C.8E: Stoichiometry
Bonding and Chemical Reactions
•  Given the amounts of reactants in a chemical equation, how can the amount of product formed
from the reaction be predicted?
Background: Stoichiometry
Stoichiometry is a method in chemistry that describes
numerical relationships in chemical reactions using
conversion factors, such as the molar masses of reactants and
products needed in a chemical reaction, which can be
determined from the balanced chemical equation. This
conversion factor is called the mole ratio. A mole ratio can be
used to show the relationship between the moles of any two
substances in the chemical equation by using the coefficients
of the chemical equation. The mole ratio can be used to
determine the moles of reactants needed to form a given
number of moles of a product. The mole ratio may also be
used to convert between any two compounds involved in the
reaction.
In order to solve these types of chemistry problems, a method called dimensional analysis
is used. Dimensional analysis uses ratios of values that are directly proportional to each
other. These ratios are represented as a "per" expressions that quantitatively show the
relationship between two values. "Per" expressions are always directly proportional to each
other.
The theoretical yield of a product can be found through the process of stoichiometry. The
theoretical yield is the ideal yield that would be produced if every atom involved in the
reaction reacted with all other atoms, producing a 100% yield of product. In reality,
however, this rarely happens. The amount of the actual product created is known as the
actual yield. Using the ratio of the actual yield to the theoretical yield, the percent yield of
the reaction can be found.
Actual Yield
Theoretical Yield
x 100 = % Yield
Continue on to the next page.
1 C.8E: Stoichiometry
Bonding and Chemical Reactions
•  Given the amounts of reactants in a chemical equation, how can the amount of product formed
from the reaction be predicted?
Background: Stoichiometry, continued
Let us work through one example together. You are given the following question:
A 2.8 g piece of sodium metal is dropped into a test tube containing water, and vigorous
bubbling occurs as the reaction produces sodium hydroxide and hydrogen gas. How many
grams of sodium hydroxide were produced?
First: Write and correctly balance the chemical equation. This gives you the required mole ratios
that you will need for your stoichiometric calculations.
2 Na (s) + 2 H2O (l) è 2 NaOH (aq) + H2 (g)
Second: Write what you know or what you are given (2.8 g Na) and what you want (grams of
NaOH).
Third: Set up your dimensional analysis problem to solve. Remember to include the correct
chemical formula with each step in the problem. Units and chemical formulas should cancel,
leaving only the unit and chemical formula that you want. You will use two specific conversion
factors in this problem to help you get from your given to your wanted. You will use the molar
masses of the chemicals involved, and you will use the mole ratio provided from the balanced
chemical equation to convert from one chemical to the other.
Given
2.8 g Na
Molar Mass Na
x
1 mol Na
22.99 g Na
Mole Ratio
x
2 mol NaOH
2 mol Na
Molar Mass NaOH
x
40.00 g NaOH = 4.87 g NaOH
1 mol NaOH
Multiply all of the numerators together and then divide by the denominators. This calculation
gives you the theoretical yield of NaOH that could be produced.
Continue on to the next page.
2 C.8E: Stoichiometry
Bonding and Chemical Reactions
•  Given the amounts of reactants in a chemical equation, how can the amount of product formed
from the reaction be predicted?
Background: Stoichiometry, continued
So what does theoretical yield mean? It means that you have calculated how many grams
of sodium hydroxide (the yield) could theoretically be produced with that amount of
sodium metal if every atom reacted perfectly with every other atom in the reaction, giving
you a 100 % yield. As stated earlier, this does not occur in real life. Most chemical
reactions produce what is known as an actual yield, which refers to the amount of
product that was actually created.
Let us say that in your real world reaction, you “actually” only produced 3.87 grams of
sodium hydroxide. This is different than the amount that could have theoretically been
produced. You lost some potential product during the reaction. Is this a good yield or a
bad yield for this reaction, and how can you find out? You can calculate the percent yield
of your reaction by using your theoretical yield and your actual yield. Remember:
Actual Yield
Theoretical Yield
x 100 = Percent Yield
For this reaction simply place your actual and theoretical yields in the correct spots in the
formula. Include the correct chemical formula.
3.87 g NaOH
4.87 g NaOH
x 100 = 79.5 %
Therefore, for your actual reaction, you had a 79.5 % yield of NaOH. Some factors that
could have affected your percent yield include, but are not limited to, the temperature of
the water, purity of the water used, or the time that the reaction was allowed to occur.
Complete the Background portion of your Student Journal before moving on to the investigation.
3 C.8E: Stoichiometry
Bonding and Chemical Reactions
•  How is the percent yield of a reaction calculated?
Part I: Plan Your Investigation
Your Mission: Perform stoichiometric calculations, including determination of mass
relationships between reactants and products and calculating percent yield.
You will plan and implement an investigation to perform a stoichiometric calculation in order to determine the percent yield for the reaction. Use Part I of your Student Journal to create a Question of Inquiry that you will research. Also, choose the equipment and any technology you need, list the important
safety steps you will follow, and identify the steps of your procedure. In your groups and with your teacher, determine how you will find the percent yield of the reaction.
Part II: Implement Your Investigation
1. 
Obtain the materials and equipment needed for your investigation. You will add 1 M
hydrochloric acid to sodium bicarbonate (baking soda) to see how much sodium
chloride is produced.
2. 
In Part II of your Student Journal, write the balanced chemical equation for the
reaction and use your knowledge of molar masses and balanced chemical equations
to understand chemical ratios and proportions.
3. 
Prepare to perform an experiment in which you will heat the specific reactants in
order to form products. Calculate the theoretical yield of this reaction before you
begin the actual procedure.
4. 
Measure the mass of the final product. Record the mass of the sodium chloride that
was actually produced in Part II of your Student Journal. Use this data to calculate
the percent yield of your specific reaction.
Complete the remaining questions in Part II of your Student Journal.
4 C.8E: Stoichiometry
Bonding and Chemical Reactions
Procedure:
1. 
Set up the ring stand, clamp, heating triangle, and the Bunsen burner.
2. 
Put on your safely gloves and goggles before you handle any chemicals.
3. 
Set the evaporating dish on the scale and tare (zero) the scale.
4. 
Measure out close to 2.00 g of sodium bicarbonate in the evaporating dish and set
aside. Make sure that there are no lumps or hard pieces in your sample. Record the
mass of the sodium bicarbonate that you actually measured. Record all mass
measurements to the nearest hundredth.
5. 
Place the evaporating dish on the clay triangle on the ring on the ring stand. Do not
turn on the Bunsen burner at this point.
6. 
Place a glass shield or watch glass over the evaporating dish. Do not cover the spout
on the evaporating dish as you will add the HCl in that opening.
7. 
Fill the dropper with the 1 M HCl. Add the HCl to the baking soda, drop by drop,
allowing the HCl to slide down the spout.
8. 
Continue to add the HCl until the baking soda stops fizzing. Record the number of
drops that were needed to complete the reaction.
9. 
Turn on the Bunsen burner, do not remove the watch glass, and gently heat the
mixture. Boil off all the liquid until only NaCl remains in the evaporating dish.
Make sure to monitor the heat of the burner and adjust as necessary so that the
reactants do not boil over.
10.  Allow the dish to cool (about 5 minutes), remove the watch glass, and then measure
the mass of the NaCl left in the dish. Record the mass of the NaCl and record all
measurements to the nearest hundredth.
When done, clean the evaporating dish and watch glass with soap and water and put away your materials.
5