Stoichiometry
…the basis of chemical calculations
What the heck is
Stoichiometry
• Stoichiometry is the study of quantities of
materials produced and consumed in chemical
reactions. Or, in a way normal people (not
honors students) could understand…
…solving
problems using
relationships found in
balanced, chemical equations!
How could this be possibly used?
2NaN3 → 2Na + 3N2
To get the correct amount of N2 we must start with
the correct amount of NaN3 . Stoich is used to
calculate this amount.
The MOLE Concept –How to count
things that are too small to see
• A mole is a counting number used by chemists
…Kind of like 1 dozen eggs – 12 eggs
A mole is the name of a number!
The mole is defined as the number of Carbon atoms in exactly
12 grams of pure Carbon – 12 (think the graphite in a pencil!)
It was determined that
1 mole = 6.02 x 1023 units
The units are usually atoms or molecules
This number is called AVOGADRO’S NUMBER!
A Brief History
In 1811 Amadeo Avogadro proposed that the number of
particles in a given volume of a gas (at a specific temperature
and pressure) is always the same, regardless of the identity of
the gas.
In 1865, Loschmidt estimated a value for what
Specific number this would be.
Lorenzo Romano Amedeo Carlo Avogadro di Quaregna e di Cerreto
Just How Big is This Number?
• How big is 6.02 x 1023 ? A huge number!
• 1 mole of marbles would cover the entire earth a depth of 50
miles!
• 1 mole sheets of paper would
go to the moon and back 80
billion times.
• 1 mole of pennies- each of us
on earth could spend 1 million
dollars each hour (day and night) and
have ½ of it left if we died at age 70!
(Much too big a number to comprehend)
So what is the BIG DEAL about a mole?
We know that 1 mole of anything is 6.02 x 1023
anythings.
So 1 mole of atoms of a given element would be
6.02 x 1023 atoms of that element.
We also know that the mass of an atom is given in
amu’s. (The mass of a N atom is 14 amu).
The true wonderfullness of the mole concept is that:
If we have 6.02 x 1023 atoms (1 mole) of an
element, the mass of that element will be equal
to it’s atomic mass IN GRAMS!
So 1 mole of N has a mass of 14 g.
And since atomic masses
are relative to each other,
this works for
ALL ELEMENTS!
Comparisons of 1 mole of different
elements
Aluminum
6.02 x 1023 atoms
26.98 grams
Gold
6.02 x 1023 atoms
196.97 grams
Boron
6.02 x 1023 atoms
10.81 grams
Essentially, this makes the periodic table a
huge list of conversion factors!!!
N - 14.01 g / 1 mol
Si - 28.09 g / 1 mol
C-
Fe -
Au This is why we love the periodic
table!
You have 10 g of Al. Calculate the
number of moles AND the number of
atoms .
• Remember: the mass of 1 mole of Al equals 26.98 or 27 grams (the at. wt.)
Mole Map
6.02 x 1023
Atomic mass
grams
1 mole
Atoms,
molecules,ions
A silicon chip used in your cell phone has a
mass of 5.68 mg (which is 0.00568g). How
many Si atoms are present in this chip?
Co is a metal added to steel (a mixture) to help
the steel resist corrosion. You have 5.00 x 1020
atoms of Co. Calculate:
a. The number of moles:
b. The mass of Cobalt:
Molecular or Formula Weights
• This concept can be extended to compounds.
• Since a molecule is made up of a specific
combination of atoms, if we add up the
masses of the atoms, we will get the mass of
the compound!!!
• Find the formula weight of CH4
(a byproduct of Chipotle)
Calculate the formula weight of NaCl
Calculate the formula weight of C10 H6 O3 , a
natural herbicide (weed killer) and dye in Rit
Dye.
Now, nerds of the world…calculate the
number of moles of C10 H6 O3 in
0.0156 g
Isopentyl acetate (C7H14O2) is the compound responsible for
the scent of bananas. Bee’s also release this same compound
when they sting in order to attract other bees (to the buffet)
and join in the attack. They release 1 microgram in each bee
sting.
1. How many molecules of C7H14O2 are released in one bee
sting?
How many atoms of C are present
in one bee sting?
(a fact you have always wanted to know)
Percent Composition
or Rabbit and Cow stew
• Ethanol, an additive to gas (and alcoholic drinks) has the
formula C2H5OH. Find the % of each element in ethanol.
STEP 1: Calculate the formula weight
STEP 2: Divide the total mass of the individual element by
the formula weight. Times 100 to get %.
Find the mass % of each element in
C10H14 O
Penicillin, an antibiotic, has the formula
C14H20N2SO4.
1. Find the % of Nitrogen in penicillin.
2. Find the number of moles of penicillin if you give a
patient a 250 mg tablet.
How many grams of potassium can be
produced when 150.6 g of potassium
sulfate is completely decomposed?
First, find the % of K in K2SO4
Next, multiply the % (decimal) by the mass given.
How many grams of Cu are needed to
make 16.6 g of copper II phosphate?
First, find the % of Cu:
Second, multiply the % by the mass given:
• A empirical formula is the simplest, whole
number ratio of atoms. Most formulas for
ionic compounds are empirical. What is the
key? If it CAN’T be reduced, it is empirical. If
it CAN be reduced, it is a molecular formula.
What are these?
Na2SO4 C12H22O11
C6H12O6 N4O12
To find the Empirical Formula (from %)
1. Convert % data to grams (% means
“parts per hundred”).
2. Convert the grams to moles (÷ by
the atomic weight)
3. Divide by the smallest to the get
the simplest, whole-number ratio.
The junior scientist was given 38.67% C, 16.23%
H and 45.1% N. Find the empirical formula.
A white powder was found near where mole died and was
found to contain 43.64% P and 56.36% O. Find the
empirical formula and name this molecule.
Molecular Formulas
• A molecular formula is NOT the simplest
formula (it can be reduced…but don’t do it!)
• Formula:
MolForm = X(EmpForm)
–Where X is an integer
To get X, divide the Mol FW by the
Emp FW
Ex:
Emp Form
CH2 Mass =14
X
2
Mol Form
C2H4 mass =28
CH2
3
C3H6 mass =
CH2
4
C4H8 mass =
A compound with an empirical formula of CH5N
has a molecular weight of 93 g/mole. Find the
molecular formula.
Find the empirical and molecular formulas
(normal kids don’t get to do
for a compound that is 71.65% Cl, 24.27% C, and
4.07% H. The molecular formula weight is 98.96 g/mol.
this….you’re lucky!)
Four Steps to find the Molecular Formula:
1. Find the empirical formula:
2. Find the empirical formula weight:
3. Find X (X = Molec.fwt/Emp. Fwt)
4. Multiply X by the empirical formula to get the
molecular formula.
• CH4 + O2 → CO2 + H2O
• In a chemical reaction, atoms have been
rearranged but have NOT been created or
destroyed. This is why we have to balance every
chemical reaction/equation. Java-Balancing!
• Balance CH4 + O2 → CO2 + H2O
• Balance HCl + NaHCO3 → CO2 + H2O + NaCl
• When balancing equations, the formulas must
NEVER be changed. We may just add coefficients.
Tips on Balancing!
1. Leave lone elements until the end.
2. If you get stuck, start over with a different
element.
2. If a group (polyatomic) does not change from
one side to the other, balance it as a group.
3. Temporarily use fractions to balance and then
multiply to get rid of them.
Balance the following reactions:
• (NH4 )2Cr2O7 → Cr2O3 + N2 + H2O
• C3H8 + O2 → CO2 + H2O
• NH3 + O2 → NO + H2O
Na2SO4 + Ba(NO3)2 → NaNO3 + BaSO4
C2H5OH + O2 → CO2 + H2O
C4H10 + O2 → CO2 + H2O
Diatomic Elements
•
•
•
•
N2 , Cl2 , O2 , H2 , I2 , Br2 , F2
Memory aid: I Bring Clay For Our New House
BOFINCH or H. BrOFINCl
Diatomic elements always combine with a like
atom when they are a free element.
• An arrow pointing up (↑) indicates a gas. The
abbreviation ppt. or a down arrow (↓)
indicates a precipitate.
Chemical equations indicate a…
• Chemical change. Remember, they must obey
the Law of Conservation of Mass – the
number of atoms of the reactants MUST
equal the number of atoms of products.
• The coefficients from the balanced equation
tell us the mole ratios of reacting substances
(this will be important later!!!)
• 2K + S → K2S
• 2FeCl3 + 3Ca(OH)2 → 2Fe(OH)3 + 3CaCl2
Five Types of Chemical Reactions
• 1. Composition (or Combination) Reactions
A + B → AB
• Sodium and sulfur yields _____________
1. complete the word equation
2. write the correct formulas
3. Balance that bad boy!
Aluminum and iodine yields ??
2. Decomposition Reactions (6 types)
• Generic equation AB → A + B
• These are the opposite of Composition
Reactions.
• Here, one thing is HEATED and it breaks down
into two things.
• The best way to learn these
six types of equations is to
MEMORIZE them!!!
1. Binary Compound into Elements
• Arsenic III oxide yields when heated
• Water yields with electricity
2. Metal Carbonates form metal oxides
and carbon dioxide
• Magnesium carbonate yield when heated
• Sodium carbonate yields when heated
3. Metal hydroxides yield metal oxides
and water
• Potassium hydroxide yields when heated
• Calcium hydroxide yields when heated
4. Metal chlorates yield metal
chlorides and oxygen
• Potassium chlorate yields when heated
5. Metallic bromates yields metal
bromides and oxygen
• Calcium bromate yields when heated
• Iron II bromate yields when heated
6. Acids when heated form nonmetal
oxides and water
• Carbonic acid when heated yields
3. Single Replacement
Reactions (two types)
• A. Positive Replacement (3 types)
• AX + B → BX + A
• The single element will replace the
positive element in the compound
1. Replacement of a Metal by a more
active metal
• Magnesium and calcium nitrate yields
2. Replacement of hydrogen in
water by a metal (to form a metallic hydroxide and
hydrogen gas)
• Aluminum and water yields
3. Replacement of H in an acid by a
metal
• Aluminum and hydrochloric acid yields
B. Negative Replacement Replacement of Halogens (1 type)
• A + MB → MA + B
• The halogens are
,
,
,
and I-1 and they are all diatomic
when they stand alone (Br2 , Cl2 ,
F2 , and I2 )
-1
Br
-1
Cl
-1
F
Hydrochloric acid and bromine yields…
4. Ionic/Exchange/Double
Replacement Reactions
• AB + CD → AD + CB
• The product MAY produce a solid or a
precipitate
• PRECIPITATE: an insoluble substance
formed in solution.
•a liquid + liquid → solid
• but only one of the products may be a
precipitate
Solubility Rules
• See chart in your book, the
chart in your handouts
section or on the back of
your Chem Helper.
Sodium hydroxide + iron III chloride →
Lead II nitrate + potassium iodide →
Sodium sulfate + lead II nitrate →
• Combustion reactions are the burning of
a fuel (usually a hydrocarbon like CH4)
with oxygen (O2 ) to form CO2 and H2O.
• Methane gas (CH4) is burned in a
furnace
Propane gas (C3H8) is used to cook in
most campers.
First 10 Hydrocarbons
•
•
•
•
•
•
•
Methane
Ethane
Propane
Butane
Pentane
Hexane
Heptane
CH4
C2H6
C3H8
C4H10
C5H12
C6H14
C7H16
• Octane C8H18
• Nonane C9 H20
• Decane C10 H22
MASS/MASS Problems
• Five Steps:
1. Balance the equation
2. Find the formula weights of given and
wanted
3. Convert the known mass to moles
4. Use the mole ratio to get from the given to
the wanted
5. Convert the moles to grams of the wanted
Given grams of
substance A
Formula weight
of A
Convert grams of A to
moles of A
Mole Ratio
Convert moles of A to
moles of substance B
Formula
weight of B
Convert moles of B to
grams of B
Lithium hydroxide is used in the space shuttle to remove
exhaled carbon dioxide from the living environment to form
lithium carbonate and water. What mass of CO2 can be
absorbed by one Kg of lithium hydroxide?
Sodium bicarbonate is often used as an antacid because it neutralizes
HCl secreted by the stomach to form sodium chloride, water and
carbon dioxide. How much baking soda is required to neutralize 6.8 g
of HCl?
Summer Job: Preparing lunch at Boy
Scout camp
• 400 hungry boy scouts need lunch.
• You have 35 loaves of bread, 17 jars of jelly
and 2 jars of peanut butter.
• During the sandwich-making process (the
“reaction”) which “reactant” will you run out
of first (bread, jelly, or pb?)
• The “reactant” that you run out of 1st will
stop the sandwich-making process.
• And, 300 boy scouts are going to tie you up in the knots they just learned
to tie and hang you in an oak tree (they just learned to identify trees)
Limiting Reagent Problems
X + Y → XY
3g + 4g → both are used up
completely
What would happen if we reacted 3g
of X with 5 grams of Y?
The limiting reactant is the reactant that is used up first
in a reaction. The other reactant that still remains is said
to be “in excess”. Think PB+J sandwiches.
To solve a limiting reactant problem:
-Do 2 mass-mass problems, one with each reactant.
-The product for each mass-mass must be the same.
-The reactant that gives the least mass of product is the
limiting reactant.
Ammonia is an important fertilizer used by farmers. It is produced by a
famous reaction called the Haber process. Suppose 25,000 grams of N2
and 5000 grams of H2 are mixed to form NH3 . Find the limiting
reactant and calculate the amount of ammonia produced.
First, write the balanced equation:
Mass-mass 1
Mass-mass 2:
Compare the 2 results, the lower mass will be the
amount produced and the reactant that gives that
amount is the limiting reactant
Ammonia and copper II oxide yields nitrogen gas, copper, and water
vapor. If 18.1 g of ammonia is reacted with 90.4 g of copper II oxide,
which is the limiting reagent? How many g of nitrogen will be
formed?
Wood alcohol is methanol (CH3OH). If hydrogen +
carbon monoxide yields methanol, write the balanced
equation.
If 68.5 Kg of CO is reacted with 8.60 Kg of hydrogen,
find the limiting reagent:
Now, calculate the mass of methanol
produced.
If the actual yield is 35,700 g, calculate
the % yield of CH3OH.