4.06 Stoichiometry
My Goals for this Lesson:

Apply the mole ratio to solve stoichiometry problems.
I’m preparing to understand the mole ratio and how to solve stoichiometry problems.
Stoichiometry
Fill in the blanks using the lesson.
Stoichiometry is the use of dimensional
to calculate relationships
between the amounts of
and
in chemical reactions. The word stoichiometry comes
from two Greek words: stoichion, which means
, and metron, which means
Don’t let the word stoichiometry intimidate you. This process uses the
equations as just another fraction in the dimensional
.
from balanced
you have seen in previous lessons.
As you saw in the introduction to this lesson, you practice stoichiometry skills when you make
calculations using
from recipes.
You can apply the same skills to a chemical equation instead of a recipe. You will be using the coefficients
to represent ratios just as you did with the s’mores recipe; the only difference is that the coefficients
are followed by chemical formulas instead of food.
Helpful Hints
Stoichiometry is dimensional analysis (math conversions) that starts with one substance and ends with a
different substance. This means that all stoichiometry problems involve:
1. A balanced equation (the recipe for the chemical reaction).
2. A mole ratio (the coefficients in the balanced equation provide a ratio, or proportion, that can be
used as a conversion factor in our dimensional analysis).
3. Dimensional analysis (the math setup that you’ve already learned and practiced in this course).
Steps to Solve
Fill in the blanks using the lesson.
Steps for solving a basic stoichiometry problem:
1. Write a balanced chemical equation. (You need the coefficients for the mole ratio.)
2. Identify the given information and what you have been asked to solve for in the question.
3. Starting with the given information (measurement and unit), set up the dimensional analysis
problem. Use the units as a guide, and remember that a mole ratio is the only way to convert
from one substance to another in a calculation.
4. Solve the dimensional analysis calculation by multiplying everything in the numerators and
dividing by everything in the denominators. Remember to check your setup by making sure
that all the units cancel except the unit for your final answer.
Complete each example step by step using the tabs in the lesson as a guide.
Example One:
How many moles of hydrogen gas (H2) would be needed to form 8.6 moles of ammonia (NH3)?
N2 + H2 → NH3
1) The first thing you need to always make sure you do is balance the chemical equation. An
equation needs to be balanced so that you can use correct coefficients in your ratios.
Balance the equation below.
2) Next, look at the question you are being asked to solve. The question usually includes a given
amount of one substance as well as the unit of another substance you are being asked to solve
for. Once you have identified both pieces of information, you have identified the starting point
and the destination of your calculation.
In this question, what is the given measurement?
What are you being asked to solve for in this question?
3) Always start your dimensional analysis problem with the amount given in the word problem. Then,
use the units to help you determine the path you will take to reach your destination.
Because the given information is already in moles, you are ready to use the mole ratio as the next
step. Use the coefficients from the balanced equation as the mole ratio to convert from moles of
one substance to moles of another substance.
Set up the problem below. Show all the work, steps and units!! (This is required in the
assessments for you to earn full credit.)
Because you were asked to solve for moles of H2, the mole ratio was the only conversion factor
needed for this problem. If you needed to convert from moles of H2 to grams, you would do so by
adding more conversion factor steps after the mole ratio.
4) Solve the calculation by multiplying all numerators and dividing by all denominators. Do not forget
to check that all of the units cancel out diagonally except for the unit of the final answer.
Solve the problem below showing all the cancelations, units and have the final answer in the
correct sig. figs.
Example Two:
How many moles of aluminum metal would be needed to produce 35.6 grams of aluminum oxide in the
following synthesis reaction?
Al + O2 → Al2O3
1) Balance the chemical equation.
2) Identify the given amount as well as what you are being asked to solve for in the question.
Given:
Asked for:
3) Start your dimensional analysis problem with the amount given in the word problem and use the
units to help you determine the path you will take to reach your destination.
Remember to use the periodic table to help you determine the molar mass of elements and
compounds.
Determined the molar mass of aluminum oxide below.
Set up the dimensional analysis calculation below. Show all the work, steps and units!! (This is
required in the assessments for you to earn full credit.)
4) Solve the calculation, being sure to check that all of the units cancel except the unit of the final
answer.
Example Three:
How many grams of sodium metal are needed to react completely with 45.6 grams of water?
Na + H2O → NaOH + H2
1) Write out the balanced equation for the reaction.
2) Identify the given information and what you have been asked to solve for from the word problem.
Given:
Asked for:
3) Set up your dimensional analysis calculation, starting with the measurement given in the problem.
Determined the molar mass of water below.
Set up the dimensional analysis calculation below. Show all the work, steps and units!! (This is
required in the assessments for you to earn full credit.)
4) Solve the calculation, checking your set up by making sure all units cancel except the unit for your
final answer.
Be sure to do the Let’s Practice and print the practice sheet on the Activity page of the lesson.