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The Mole
Relative atomic mass
Relative atomic mass is a weighted average
of all the naturally occurring isotopes of an
element. The standard upon which it is
based is one-twelfth the mass of carbon-12.
For molecules (covalently bonded), the term
relative molecular mass is used.
For ionic compounds, use relative formula
mass.
Both of these are calculated as sums of the
relative atomic masses of the elements in the
chemical formula.
How to Find the
Formula/Molecular Mass
1. Write a CORRECT formula for the
compound
2. Look up the atomic mass of each element
from the periodic table in the compound
and round it to a whole number
(EXCEPTIONS: round Cl to 35.5 and Cu
to 63.5).
3. Multiply the rounded atomic mass by the
subscripts, if any.
4. Add all masses of elements together
Example
Ex: Find the formula mass of calcium
phosphate
1.
2.
3.
Ca3(PO4)2
Ca = 40 x 3 = 120
P = 31 x 2 = 62
O = 16 x 8 = 128
310
Examples
l. Find the formula mass of ammonium
sulfate
2. Find the molecular mass of dichlorine
heptoxide
Answers
1. (NH4)2SO4 = 132
2. Cl2O7 = 183
Units?
Same numbers can be used to calculate the
mass of a single molecule or a MOLE of
molecules.
Single molecule unit: amu
MOLE of molecules: grams per mole
Relative Masses
• A golf ball weighs 17 times more than a
ping pong ball.
• 10 golf balls will be 17 times heavier than
10 ping pong balls
• 1000 golf balls will be 17 times heavier
than 1000 ping pong balls
• The relative mass is the same no matter
how many you have.
Relative Masses
• The masses on the periodic table are
relative masses.
• One carbon atoms weighs 12 times more
than one hydrogen atom
• 10 carbon atoms weighs 12 times more
than 10 hydrogen atoms
• 1000 carbon atoms weighs 12 times more
than 1000 hydrogen atoms
Molar Mass
12 g of carbon-12 contains 6.02 x 1023
atoms of carbon-12. This number is known
as Avogadro’s constant.
One mole of any substance contains
6.02 x 1023 representative particles. The
molar mass (M) of any substance is
calculated using the same numbers as
relative atomic/molecular/formula mass with
the units of grams per mole.
Measuring Matter
There are three ways to measure matter: by
counting representative particles (typically
molecules or formula units), by mass (in
grams), or by volume (in liters for gases).
The method used is usually chosen by the
ease of each method and the information
needed. Once a measurement has been
made, it is possible to convert between the
units for the other methods.
Measuring Matter
1 mole of a compound
1 mole of an element
1 mole of a compound
1 mole of a gas
= molar mass in grams
= 6.02 x 1023 atoms of that element
= 6.02 x 1023 representative particles
=
22.4 liters
The mole is the link between grams, the number of
representative particles and liters!
For neutral compounds, the representative particle is either the
molecule (covalent) or the formula unit (ionic).
Mole Road
Map
Mole Practice: One Step Conversions: Remember to
show all work using dimensional analysis.
1. How many moles are in 18.0 grams of
sugar (C6H12O6)?
Mole Practice: One Step Conversions: Remember to
show all work using dimensional analysis.
2. What is the weight in grams of 4.50
moles of barium sulfide?
Mole Practice: One Step Conversions: Remember to
show all work using dimensional analysis.
3. How many moles are in 3.90 x 1028
molecules of methane (CH4)?
Mole Practice: Multi Step Conversions:
7.How many formula units are in 198.5
grams of sodium chloride?
Mole Practice: Multi Step Conversions:
9. How many grams are in 3.21 x 1024
formula units of potassium hydroxide?