Formula
What is the formula for a hydrate if the
compound contains 79.07% CaSO4 and
20.93% H20?
Step 1
Step 2

Step 3
Calculating Molecular Formulas

Known:
◦ Empirical formula
(and therefore empirical formula mass)
◦ Molecular mass

Steps
1.
Calculate empirical formula mass (same as
calculating a molecular mass but use empirical
formula)
2. Divide molecular mass by empirical formula mass to
get a whole integer
3. Multiple all elements in the empirical formula by the
whole integer to get the molecular formula.
Calculating Molecular Formulas

If the empirical formula of hydroquinone
is C3H3O and the molecular mass is 110
g/mol, what is the molecular formula?
Step 1
Step 2
Step 3