How do we use the mole?
Chemistry Unit 9: Guided Notes
New Skills






Convert mass, moles and number of atom/molecules
Use compound formulas to calculate molar mass
Determine percent composition of a compound
Determine empirical and molecular formulas of a compound
Calculate the formula of a hydrate
Apply mass and mole conversions to balanced reactions
Academic Language
Avogadro’s number
Conversion factor
Crystal lattice
Empirical formula
Hydrate
Molar mass
Mole
Molecular formula
Molecule
Percent by mass
Percent composition
Representative particle
1
Unit 9 Homework and book practice problems:
CALM: http://calm.indiana.edu/
9.1 Measuring Matter
 10 CALM questions
 p322 #1-4
 p324 #5-6, 7-14
9.2 Mass and the Mole





10 CALM questions
p328 #15-16
p329 #17-18
p331 #19-21
p332 #22-27
9.3 Moles of Compounds




10 CALM questions
p335 #29-36
p336 #37-41
p339 #42-46
9.4 Empirical and Molecular Formulas and Hydrates






10 CALM questions
p344 #54-57
p348 #58-61
p350 #62-66
p353 #74-75
p354 #76-82
9.5 Accumulating Content and Skills
 10 CALM questions
2
Chemistry Unit 9: Learning Goals and Objectives
9.1 Measuring Matter – Chemists use the mole to count atoms, molecules, ions and formula
units.
o Explain how a mole is used to indirectly count the number of particles of matter.
o Relate the mole to a common everyday counting unit.
o Convert between moles and number of representative particles.
9.2 Mass and the Mole – A mole always contains the same number of particles, however,
moles of different substances have different masses.
o Relate the mass of an atom to the mass of a mole of atoms.
o Convert between number of moles and the mass of an element.
o Convert between number of moles and number of atoms of an element.
9.3 Moles of Compounds- The molar mass of a compound can be calculated from its
chemical formula and can be used to convert from mass to moles of that compound.
o
o
o
o
Recognize the mole relationships shown by a chemical formula.
Calculate the molar mass of a compound.
Convert between the number of moles and mass of a compound.
Apply conversion factors to determine the number of atoms or ions in a known mass
of a compound.
9.4 Empirical and Molecular Formulas and Hydrates – A molecular formula of a
compound is a whole-number multiple of its empirical formula
o Explain what is meant by the percent composition of a compound.
o Determine the empirical and molecular formulas for a compound from mass percent
and actual mass data.
o Explain what a hydrate is and relate the name of the hydrate to its composition
o Determine the formula of a hydrate from laboratory data.
9.5 Accumulating Content and Skills:– Chemistry content is continuous and builds on prior
knowledge and skills. This section will combine this unit with previous units.
o
Apply knowledge and skills from previous units to content learned in this unit.
3
9.1 Measuring Matter – Chemists use the mole to count atoms, molecules, ions and
formula units.
Objective: Explain how a mole is used to indirectly count the number of particles of matter.
Objective: Relate the mole to a common everyday counting unit.
Mole – (mol) – SI base unit for measure of amount of a substance. 6.0221367 x 1023



Called Avogadro’s number – Italian physicist who in 1811, determined the volume of
1 mol of gas.

The Mole: A good comparison
The mole is a number. What other unit is used in a similar manner?



A dozen flowers, doughnuts or eggs.
A baker’s dozen of cookies or bagels.
A pair of socks or friends
If you have a dozen flowers and a dozen eggs, do they weigh the same?
Objective: Convert between moles and number of representative particles.
The mole as a conversion factor - In order to convert between moles and number of
particles we need to use the ratio of equivalent values (conversion factor) to express the
same quantity in different units.
Example Problem 1: How many particles are in 3.5 mols?
Example Problem2: How many moles of atoms are in 9.63 x 1026 atoms?
***p322 #1-4, p324 #5-6, 7-14***
4
9.2 Mass and the Mole – A mole always contains the same number of particles, however,
moles of different substances have different masses.
Objective: Relate the mass of an atom to the mass of a mole of atoms.
Objective: Convert between number of moles and the mass of an element.
Molar mass - the mass in grams of one mole of any pure substance.


Example Problem 3: If I need 3 mols of Cu, how do I measure the amount?
Example Problem 4: I measured 5.0g of Iron, how many atoms do I have?
Objective: Convert between number of moles and number of atoms of an element.
grams
= molar mass
mol
moles grams grams
x
=
mol
grams moles moles
x
=
gram
5
Example Problem 5: How many atoms of gold are in a U.S. Eagle bullion coin with a
mass of 31.1g?
Example Problem 6: How much does 5.8 x 1015 atoms of lead weigh?
***p328 #15-16; p329 #17-18, p331 #19-21; p332 #22-27***
9.3 Moles of Compounds- The molar mass of a compound can be calculated from its
chemical formula and can be used to convert from mass to moles of that compound.
Objective: Recognize the mole relationships shown by a chemical formula.
Objective: Calculate the molar mass of a compound.
Steps:
1.
2.
3.
4.
Count the number of atoms in each molecule.
Find the molar mass of each atom.
Multiply the molar mass of each atom to the number of atoms in a compound.
Add the total molar masses together.
Example Problem 7: Find the molar mass of the following compounds/molecules.
H2O
NaCl
H2SO4
Al2O3
6
Fe2(SO4)3
CCl2F2
Objective: Convert between the number of moles and mass of a compound.
Objective: Apply conversion factors to determine the number of atoms or ions in a known mass
of a compound.
Steps:
1. Find the molar mass of the compound.
2. Use molar mass and the mole as conversion factors to get the units needed.
Example Problem 8: What is the mass of 2.5 mols of (C3H5)2S?
7
Example Problem 9: Calculate the number of moles of Ca(OH)2 in 325 g of the
compound?
Example Problem 10: How many atoms are in 212g of water?
**p335 #29-36; p336-6 #37-41; p339 #42-46***
9.4 Empirical and Molecular Formulas – A molecular formula of a compound is a
whole-number multiple of its empirical formula
Objective: Explain what is meant by the percent composition of a compound.
Percent Composition – the percent by mass of each element in a compound.
Steps to determine percent composition of a compound:
1. Assume 1 mole of a compound.
2. Calculate molar mass of each element in the compound.
3. Use each element’s molar mass to calculate percent by mass.
Percent by Mass is a description of the amount of an element in a compound
æ mass of 1 mole of element ö
%mass = ç
è molar mass of compound ÷ø
8
Example Problem 11: What is the percent by mass of each element in NaHCO3?
Objective: Determine the empirical and molecular formulas for a compound from mass percent
and actual mass data.
Empirical formula – smallest whole number ratio of elements in a compound.


May or may not be the same as the actual molecular formula.
o
o Hydrogen peroxide: HO- empirical formula, H2O2 – actual formula (molecular
formula)
Steps to figure empirical formula from percent composition:
1. Assume an overall 100g sample of the compound.
2. Each element’s percentage can be used as mass in calculations.
3. Use this ‘mass’ to convert to moles. This provides a ‘mole ratio’ for the compound.
o Since these mole ratios are not whole numbers, we convert them to whole
numbers that can be used as subscripts by dividing them all by the smallest
ratio. (We assume the smallest mole ratio is a 1 in the compound)
Example Problem 12: A compound has the following mass percentages: C – 48.64%,
H – 8.16%, O – 43.20%
What is the empirical formula for this molecule?
9
Molecular formula – specifies the actual number of number of atoms of each element in one
molecule/formula unit of the substance.
Steps to determine the molecular formula:
1. Determine the molar mass of the empirical formula.
2. Determine the molar mass of the actual compound. (might be given to you)
3. Divide the molar mass of the actual compound by the molar mass of the empirical
formula
4. Multiply all subscripts of the empirical formula by this molar mass ratio.
Example Problem 13: The mass of benzene has been experimentally determined to
be 78.12g. We know that benzene is 92% C by mass and 8% H by mass. What is the
molecular formula of benzene?
***p344 #54-57, p348 #58-61, p350 #62-66***
10
Objective: Explain what a hydrate is and relate the name of the hydrate to its composition.
Hydrates – solid ionic compounds in which water molecules are trapped.



The number of water molecules associated with each molecule is written following a
dot after the molecular formula:
o Na2CO310H2O

Names of these compounds are named with a prefix representing the number of
water molecules and the word hydrate.
o Na2CO3  10H2O – sodium carbonate decahydrate
o Prefixes are the same as the ones used in naming covalent compounds.
Objective: Determine the formula of a hydrate from laboratory data.
Anhydrous – a compound without water.

11
Steps to determining the formula of a hydrate:
1. Determine the initial mass of the compound prior to heating.
2. Determine the final mass of the compound after heating.
3. The final mass is used to determine the number of moles of the anhydrous
compound. Grams to moles
4. Calculate the difference of the initial mass and the final mass and use this mass to
determine number of moles of water, grams to moles
5. Number of hydrates per compound molecule (molar ratio – hydrate: anhydrous) =
moles of H2O/moles of compound
Example Problem 14: A mass of 2.50g of blue, hydrated copper sulfate (CuSO4 
xH2O) is placed in a crucible and heated. After heating, 1.59g of white anhydrous
copper sulfate (CuSO4) remains. What is the formula for the hydrate? Name the
hydrate.
***p353 #74-75; p354 #76-82***
12
9.5Accumulating Content and Skills:– Chemistry content is continuous and builds on
prior knowledge and skills. This section will combine this unit with previous units.
Objective: Apply knowledge and skills from previous units to content learned in this unit.

How does the mole apply to balanced equations?

Example Problem 15: How many grams of each reactant are needed to run the
following reaction? How many grams of each product are produced? (Hint:
complete, balance, convert)
CuSO45H2O(aq) + CaCl2(aq) 

Example Problem 16: From the reaction above, how much is needed of each reactant
in the net ionic equation to produce the balanced amount of the precipitate? How
much precipitate is produced?
13
Key Concepts

The mole is a unit used to count particles of matter indirectly. One mole of a pure substance
contains Avogadro’s number of particles.

Representative particles include atoms, ions, molecules, formula units, electrons, and
other similar particles.

One mole of carbon-12 atoms has a mass of exactly
12 g.

Conversion factors written from Avogadro’s relationship can be used to convert
between moles and number of representative particles.

The mass in grams of 1 mol of any pure substance is called its molar mass.

The molar mass of an element is numerically equal to its atomic mass.

The molar mass of any substance is the mass in grams of Avogadro’s number of
representative particles of the substance.

Molar mass is used to convert from moles to mass. The inverse of molar mass is used
to convert from mass to moles.

Subscripts in a chemical formula indicate how many moles of each element are
present in 1 mol of the compound.
14

The molar mass of a compound is calculated from the molar masses of all of the
elements in the compound.

Conversion factors based on a compound’s molar mass are used to convert between
moles and mass of a compound.

The percent by mass of an element in a compound gives the percentage of the
compound’s total mass due to that element.

The subscripts in an empirical formula give the smallest whole-number ratio of
moles of elements in the compound.

The molecular formula gives the actual number of atoms of each element in a
molecule or formula unit of a substance.

The molecular formula is a whole-number
multiple of the empirical formula.

The formula of a hydrate consists of the formula of the ionic compound and the
number of water molecules associated with one formula unit.
The name of a hydrate consists of the compound name and the word hydrate with a
prefix indicating the number of water molecules in 1 mol of the compound.


Anhydrous compounds are formed when hydrates are heated.
15