RELATING AMU TO
GRAMS
Atomic Mass vs. Molar Mass
• Atoms, ions, and molecules are too
small to directly measure in atomic
mass units
• A mass in grams is an easy unit to
measure!
• The number of atoms of any element
needed to equal its atomic mass in
grams will always be 6.022 x 1023
atoms
Why Does This Relationship Between
amu and Grams Work?!
• We know that by definition, a mole of carbon-12 atoms
weighs exactly 12 grams
• Therefore, to measure out a mole’s worth of carbon-12 atoms, you
would weigh out exactly 12.00 g of carbon-12
• This number is called the molar mass
• So, molar mass is the mass of a mole of atoms in grams
• Units are grams/mole (g/mol)
For any element:
Atomic mass (in amu) = Molar mass (in grams)
In Summary…
1 atom of 12C = 12 amu
1 mole of 12C atoms = 12 g
12 g of 12C = 6.022 x 1023 atoms
Moles and Masses
• Note, atoms come in different sizes and masses
• As a result, a mole of atoms of one type would
have a different mass than a mole of atoms of
another type
H
1.008 grams / mol
O
16.00 grams / mol
Pb
207.2 grams / mol
• We rely on a straight forward system to relate mass and
moles
Molar Mass from the Periodic
Table
Basic Chemistry
Copyright © 2011 Pearson Education, Inc.
Practice – Determining the Molar Mass of
an Element
• What is the molar mass of the following elements?
• Strontium
• Silver
• Tellurium
• Lithium
• Chlorine
DETERMINING THE MOLAR
MASS OF COMPOUNDS
Some One Mole Quantities
Determining the Molar Mass of a
Compound
• Scientists also use the Periodic Table to determine the molar
mass (also known as the formula weight) of compounds
• We just observed that one mole of different substances look
different
• Note that even though there is the same quantity of atoms or
molecules in a mole of any substance, each substance has a unique
molar mass!
• Why is that?
• It’s because the molar mass of the compound is the sum of the molar
masses of each element times the number of atoms of that
element in the compound
• Let’s review on how to count number of atoms in a compound!
• Remember, superscripts are charges and subscripts tell you how
many atoms you have
Practice with Counting Atoms in a
Compound
2 Sr3(PO4)2
Sr =
P=
O=
3 N4O10
N=
O=
Ca(C2H3O2)2
Ca =
C=
H=
5 Al2(SiO3)3
Al =
Si =
O=
O=
12
How do We Calculate the Molar Mass of a
Compound?
Basic Chemistry
Copyright © 2011 Pearson Education, Inc.
Practice!
• Calculate the molar mass of CaCl2
Element
Number
of Moles
Molar Mass
Total Mass
Ca
1
40.08 g/mol
40.08 g
Cl
2
35.45 g/mol
70.90 g
CaCl2
110.98 g
Practice!
• Calculate the molar mass for aluminum hydroxide with 4
sig figs:
Solution
STEP 1 Obtain the molar mass of each element.
• Al 26.98 g/mol
O 16.00 g/mol
H 1.008 g/mol
STEP 2 Multiply each by the subscript
• 1 mol Al x 26.98 g/mol Al
=
26.98 g
• 3 mol O x 16.00 g/mol O
=
48.00 g
• 3 mol H x 1.008 g/mol H
=
3.024 g
STEP 3 Calculate the molar mass by adding the
masses of the elements.
1 mol of Al(OH)3 = 78.00 g
USING DIMENSIONAL
ANALYSIS TO CALCULATE
MASS, MOLES, AND # OF
PARTICLES
Calculating with Moles
• As we have seen so far, the amount of a substance and
its mass is related
• Therefore, it is often useful to convert from grams to moles to
particles!
• You can use conversion factors to relate units (aka
dimensional analysis)!
• Depending on which conversion factor you use, you can solve for
either the amount of a substance (in particles or moles) or its mass!
General Format For Dimensional Analysis
Unit to Keep
Unit to Keep
Given Unit ×
×
× …
Unit to Discard
Unit to Discard
= Wanted Unit
How Do We Convert Using the Mole?!
• We use molar mass conversion factors!
• These conversion factors relate grams and moles of an element or compound
• Derived from EQUIVALENT STATEMENTS of molar mass
• Example – What is the molar mass of methane, CH4?
1 mole CH4 = 16.04 g
• Each equivalent statement produces TWO conversion factors
• Example:
1 mol CH4
16.04 g
OR
16.04 g
1 mol CH4
• You now need to decide which conversion factor you should use in
your calculation
• Are you trying to convert to moles? Or are you trying to convert to grams? Or
are you tying to convert to number of particles?
• Choose the one that will help you discard the unwanted unit!
A Handy Guide for Converting Between
Mass and Moles
Moles are the
HEART of
chemistry!
TIME FOR PRACTICE!
Pages 87-88