Stoichiometry
Chapter 11 & 12
I. Things you should remember
• From the Moles Unit:
• Identify particles as atoms, molecules (mc), and
formula units (fun)
• 1 mole = 6.02 x 1023 atoms, molecules, or
formula units
• 1 mole atom = mass (in grams) from the
periodic table
• From the Naming & Formulas Unit:
• How to write a formula given a chemical name
• From the Chemical Reactions Unit:
• How to write a chemical equation given words
• Balancing equations
Example 1:
Determine the moles of magnesium there
are in 1.23 x 1024 atoms of magnesium
G: 1.23 x 1024 atoms Mg
W: moles Mg
R: 1 mole Mg = 6.02 x 1023 atoms Mg
1.23 x 1024 atoms Mg
1
1 mole Mg
6.02 x 1023 atoms Mg
= 2.04 mol Mg
Example 2: Determine the mass of 3.50
mol of copper.
From the p.t  Copper: 63.546g Cu = 1 mol Cu
3.50 mol Cu 63.546g Cu
1
1 mol Cu
= 222 g Cu
Example 3: Calcium and sodium
carbonate react together to form
calcium carbonate and sodium
Ca +
+ Na
+
Na
2CO3
Na2CO3
2+
Ca
2CO3
CaCO3
Example 4: Balance
Li2O(s) + H2O(l)  LiOH(aq)
Li2O(s) +
H2O(l)  2 LiOH(aq)
Practice
1. Determine the number of water
molecules in 11.2 moles of water.
G: 11.2 moles water
W: mc water
R: 1 mole H2O = 6.02 x 1023 mc H2O
11.2 moles H2O
1
6.02 x 1023 molecules H2O
1 moles H2O
= 6.74 x 1024 molecules H2O
Practice
2. Determine the number of moles in
11.9 kg of aluminum.
G: 11.9 kg Al
W: mol Al
R: 1 mole Al = 26.982g Al
1 kg Al = 1000g Al
From the p.t  Aluminum: 26.982g Al = 1 mol Al
11.9 kg Al
1
1000g Al
1 mol Al
1 kg Al
26.982g Al
= 441 mol Al
Quick Review:
Writing Formulas for
Ionic Compounds
A reminder of what a chemical formula
tells us:
EX: aluminum carbonate
3+
Al
2CO3
Al2(CO3)3
in 1 formula unit of
aluminum carbonate:
Al3+
Al2(CO3)3
CO32-
CO32Al3+
2CO
2 moles aluminum 2 atoms aluminum
3
3 moles carbons
9 moles oxygens
3 atoms carbons
9 atoms oxygens
Al2(CO3)3
Al
Al
2 moles Al
3 moles CO3
II. Quantifying
Chemical
Compounds
II. Quantifying Chemical Compounds
We will now do the same thing, but we will
have questions that are more detailed about
the compounds. First we need to know how
to find the molar mass of the whole
compound.
I. Molar Mass
1. Definition
• Remember, molar mass is the mass (in
grams) of one mole of an element or
compound.
2. Determining Molar Mass
• The molar mass of the compound is the sum
of the molar masses of each atom in the
compound.
• The units for molar mass are grams per mole
(g/mol).
Grams
mole
Ex: Determine the molar mass of
sulfur trioxide.
SO3
+
X3
=
(47.997 g)
80.063 g/mol or 80.063 g = 1 mole SO3
For the sake of consistency…
When calculating molar masses, use all the sig
figs presented on your periodic table
Let’s show all of our molar masses to the
thousands place (you should know it’s not
going to make a huge difference)
Ex: 18.015g/mol
But, show our final final stoichiometry answers
in 3 sig figs
Ex: Determine the molar mass of
calcium nitrate.
Ca(NO3)2
+
2
=
164.088 g/mol or 164.088 g = 1 mol Ca(NO3)2
Ex: Determine the molar mass of
calcium nitrate.
Ca(NO3)2
+
X 2+
X6=
164.088 g/mol or 164.088 g = 1 mol Ca(NO3)2
Ex: Determine the molar mass of
Iron(III) sulfate.
3+
Fe
3+
Fe
First you need the right
formula right?
2)
(SO4
2(SO4)
2(SO4)
Fe2(SO4)3
Ex: Determine the molar mass of
Iron(III) sulfate.
Fe2(SO4)3
X 2+
X 3+
X 12
399.88 g/mol or 399.88 g = 1 mol Fe2(SO4)3
Practice:
1. Determine the molar mass of each of
the following.
a. Li2S 45.948 g/mol Li2S
b. (NH4)2CO3 96.086 g/mol (NH4)2CO3
C. magnesium hydroxide
58.319 g/mol Mg(OH)2
d. copper (II) iodide
317.354 g/mol CuI2
3. Using Molar Mass in Calculations
You would use the molar mass of a compound
in dimensional analysis just like you did with
elements.
Ex: Determine the mass of 48.6 moles
of sodium chloride.
Step 1: Write the formula for sodium chloride.
+
Na
Cl
NaCl
Ex: Determine the mass of 48.6 moles
of sodium chloride.
Step 2: set up a dimensional analysis problem
G: 48.6 moles NaCl
W: Mass (g) NaCl
R:
Ex: Determine the mass of 48.6 moles
of sodium chloride.
Step 3: Find the molar mass for NaCl because
the wanted is MASS
Na = 1 X
g = 22.990g
Cl = 1 X
g = 35.453g
58.443g/mol NaCl
Ex: Determine the mass of 48.6 moles
of sodium chloride.
Step 3: Plug it all in
48.6 mol NaCl
58.443g NaCl
1 mol NaCl
= 2.84 x 103g NaCl
= 2840g NaCl
Tips for working molar mass problems
1. If mass is involved in your problem, determine molar mass of compound.
1 mole X = ____________ g X
2. If you see the terms: atoms, molecules, or formula units, use Avogadro’s
number.
1 mol X = 6.02 X 1023___*___ X
* atoms, molecules or formula units
Ex: Determine the number of moles in
582 g of magnesium nitrate
Step 1: Write the formula for magnesium nitrate
+2
Mg
NO3
-
Mg(NO3)2
Ex: Determine the number of moles in
582 g of potassium nitrate
Step 2: set up a dimensional analysis problem
G: 582 g Mg(NO3)2
W: Moles Mg(NO3)2
R:
Ex: Determine the number of moles in
582 g of magnesium nitrate.
Step 2: Find the molar mass for Mg(NO3)2
Mg = 1 X 24.305 g = 24.305 g
N = 2 X 14.007 g = 28.014 g
O = 6 X 15.999 g = 95.994 g
148.313 g/mol Mg(NO3)2
Ex: Determine the number of moles in
582 g of magnesium nitrate.
Step 3: Set up a dimensional analysis problem.
582g Mg(NO3)2
1 mol Mg(NO3)2 = 3.92 mol
Mg(NO3)2
148.313g Mg(NO )
3 2
Reminder:
How to type in Scientific Notation into a
scientific calculator
6.02 x 1023
Keystrokes:
6.02
EE or EXP
(2nd X-1)
23
Ex: Determine the number of molecules in
47.3 g of sulfuric acid.
G: 47.3 g H2SO4
W: Molecules of H2SO4
R: 1 mol H2SO4 = 98.078 g H2SO4
1 mol H2SO4 = 6.02 x 1023 mc H2SO4
47.3g H2SO4
1 mol H2SO4
6.02 x 1023 molecules H2SO4
1
98.078g H2SO4
1 mol H2SO4
= 2.90 x 1023 molecules H2SO4
Practice
1. Determine the number of moles in 27.4 g
of TiO2. 0.343 mol TiO2
2. Determine the mass of 9.45 mol of
dinitrogen trioxide. 567g N2O3
3. Determine the mass (in kg) of 5.83 x 1023
molecules of HCl. 0.0353 kg HCl
3. Determine the number of molecules in 782g
of N2O3. 6.19 x 1024 molecules N2O3
II. Percent Composition
II. Percent Composition
• The relative amounts of each element in a
compound are expressed as percentages.
• The percent by mass of an element in a
compound is the total number of grams of
the element divided by the molar mass of
the compound multiplied by 100%.
II. Percent Composition
Identify all of the atoms in the compound and
solve using the equation below to find the
abundance of each element.
% composition =
Mass element
Molar Mass
(Mass of Compound)
X 100
Ex: Calculate the % composition of propane
(C3H8). (In other words, what percent of
propane is carbon and what percent is
hydrogen?)
Carbon
3x
Hydrogen 8 x
= 36.033g/mol
= 8.064g/mol
Molar mass = 44.097 g/mol
Ex: Calculate the % composition of
propane (C3H8)
81.7% C
% compositionC =
100
36.033g/mol
44.097 g/mol
18.3% H
% compositionH =
100
8.064g/mol
44.097 g/mol
Ex: Calculate the % composition of
propane (C3H8)
These calculations tell us that 81.7% of
propane is composed of carbon and 18.3% is
made up of hydrogen
Note: Percentage is part over whole.
Mass of one type of element in a compound /
mass of the whole compound
Ex: Determine the percent nitrogen in
zinc nitrate.
Zn2+ NO3Zn(NO3)2
To Calculate Molar Mass:
Zn = 1 x 65.39
g/mol = 65.39 g/mo
N = 2 x 14.007 g/mol = 28.014 g/mol
O = 6 X 15.999 g/mol = 95.994 g/mol
Molar mass: 189.398 g/mol
14.8% of N in zinc nitrate = %N = 28.014 g/mol
100
189.398 g/mol
EX: Determine the mass of sodium
in 450 g of sodium chloride.
Na = 22.990 g/mol
Cl = 35.453 g/mol
58.443 g/mol
39.3% of Na in sodium chloride = %Na = 22.990 g/mol
100
58.443 g/mol
0.393 x 450g = 176g Na
Practice
1. Calculate the percent composition of
a. calcium chloride 36.1 %Ca, 63.9 % Cl
b. potassium nitrate
38.7 % K, 13.9 % N, 47.5 %O
2. Determine the percent oxygen in calcium
carbonate 48.0 % O
3. Calculate the mass of hydrogen in
a. 350 g of C2H6 70.4 g H
b. 2.14 g NH4Cl
0.161 g H