Stoichiometry
(stoy-kee-ahm’-uh-tree)
This is one of the most important topics in chemistry!
Stoichiometry – The area of chemistry that deals with quantitative relationships among reactants and
products in a chemical reaction. You must always have a balanced chemical equation to do stoich
conversions.
Stoichiometry problems give you a certain amount of a reactant and then ask how much of a product can
be formed. For example:
You are given 20.0g of compound A and a sufficient amount of B, how many g of
C are produced?
In the laboratory or in commercial processes, the chemist or engineer must often find the quantity of one or more
products that can be obtained form a given quantity of reactants. In order to solve any problem related to a chemical
reaction you must:
 Start with a balanced chemical equation. Crucial step! If this is not done correctly, the rest will be wrong
 Label what is given above the elements or compounds in the equation
 Label the unknown element or compound with an x. Include units wanted
 Set up calculations with your given and cancel units.
Mole Ratios in Equations
Propane is burned in many rural homes for heat in the winter. Below is the balanced equation
for the combustion of propane (C3H8).
C3H8 + 5 O2  3 CO2 + 4 H2O
For each molecule of propane that is burned, there needs to be five molecules of oxygen present.
Likewise, if there were a dozen molecules of propane, five dozen molecules of oxygen would be
required. Similarly, for each mole of propane, five moles of oxygen are needed. Also, for each
mole of propane burned three moles of carbon dioxide and 4 moles of water are produced. The
numbers of moles of each substance in a chemical equation are related by the ratio of the
coefficients of each substance.
Questions
Note: For questions 1-6, refer to the balanced equation for the combustion of propane.
1. a) How many moles of water are produced when 1.45 moles of propane are combusted?
b) How many molecules of water is this? (Remember each mole has 6.02x1023 molecules.)
2. If 2.35 moles of CO2 are produced in a reaction, how many moles of H2O would be
produced?
3. Why is this statement false: “If 10 grams of propane burn, you need 50 grams of oxygen.”
4. a) If 27.3 moles of carbon dioxide are produced during the combustion of a certain amount of
propane, how many moles of propane were combusted?
b) How many grams of propane was this?
5. If you have 410 grams of propane and want to know how many grams of oxygen are required
to burn it, you can follow these steps…
a) Find the number of moles of propane that you have. Convert grams to moles!
b) The moles of propane are related to the moles of oxygen by the ratio of coefficients in the
balanced chemical equation. Find the number of moles of oxygen you need given the
moles of propane from part a.
c) Find the grams of oxygen from the moles of oxygen. Convert the moles of oxygen
(answer to part b) to grams of oxygen (O2)! (Note: use the molar mass for O2, not just O.
You should get approximately 1490 g of oxygen.)
6. Verify that this statement is correct: If 315 grams of propane combusts, then approximately
515 grams of water are produced.
7. Consider the decomposition of ammonia: 2 NH3  3 H2 + N2. If you start with 425 g of
NH3, how many grams of H2 and N2 can be produced?
4 types of stoichiometry reactions
YOU MUST BEGIN WITH A BALANCED CHEMICAL EQUATION!!!!!!
Mole- Mole
 Quantity of the starting substance is given in moles and the quantity of the desired
substance is requested in moles
The conversion factor needed is the mole ratio from the equation.
Mole- Mass
 Quantity of the starting substance is given in moles and the quantity of the desired
substance is requested in mass
Mass- Mole
 Quantity of the starting substance is given in mass and the quantity of the desired
substance is requested in moles
Mass – Mass 
Quantity of the starting substance is given in mass and the quantity of the desired
substance is requested in mass
Questions
Mole to Mole problems: start with the moles of the given element or compound and find the moles of the unknown
element or compound.
1.
2.
Ammonia, NH3, is widely used as a fertilizer and in many household cleaners. How many moles of
ammonia are produced when 6.0 moles of hydrogen gas react with nitrogen gas?

Write a balanced chemical equation

Identify the given and unknown

Set up with problem so that units cancel
The decomposition of potassium chlorate is used as a source of oxygen in the laboratory. How many moles
of potassium chlorate are needed to produce 15.0 moles of oxygen?
Mole-Mass problems: Start with the moles of the given element of compound and find the mass in grams of the
unknown element or compound.
1.
When magnesium burns in air, it combines with oxygen to form magnesium oxide. What mass in grams of
magnesium oxide is produced from 2.00 moles of magnesium?
2.
What mass in grams of oxygen combines with 2.00 moles of magnesium in this same reaction?
Mass-Mole problems: start with the mass in grams of the given element or compound and find the moles of the
unknown element or compound.
1.
Oxygen was discovered by Joseph Priestley in 1774 when he decomposed mercury (II) oxide to its simplest
elements by heating it. How many moles of mercury (II) oxide are needed to produce 125.0 g of oxygen.
Mass-mass problems: start with the mass in grams of the given element or compound and find the mass in grams of
the unknown element or compound.
1.
Iron reacts with oxygen to produce iron (III) oxide. If 84.9 g of iron is used, what mass of oxygen is needed
for the complete reaction?
2.
In the above reaction, what mass of iron is required to produce 11.7 g of Fe 2O3?