Stoichiometry
Stoichiometry is a method used in chemistry to
determine the amount of materials used and
produced in a chemical reaction. It is a vital
part of all quantitative chemistry. Learn well.
Step by Step Example Problem
What mass of aluminum will be produced if
excess aluminum chloride is reacted with
11.23g of magnesium?
The word excess indicates that there will be
more aluminum chloride than you will need.
Step #1 – Predict the Products
What are the two products when aluminum
chloride reacts with magnesium?
Magnesium chloride and aluminum.
Step #2 – Write Proper Formulas
It is important to write the proper formulas using
subscripts. On your worksheet, write the
formulas for aluminum chloride reacting with
magnesium to form magnesium chloride and
aluminum.
AlCl3 + Mg  MgCl2 + Al
Step #3 – Write Balanced Equation
AlCl3 + Mg  MgCl2 + Al
Balance the equation on your worksheet.
2AlCl3 + 3Mg  3MgCl2 + 2Al
Step #4 – Write down your given
You are given are 11.23g of magnesium, it is
important that you write it all down; the
number, the units and the substance.
11.23g Mg
Step #5 – Convert grams to moles
You are given are 11.23g of magnesium, convert
this to moles of magnesium. Use factor label.
11.23g Mg
moles Mg
Step #5 – Convert grams to moles
11.23g Mg
1 mole Mg
24.3g Mg
0.462 moles Mg
Step #6 – Moles to Moles
This is Stoichiometry!!! Typically you should
just continue on with the problem from step
#5, but since we are breaking them up, start
this step by writing down what you calculated
in step #5.
0.462 moles Mg
Now use factor label to convert to moles of Al.
(remember to use your balanced equation)
Step #6 – Stoichiometry
0.462 moles Mg
2 moles Al 0.308 moles Al
3 moles Mg
Step #7 – Convert moles to grams
0.308 moles Al
27.0g Al
1 mole Al
8.32g Al
So we have determined that if 11.23g of
magnesium reacts with excess aluminum
chloride, it will produce 8.32g of aluminum.
Practice Problem #1
Do this practice problem and show all of your
work on your worksheet. When you finish,
go to the next screen to check your answer.
How many grams of product will be produced
from the composition of 34.21g of aluminum
with excess chlorine?
2Al + 3Cl2  2AlCl3
Practice Problem #2
Do this practice problem and show all of your work on
your worksheet. When you finish, go to the next
screen to check your answer.
How many grams of sodium will react with 0.0045g of
magnesium chloride?
Notice that this problem wants to know the amount of
the other reactant. No worries, it is done the same
way as the other stoichiometry problems.
2Na + MgCl2  2NaCl + Mg
Practice Problem #3
If you decompose 1.25 moles of aluminum
oxide, how many grams of aluminum can you
produce?
Notice this problem does NOT start with grams,
therefore you can skip step #5. Show all your
work on your worksheet and then go to the
next frame to check your answer.
2Al2O3  4Al + 3O2