Grouping!
• When with many
items in a large set
need to be
counted, it is often
useful to work with
groups of items
rather than
individual items.
Item
Quantity Amount
Gloves
Pair
2
Cans
of pop
Eggs
Six-pack
6
Dozen
12
Pens
Gross (12
dozen)
Paper
Ream
144
500
• Chemists use a quantity to group atoms and
molecules that is much larger than a dozen or a ream
- THE MOLE (mol)
• The mole is defined as the amount of substance that
contains as many entities (atoms, molecules, or
units) as exactly 12 g of Carbon.
• One mole of a substance contains
• 6.022 141 99 x 1023 particles
• This number is Avogadro’s constant and for the
purposes of this class we will just know it as
• 6.022 x 1023 particles.
• The mass on the periodic table is called the
molar mass.
• 55.847 g of iron is one mole.
• One mole of iron has 6.022 x 1023 atoms.
• So 55.847 g of iron has 6.022 x 1023 atoms.
Mole Conversions
• The Mole formula! Be able to
rearrange it to solve for each of the
variables!
Number of
moles (mol)
mass (g)
n=m
MM
Molar mass
(g/mol)
Mole to Mass Example
• 2.001 mol KCl is how many grams?
• First you need the molar mass of KCl from the
periodic table.
• 39.0983 + 35.4527 = 74.5510 g/mol
• The you need to do the conversion.
• 2.001 mol x 74.5510 g = 149.2 g
1 mol
Another Mole to Mass Example
• How many grams is in 0.42 mol of Mg(OH)2?
• Molar mass of Mg(OH)2 is
• 24.3050 + 2(15.9994) + 2(1.00794) = 58.3197 g/mol
• 0.42 mol x 58.3197 g = 24 g
1 mol
Mass to Mole Example
• You are given 157 g NaOH. How many moles is
this?
• To solve this, you need the molar mass from
the periodic table.
• 22.989768 + 15.9994 + 1.00794 = 39.9971 g/mol
• 157 g NaOH x 1 mol = 3.93 mol
39.9971g
Another Mass to Mole example
• How many moles are in 1526 g K2CO3?
• Molar mass of K2CO3 is
• 2(39.0983) + 12.011 + 3(15.9994) = 138.197 g/mol
• 1526 g x 1 mol = 11.04 mol
138.197 g
Moles to Particles Example
• How many particles are in 10.8 mol FeO?
• The only thing you need for this conversion is
Avogadro’s constant (6.022 x 1023)
• 10.8 mol FeO x 6.022 x 1023 particles
1 mol
• 6.50 x 1024 particles
Another Mole to Particle Example
• How many atoms are in 17.6 mol of CaCl2?
• Avogadro’s constant is 6.022 x 1023
• 17.6 mol CaCl2 x 6.022 x 1023 atoms
1 mol
• 1.06 x 1025 atoms
Moles to Volume at STP Example
• Volume at standard temperature (25oC) and
pressure (1 atm) is a constant of 22.4 L.
• How many L are in 25.5 mol Al2O3?
• All you need to do the conversion is the
constant of 22.4 L.
• 25.5 mol x 22.4 L = 571 L
1 mol
Volume to Mole Example at STP
• How many moles in 6.85 L of Cl2 gas?
• All you need is the constant.
• 6.85 L
x 1 mol = 0.306 mol
22.4 L
• When you calculate and use the molar mass of a
compound, you are making an important
assumption.
• Say you are working with water, you are assuming
that every sample of water contains two hydrogen
and one oxygen and the mass ratio is 2(1.00794) to
1(15.9994).
• At this point, now that we have the periodic
table, this two-to-one ratio seems obvious.
But when scientists first discovered that
compounds contained elements in fixed mass
proportions, they did not have the periodic
table.
• In fact, the discovery of fixed mass
proportions was an important step toward the
development of atomic theory.
FYI
• There are an infinite number of
significant digits in a mole, so don’t
assume it to have only 1!
• You MUST convert ALL quantities into
standard units (L, g, etc) to use in all
formulas
• Molar masses for molecules are
calculated based on their FORMULA
• For full marks you MUST SHOW
ME ALL STEPS, FORMULAS,
INCLUDE FINAL STATEMENTS
and correct SIGNIFICANT DIGITS
• My advise… carry your units through
all calculations so that you can be
sure your answer is correct when
they cancel out properly