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Stoichiometry/Moles Tutorial
OR Chemistry for Dummies Ch. 10. pp166-171
A Deli has a Super Bowl sandwich special. You can get a foot long sub sandwich for $2
as long as it is just meat and cheese. The sandwich is assembled this way:
1 split bun + 6 meat slices + 5 cheese wedges = 1 sub sandwich
If 4 sandwiches were wanted:
4 split buns + 24 meat slices + 20 cheese wedges = 4 sub sandwichs
Assembling a final product using known ratios of numbers of individual parts is called
STOICHIOMETRY.
Some of the stoichiometries for the above sub sandwich are:
1 bun requires 6 meats slices to make 1 sandwich
1 bun requires 5 cheese wedges
1 bun is required to make 1 sandwich
6 meat slices require 5 cheese wedges to make 1 sandwich
If the number of slices of meat are known, the possible number of sandwiches that can
be made can be estimated. Notice that this does not work if instead of slices, the number of ounces of meat are known. The only way this can work is if the weight of an individual slice is known.
Stoichiometry Ratios In Chemical Reactions
In a chemical reaction, all chemicals are related as shown in the balanced formula
equation. The relationship is mathematical by particles.
2 K3PO4
+
3 CaCl2
→
Ca3(PO4)2
+
6 KCl
2 particles of K3PO4 ALWAYS requires 3 particles of CaCl2 for the reaction.
2 particles of K3PO4 ALWAYS produces 1 particle of Ca3(PO4)2
2 particles of K3PO4 ALWAYS produces 6 particles of KCl
3 particles of CaCl2 ALWAYS requires 2 particles of K3PO4 for the reaction.
3 particles of CaCl2 ALWAYS produces 1 particle of Ca3(PO4)2
3 particles of CaCl2 ALWAYS produces 6 particles of KCl
1 particle of produced Ca3(PO4)2 ALWAYS requires 2 particles of K3PO4
1 particle of produced Ca3(PO4)2 ALWAYS requires 3 particles of CaCl2
1 particle of Ca3(PO4)2 ALWAYS has 6 particles KCl of produced as well
6 particles of produced KCl ALWAYS requires 2 particles of K3PO4
6 particles of produced KCl ALWAYS requires 3 particles of CaCl2
6 particles of KCl ALWAYS has 1 particle of Ca3(PO4)2 produced as well
The above are the basic stoichiometric ratios at work in the equation.
In the sub sandwich example, the individual parts of the sandwich all have different
names, ie bun, slice, wedge. In a chemical reaction, nearly all parts or particles will be
molecules. Even if some particles turn out to be atoms, they will still be treated as
though they are molecules. The sub sandwich recipe uses parts that are easy to see.
Multiples of the recipe use fairly small numbers. The chemical reaction does not have
parts that are easy to see. In a chemical reaction the actual particle amounts need to
be multiplied by a rather large number in order to have actual amounts that can be used
in a lab situation.
2 K3PO4
+
3 CaCl2
→
Ca3(PO4)2
+
6 KCl
In this equation, 2 K3PO4 molecules are required to produce one Ca3(PO4)2 molecule.
In this equation, 4 K3PO4 molecules are required to produce 2 Ca3(PO4)2 molecule.
In this equation, 6 K3PO4 molecules are required to produce 3 Ca3(PO4)2 molecule.
In this equation, 8 K3PO4 molecules are required to produce 4 Ca3(PO4)2 molecule.
The pattern of this process is obvious. The ratio of the coefficients is the same as the
ratio of the actual molecules. It would not matter what unit of counting molecules was
being used, as long as the ratio of the molecule numbers matches the ratio of the coefficients in the balanced equation. So a counting unit of dozen could be used in the
equation above.
2 dozen K3PO4 molecules are required to produce one dozen of Ca3(PO4)2 molecules.
4 dozen K3PO4 molecules are required to produce 2 dozen of Ca3(PO4)2 molecules.
6 dozen K3PO4 molecules are required to produce 3 dozen of Ca3(PO4)2 molecules.
8 dozen K3PO4 molecules are required to produce 4 dozen of Ca3(PO4)2 molecules.
OR
2 gross K3PO4 molecules are required to produce one gross of Ca3(PO4)2 molecules.
4 gross K3PO4 molecules are required to produce 2 gross of Ca3(PO4)2 molecules.
6 gross K3PO4 molecules are required to produce 3 gross of Ca3(PO4)2 molecules.
8 gross K3PO4 molecules are required to produce 4 gross of Ca3(PO4)2 molecules.
The counting units used in chemistry are called MOLES. The main reason for using
this unit is that 1 mole of any chemical has the exact same amount of particles as 1
mole of any other chemical.This unit relates the amount of particles with the formula
weight. The mathematics for doing this looks like:
grams of the chemical = moles of the chemical
formula weight of the chemical
Stoichiometry Calculation Methods
To use the formulas below, there must be a correctly written and balanced symbolic
equation. Chemical B is the chemical in an equation for which information is desired.
Chemical A is the chemical for which information is available.
All formulas below are mathematically identical. On the same problem, all methods will
produce identical answers. They are arranged in various ways because of the many
ways that people understand ratio calculations. The method which works best or is easiest to remember is the one that should be chosen. These are just the most common.
There are other methods.
Notice that the formulas contain amounts of chemicals IN MOLE UNITS ONLY.
These formulas DO NOT WORK using gram amounts.
1) known moles(chemical A) x
coefficient chemical B
coefficient chemical A
= ? moles chemical B
2) coefficient chemical A =
coefficient chemical B =
known moles(chemical A)
? moles chemical B
3) known moles(chemical A)
coefficient chemical A
=
? moles chemical B
coefficient chemical B
4) coefficient chemical A =
known moles chemical A
coefficient chemical B
? moles chemical B
------------------------------------------------Additional useful formulas
grams of chemical A
F. W. of chemical A
= moles of chemical A =
# particles of chemical A
6.02 x 1023
(Avogadro's Number)