STOICHIOMETRY
Moles and Molar Mass
You should already be able to…
• Identify elements by name and symbol
• Calculate molar masses of elements and
compounds
By the end of this video you should be able to….
• Use dimensional analysis to calculate the number
of a moles of particles in a sample.
• Use dimensional analysis to calculate the mass of a
given sample.
The Mole represents
6.02x1023 particles
such as atoms and
molecules.
The mole is based on
the fact that 12 grams
of Carbon-12 has a
mole of atoms.


Molar Mass (aka gram formula mass or
molecular mass) is the mass of one mole of a
substance.
Element’s molar masses are reported on the
periodic table.
1.
What is the molar mass of iron?
2.
What is the molar mass of copper?
55.85 g/mol
63.55 g/mol
1. What is the molar mass of water?
H2O = 2(1.008) + 16.00 =
18.016g/mol
2. What is the gram-formula-mass of calcium
chloride?
CaCl2 = 40.1 + 2(35.5) =
111.1 g/mol
1.
Calculate the mass of 6.70 moles of Neon.
2. Calculate the mass of 0.023moles of NH3.
3. Determine the number of moles in
0.567grams of Helium.
4. Determine the number of moles in 8.0 grams
of CCl4.
Can you calculate the moles of an element or
compound using dimensional analysis?
Don’t cheat yourself of understanding… take
out a pen and paper and calculator and really
try to set this up!




Start with the number given as a numerator
over 1.
MULTIPLY by a new fraction
Place a conversion with the same unit on the
denominator of the new fraction.
Multiply numerators, divide denominators.
(maybe pause
and copy all
this?)
Use dimensional analysis to calculate the number
of a moles of particles in a sample.
• Use dimensional analysis to calculate the mass of a
given sample.
•
Molar Volume and Avogadro
You should already be able to…
• Identify elements by name and symbol
• Use dimensional analysis to calculate moles and
masses of elements and compounds
By the end of this video you should be able to….
• Use dimensional analysis to calculate the number
of a moles of particles in a sample given the
number of atoms or molar volume.
• Use dimensional analysis to calculate the molar
volume or number of particles of a given sample.
mass
• Gram
formula
mass
volume
particles
• Gases at
STP: use
22.4L
• Atoms or
molecules:
use 6.02x1023
1. Calculate the number of molecules in 4.37
moles of Lithium sulfide.
2. Calculate the number of atoms in question 1.
3. What is the volume of 7.89 moles of He gas at
STP?
4. How many moles of atoms are present in
79.80 Liters of diphosphorous pentoxide gas at
STP?
5. How many moles are present in 5.67x1024
molecules of H2O(l)?
Can you calculate the moles, liters, or number
of particles of an element or compound using
dimensional analysis?
Don’t cheat yourself of understanding… take
out a pen and paper and calculator and really
try to set this up!
Why do chemists use moles to measure
substances? Why aren’t grams, liters and
molecules enough?
Use dimensional analysis to calculate the number
of a moles of particles in a sample given the
number of atoms or molar volume.
• Use dimensional analysis to calculate the molar
volume or number of particles of a given sample.
•
Stoichiometry
You should already be able to…
• Use dimensional analysis to calculate the number
of a moles of particles in a sample.
• Use dimensional analysis to calculate the mass,
volume or number of particles of a given sample.
By the end of this video you should be able to….
• Use dimensional analysis to calculate multistep
mole problems.
1. Calculate the volume of 13.80 grams of Ar gas
at STP.
2. Calculate the mass of 2.5L of oxygen gas at
STP.
3. Calculate the number of atoms of P in 120.0
grams of P2O5.
Can you calculate the moles of an element or
compound using dimensional analysis?
Don’t cheat yourself of understanding… take
out a pen and paper and calculator and really
try to set this up!

Many AP questions will give you information
about a reactant used in a reaction and ask
you about the product formed. Use the
balanced equation to create a mole ratio to
convert between moles of substances.
4. Water decomposes into its elements when
electrified at high voltages. If 24.0 grams of
water is completely decomposed, how many
grams of oxygen will be produced?
5. Lithium and magnesium chloride react
according to the equation below. Calculate the
number of atoms of magnesium that form
when 2.50 moles of lithium are used.
Li + MgCl2  LiCl + Mg
Can you calculate the moles of an element or
compound using dimensional analysis?
Don’t cheat yourself of understanding… take
out a pen and paper and calculator and really
try to set this up!
•
Use dimensional analysis to calculate the hardest
AP stoichiometry problems 
Theoretical and Percent Yield
You should already be able to…
• Use dimensional analysis to calculate the number
of a moles of particles in a sample.
• Use dimensional analysis to calculate the mass,
volume or number of particles of a given sample.
By the end of this video you should be able to….
• Use dimensional analysis to calculate the
theoretical and percent yield of a product in a
reaction.

Given:
20

1.
11
36
1 sandwich =
2
+1
2.
+3
How many
sandwiches can I
make if I used all of
the bread with
unlimited other
resources?
How many
sandwiches can I
make if I used all of
the cheese with
unlimited other
resources?

Given:
20

11
36
1 sandwich =
2
+1
+3
3. How many
sandwiches can I
make if I used all of
the ham with
unlimited other
resources?
4. How many
sandwiches can I
make if I used only
what is given?
5. What ingredient was
the limiter? Why?
1. S + 3F2  SF6
Suppose you have 4 moles of sulfur and 2
moles of fluorine, which is the limiter and
which is in excess? How many moles of SF6
can be produced?
4S produces 4 SF6
2F2 produces 0.6 moles of SF6
(F2 Limiter!)
2. 2NH3 + CO2  (NH2)2CO + H2O
If 637.2 grams of ammonia react with 1142
grams of carbon dioxide, find the limiter and
the amount of urea produced.
3. In question 2, how much of the excess is left
over after the reaction goes to completion?
4. 5Ca + V2O5  5CaO + 2V
1.96x103 grams of calcium react with 1.54x
103 grams of V2O5. Find the limiter and the
amount of V produced.
5. If 803 grams of V are actually made calculate
the percent yield.
Can you calculate the theoretical and percent
yield of a product given a reactant?
(I hope you have these
two words written with
definitions on your notes)
•
Use dimensional analysis to calculate the
theoretical and percent yield of a product in a
reaction.
Percent Composition, Empirical and Molecular Formulas
You should already be able to…
• Calculate the percent composition of an element in
a compound.
• Reduce formulas to find the empirical formula.
• Calculate the molecular formula of a compound.
By the end of this video you should be able to….
• Calculate the empirical and molecular formula of a
compound given the percent by mass of its
elements.
% By mass: mass part x 100
total mass
1.
Find the % by mass of phosphoric acid.
H3PO4
3(1.0)
x 100
3(1.o)+32.1+4(16.0)
= 3.02%
Find the percent by mass of Ca in Ca(OH)2.
40.0
x 100
40.0+2(16.0)+2(1.0)
= 54.1%
A sample of a substance containing only
magnesium and chlorine was tested in the
laboratory and found to be composed of
74.5% chlorine by mass. If the total mass
of the sample was 190.2 grams, what is
the mass of the magnesium?
0.745 (190.2) = 141.7 grams
Can you calculate the percent composition of a
sample?

Empirical formula refers to any molecular
formula in it’s reduced form.
Are these empirical? If not, reduce them:
1.
2.
3.
4.
C4H 8
C6H12O6
N3O6
Na2(OH)2
1. CH2
2. CH2O
3. NO2
4. NaOH

Molecular Formulas are some multiple of the
empirical formula.
 If the empirical formula is CH4 a molecular formula
could be CH4, C2H8, C3H12 etc.

A compound whose empirical formula is NH3
has a mass of 34 g/mol. What is the empirical
formula?
NH3 = 17.0 g/mol 34/17 = 2
2NH3 = N2H6




Find the mass of the empirical formula.
Divide the mass given by the empirical mass.
Distribute your answer through the empirical
formula.
If a compound has a mass of 45 g/mol and an
empirical formula of CH3, what is the
molecular formula?
CH3 = 15 g/mol 45/15 = 3
3CH3 = C3H9
Copy this?
Rules:
1.
Convert the % to grams,
assuming the total mass of
the sample is 100 grams.
2.
Convert mass to moles,
using atomic masses.
3.
Divide each element’s moles
by the smallest number of
moles.
4.
Find a whole number ratio.
Find the empirical formula of a
compound with 40.92% C,
4.58% H and 54.50% O by
mass.
40.92g C, 4.58g H, 54.50g O
40.92/12 = 3.40 moles C
4.58/ 1 = 4.58 moles H
54.50/16 = 3.4o moles O
1 mole C, 1.5 mole H, 1 mole O
C2H3O2
1.
A sample of nitrogen and oxygen contains
1.52 g N and 3.47g O and a molar mass of
about 90 grams. Find the empirical and
molecular formulas.
2. Find the empirical and molecular formula of
a 30 gram sample with 6.44 grams of Boron and
1.803 grams of hydrogen.
3. A sample of a compound contains 47.0%
silicon and the rest is oxygen. If the total
mass is 180 grams, what is a possible set of
empirical molecular formulas of the
compound?
a. SiO2 and SiO
b. Si4O4 and SiO
c. Si2O4 and SiO2
d. Si3O6 and SiO2
Can you determine the empirical and
molecular formulas of a compound?
•
Calculate the empirical and molecular formula of a
compound given the percent by mass of its
elements.
Waters of hydration
You should already be able to…
• Identify hydrate formulas.
• Calculate the molar mass of a hydrate.
• Calculate percent by mass of a sample.
By the end of this video you should be able to….
• Explain the hydrate lab set up in detail.
• Calculate the moles of water in a hydrated sample.


Hydrates contain a specific number of moles
of water attached to a molecule: CuSO4.5H2O
When the water is heated off, the new
substance is known as an anhydrate.
The words are red because they are vocab words!!!!

The reaction must be performed in a crucible
on a clay triangle on a ring stand.
• Clean and heat crucibles
before use
• Careful they get really hot!
• Always reheat your sample
to ensure all water has left
the hydrate
• Record all measurements!

Once the hydrate is heated, you can calculate
the moles of water that evaporated, and figure
out the formula. The steps are as follows:
1. Calculate the mass of hydrate, anhydrate and
water by subtraction.
2. Find the moles of anhydrate and water
separately.
3. Find the mole ratio of anhydrate to water (1 to ?).
Hint: write this down so you have it for the lab!
 Data:
 Mass of crucible: 28.00 g
 Mass of crucible and hydrate: 29.60 g
 Mass of crucible and anhydrate: 29.24 g
 Calculate the hydrate formula and
the percent water if the anhydrate is
CuSO4.







29.60-28.00 = 1.60 g of hydrate
29.24-28.00 = 1.24 g of anhydrate
1.60-1.24 = 0.36 g of water
0.36/1.60 * 100 = 23% water
0.36/18.016 = 0.1998 moles of water
1.24/159.6 = 0.007769moles of anhydrate
CuSO4.3H2O
Can you explain what a hydrate and an
anhydrate are and how to calculate the
amount of water in it?
•
•
Explain the hydrate lab set up in detail.
Calculate the moles of water in a hydrated sample.
Molarity and Dilutions
You should already be able to…
• Calculate the molarity of a solution to determine if
it is relatively concentrated or dilute.
• Use the dilutions formula to calculate the
concentration of a solution after water has been
added.
By the end of this video you should be able to….
• Use the dilutions formula to calculate the
concentration of a solution before or after water
has been added.
Concentrated
solutions contain
large amounts of
solutes dissolved in
the solvent.
 Dilute solutions
contain small
amounts of solutes
dissolved in solvent.

Which is
concentrated?



The amount of solute (dissolvable substance)
in a solvent (does the dissolving like water) is
known as concentration and can be
represented many ways.
Molarity is the moles of solute per liter of
solution. Find this on your reference tables!
The units for molarity are mol/L or mol.L-1 or
just M.
If you don’t know what this means,
maybe write a note to ask me in
class…
M=

mol of solute
L of solution
Because volume is temperature dependent,
molarity can change with temperature.
1.
If 3.0 moles of NaCl are dissolved in 6.0L of
water, what is the molarity?
3moles/6L =
2.
0.50M
If 29 grams of NaCl are dissolved in one liter of
water, what is the molarity?
29g/58g = 0.50moles
0.5moles/1L = 0.50M
3.
If 100. grams of KF are dissolved in 300.ml of water, what is
the molarity?
100g/58.1g =1.72mol
4.
1.72mol/.300L =5.70M
Calculate the volume needed to create a 2.0M solution with
3.5 moles of Li2O.
X = 1.8L
2.0M=3.5mol/x
If you have 3.12g of HCl and 250mL of H2O,
what is the molarity of the acid? If 250mL of
H2O is added again, how does the molarity
change?
M decreases by half
Can you calculate the concentration of a
solution?


If the glass of iced tea I made is too strong for
you, what could you do to make it weaker?
This process is known as dilution. It is very
important in chemistry because we often
purchase large quantities of highly
concentrated chemicals. In order to use the
chemicals in class, for labs, teachers must
dilute the chemicals so they aren’t as
dangerous.
A dilution is a procedure for preparing a less
concentrated solution from a more
concentrated (or stock) solution. Use the
formula:
M1V1 = M2V2
1.
240mL of a 3.5M solution is diluted to 300
mL of solution. Calculate the new molarity.
3.5(240) = x(300)
2.
2.8M
7.8L of 2.0M solution is diluted by the
addition of 10.0L of water. Calculate the new
molarity.
2.0(7.8) = x(17.8)
0.88M
3.
What volume of 9.3M H2SO4 is needed to
obtain 450mL of 2.0M H2SO4?
9.3(x) = 2.0(450)
4.
97mL
What always happens to the molarity when
water is added?
M decreases
What would you have to do in order to have the
molarity increase?
Increase moles or evaporate water
Can you calculate the concentration of a
diluted solution?
•
Use the dilutions formula to calculate the
concentration of a solution before or after water
has been added.
Harder Examples of Dilutions
You should already be able to…
• Calculate the molarity of a solution to determine if
it is relatively concentrated or dilute.
• Use the dilutions formula to calculate the
concentration of a solution after water has been
added.
By the end of this video you should be able to….
• Use the dilutions formula to calculate the
concentration of a solution before or after water
has been added in harder AP problems 
1.
2.
A lab calls for 100mL of 3.oM HCl per pair of
students. The stock room has a large
amount of 12.0M HCl and 1.0M HCl. Which
acid will you use to set up the lab?
If the classroom has 26 students, explain
how to create enough 3.0M acid for the lab.
Use calculations to support your answer.
2HCl + Ca(OH)2  2H2O + CaCl2
If 250mL of 5M HCl is reacted with 300mL of
4.5M Ca(OH)2 according to the equation above,
what is the limiting reactant and how many
moles of CaCl2 are produced when the product
is evaporated to dryness?
Write this!

If a question asks you how to dilute a solution make
sure you mention…
 Measure the amount of water needed using a pipet (it is
pretty accurate and if not use the cylinder). Place into the
volumetric flask (most accurate!!!)
 Start with the highest molarity of solute: Measure the
concentrated solution with the pipet (or cylinder).
 Always add the acid to the water (the acid is more dense
and will sink to the bottom resulting in less splashing and
can be added slowly to reduced the amount of heat
generated at a time).
 Add small drops of water using a dropper until the
solutions meniscus reaches the mark on the flask.
Can you explain how to dilute a solution?
•
Use the dilutions formula to calculate the
concentration of a solution before or after water
has been added in harder AP problems 
Harder Examples of AP Stoichiometry
You should already be able to…
• Calculate moles, mass, volume, number of
particles, percent yield, percent composition, and
concentration.
• Determine empirical and molecular formulas.
• Balance and identify the type of reaction.
By the end of this video you should be able to….
• Feel confident about AP Stoichiometry problems 
(a)
Calculate the mass percent of carbon in the hydrated form of the
solid that has the formula BeC2O4•3H2O.
(b)
When heated to 220.°C, BeC2O4•3H2O(s) dehydrates completely
as represented below. If 3.21 g of BeC2O4•3H2O(s) is heated to
220.°C calculate the mass of BeC2O4(s) formed and the moles of
water formed.
BeC2O4•3H2O(s)  BeC2O4(s) + 3 H2O(g)
(a)
The hydrocarbon burns completely, producing 7.2 grams of
water and 7.2 liters of CO2 at standard conditions. What is
the empirical formula of the hydrocarbon?
(b)
Calculate the mass in grams of O2 required for the
complete combustion of the sample of the hydrocarbon
described in (a).
•
Feel confident about AP Stoichiometry problems 
•
Or review your brain off and come to extra help
with specific questions!