Chapter 3 - Stoichiometry
______________ is the study of relationships that exist due to chemical formulas and equations.
The ___________ spectrometer is a device that is used to ___________ various isotopes of an element. See
diagram:
We can use the mass spectrometer to determine the average mass of an element. This is called the
________________.
Problems: Sulfur consists of 4 isotopes, 32S, 33S, 34S and 36S with percent abundance’s in the earth’s crust of
95.00%, 0.76%, 4.22%, 0.02% respectively. The atomic mass of 32S is 31.97207 amu, of 33S is 32.97146
amu, of 34S is 33.96786amu, and 36S is 35.96709amu. Find the atomic mass of sulfur
Hydrogen contains 3 isotopes: 1H (called protium); 2H (called deuterium); 3H (called tritium). 99.985% of
all hydrogen atoms in the crust of the earth are 1H. 0.0015% is 2H. Tritium makes up less than 1 in 100,000
of the hydrogen atoms found in the crust of the earth, so it won’t affect this calculation. The atomic mass of
protium is 1.007825 amu. The atomic mass of deuterium is 2.0140. Find the atomic mass of hydrogen.
The element tedonium has two isotopes, 50Td, and 51Td. The atomic mass of the element is 50.94 amu.
99.75% of the element is tedonium-51 which has an atomic mass of 50.9440. Find the atomic mass of
tedonium-50.
69.2% of all copper is copper-63, which has an atomic mass of 62.9296 amu. If all other copper is 65Cu, and
the atomic mass of copper is 63.55, what is the atomic mass of copper-65?
In chemistry, an amount of matter can be viewed as a specific mass, or a specific volume, or even a specific
number of particles. Since atoms and molecules are very small, counting them would be very difficult. But
we have to invent a unit such that this standard molecular amount is a specific number of particles which
can allow us to predict its mass and/or volume. The standard molecular amount is called a _________,
abbreviated _____. One mole is:
This is a very big number. It is called ____________
in honor of the Italian fellow who thought it up. Originally, ___________________ was the ____________
of the mass of one _________________. We now have modified it to be ___________________________.
The beauty of the system is that this allows us to convert the atomic mass unit weighted average for the
elements to the number of ________ needed to make _____________. We call this number the
______________, which I define as the number of grams needed to make 1 mole, or 6.022 x 1023 particles.
In chemistry, the particles can be ___________________________________________ . To find the molar
mass of a molecule, we __________ the subscript number by the molar mass of the element, and ________
all the elements together. See mole highway diagram:
Problems:
Find the molar mass of 1] glucose, C6H12O6
2] aluminum nitrate
3] copper (II) sulfate
4] Acetaminophen (also known as Tylenol) is C8H9O2N. Find a] molar mass
b] number of moles in 255 mg of acetaminophen c] mass of 0.450 moles
d] mass of 2.50 x 1022 molecules of acetaminophen e] number of molecules in 1.40 g of acetaminophen
f] number of carbon atoms in 2.50 grams of acetaminophen g] mass of one molecule of acetaminophen
h] what mass of acetaminophen is needed to have 1.40 grams of oxygen?
5] For ferric nitrate find a] molar mass b] mass of 0.350 moles
c] number of moles in 1.25 grams
d] number of molecules in 1.85 pg e] number of oxygen atoms in 3.44 g
f] number of nitrate ion is 2.26 ng
g] mass of ferric nitrate needed to have 2.20 g of nitrogen.
6] The density of iron is 7.87 g/mL. Find the number of iron atoms in a cylinder of iron that has a length of
25.0 cm and a diameter of 3.00 cm.
7] The density of copper is 8.96 g/mL. Find the diameter of a sphere of copper if it contains 1.55 x 10 22
atoms of copper.
Percent composition. Using molar masses, we can predict what percent a part of a compound is by mass.
Sample problems: 1] what is the percent oxygen in sulfuric acid
2] what is the percent composition
of calcium chloride
3] what is the percent nitrogen in cupric nitrate
4] what is the percent
composition of glucose, C6H12O6?
5] Lysine is an important amino acid. A molecule of lysine contains
two nitrogen atoms. If the nitrogen is 19.17% of the compound, what is the molar mass of lysine?
6] Cysteine is also an important amino acid. There are 3 carbon atoms per molecule, which represent
29.74% of cysteine. Find the molar mass of cysteine.
Chemical formulas:
There are 3 types of chemical formulas: 1] structural
2] molecular
3] empirical
Back in the previous section, we saw that we could find the percent composition from a formula. Can the
problem be worked in reverse? You can use the percent composition to determine only one type of formula,
the ________________.
Sample problems: 1] A compound is 26.2% nitrogen, 7.50% H, 66.4% Cl. Find its empirical formula.
2] In a 12.00 g sample, 5.99 g is titanium, and the rest is sulfur. Find its empirical formula
3] A compound is 81.68% carbon. The rest is hydrogen. Find its empirical formula.
Can we convert an empirical formula to a molecular formula? Only if we know the _______________.
The _____________ of the molecular formula is always a _____________________ larger than the
_____________ of the empirical formula. This is because the subscripts of the empirical formula have to be
multiplied by some ___________________ to get the ____________________ formula.
Sample problems - method 1 1] A compound is 50.0% carbon, 5.6% hydrogen, and 44.4% oxygen. The molar mass of the compound is
360.35 g/mol. Find its molecular formula.
Method 2 - assume:
2] The molar mass of a compound is 134.00 g/mol. If the compound is 34.31% sodium, 17.93% carbon, and
the rest is oxygen, find the molecular formula of the compound.
3.
The molar mass of a compound is 58.14 g/mol. If the compound has an empirical formula of C 2H5,
what is its molecular formula?
Balancing chemical equations:
Equations must be balanced because of the ________________________________. In order to balance
equations, ________________ are written in front of a substance such that:
Some general rules for balancing: 1] balance for _______________________________
2] balance polyatomic units as if:
3] Think of ___________ as hydrogen hydroxide in some cases.
4] Coefficients must be in their __________ whole number _________
5] If a ________________ is needed, _____________ all the coefficients by the ____________________
of the fraction.
Sample problems: balance the following - 1]
2] C3H8 +
O2 
CO2 +
H2O
H2O 
3]
F2 +
4]
Zn(C2H3O2)2 +
5]
NH3 +
6]
H3PO4 +
7] Al +
HF +
O2 
KOH 
H2SO4 
H2 +
O2 
C4H10 +
9]
HC2H3O2 +
10]
K+
11]
C2H2 + O2 
CO2 +
12]
Al2(SO4)3 +
BaCl2 
NaC2H3O2 +
Zn3(PO4)2
H2O
Al2(SO4)3
CO2 +
H 2O
Ca(OH)2 
H2O  KOH +
O2
H2O
K3PO4 +
8]
Ag +
O2
Na3PO4 
NO +
Ag2O 
Ca(C2H3O2)2 + H2O
H2
H2O
AlCl3 +
BaSO4
Balancing word equations: The key here is to understand that the oxidation number of a free element is
_____. But certain free elements occur in pairs under normal conditions. They are the 7 diatomics. They
are:
more problems:
1] potassium chlorate decomposes into potassium chloride and oxygen
2] phosphorus and oxygen makes diphosphorus pentaoxide
3] calcium carbonate and hydrochloric acid forms calcium chloride, carbon dioxide and water
4] acetic acid and magnesium hydroxide form water and magnesium acetate
5] ammonia decomposes into hydrogen and nitrogen
6] titanium (IV) chloride + water  titanium (IV) oxide and hydrochloric acid
7] ferric nitrate and sodium hydroxide makes ferric hydroxide and sodium nitrate
steps for solving mass mass problems:
sample problems: 1] Given the unbalanced reaction: C4H10 + O2  CO2 + H2O
If I burn 10.0 g of butane (C4H10), a] how many grams of carbon dioxide form? B] how many grams of
oxygen are consumed?
2] Given the unbalanced reaction:
KClO3  KCl + O2
If 125 g of potassium chlorate decomposes, what mass of oxygen gas is produced?
3] Given the unbalance reaction: Na + Cl2  NaCl. If 0.0456 moles of chlorine are used with excess
sodium, what mass of sodium chloride will be produced?
4] Given the unbalanced reaction:
with 2.45 g of aluminum?
Al +
F2  AlF3. How many moles of fluorine are needed to react
5] Given the unbalanced reaction: Al(s) + HF(aq)  H2(g) + AlF3(aq)
a] if 2.50 moles of aluminum are used, what mass of HF will be needed to complete the reaction?
b] if 1.25 moles of HF react with excess aluminum, what mass of hydrogen will be produced?
c] if 0.035 moles of Al react with excess hydrofluoric acid, how many moles of hydrogen will be produced?
d] if 2.65 mg of aluminum react with excess hydrofluoric acid, what mass of aluminum fluoride will be
produced?
Limiting reactants: If _____ or more reactants are needed in a reaction, one of the reactants will be used up
first. This reactant determines:
If the reactants run out equally, this is called a:
Usually, one reactant is in excess (inxs), and the other is the:
The limiting reactant determines:
sample problem: there are 3 methods to solving these. I will show all 3 methods.
Given the unbalanced reaction: Fe + S8  FeS. If 2.00 g of iron react with 2.00 g of sulfur, find
a] limiting reactant b] mass of product
c] mass of xs reactant
second sample problem:
H2 +
N2 
NH3
If 4.52 g of nitrogen react with 0.87 g of hydrogen, find
a] limiting reactant b] mass of product
c] mass of xs reactant
#3: Given the unbalanced reaction: Mg + FeCl3  Fe + MgCl2
If 1.25 g of magnesium reacts with 6.05 g of ferric chloride, find a] LR b] mass of iron that forms
c] mass of xs
Percent yield: formula:
Using problem #3 above, if 1.50 g of iron are recovered, what is the percent yield?
Combustion Analysis:
Tough problems…. They usually put one of these on the AP chem test.
TTP#1 The molar mass of a compound is 30.08 g/mol. The compound contains only carbon and
hydrogen. Combustion of 12.0g of the compound results in 35.1 g of CO2 and 21.6 g of H2O. Find the
empirical and molecular formulas of the compound.
TTP #2. A compound contains carbon, hydrogen and oxygen. When 24.0 mg of the compound undergoes
combustion, it produced 54.5 mg of CO2, and 22.3 mg of H2O (steam). Find the empirical formula of the
compound.
TTP #3 A compound contains carbon, hydrogen, and nitrogen. When 18.0 g of the compound undergoes
combustion, it produced 35.1 g of carbon dioxide and 25.2 g of steam. Find the empirical formula of the
compound.
TTP #4 When 25.0 g of a compound that contains only carbon, hydrogen and oxygen is burned, it
produces 56.8 g of carbon dioxide and 23.3 g of steam. The molar mass of the compound is 116.18 g/mol.
Find the empirical and molecular formulas of the compound.