11B: Stoichiometry

Name:________________________________Section:_________Date:_______
11B: Stoichiometry: Quantitative Precipitate
Do the principles of stoichiometry really work? How do you use stoichiometry
to determine the percentage yield of a reaction?
This experiment will give us the opportunity to see
if stoichiometry can actually predict the amount of
products formed from a certain amount of
reactants. By carrying out a precipitation reaction
that forms solid copper(II) carbonate, we can
measure the amount of product produced. This will
be our “actual” yield. We can then compare the
amount of product obtained to the calculated or
“theoretical” yield based on stoichiometry. We will
also use the data to determine the percentage yield
of the solid product copper(II) carbonate.
This lab uses a double replacement or precipitate reaction between copper(II) chloride (CuCl 2 ) and sodium
carbonate (Na 2 CO 3 ). We will be able to predict the amount of product that should be formed according to
the reaction:
CuCl 2 (aq) + Na 2 CO 3 (aq) → CuCO 3 (s) + 2NaCl(aq)
from the masses of reactants used. Before you begin, read through the entire procedure below so that you
are familiar with each of the necessary steps.
Part 1: Preparing the solutions
CuCl 2 (s)
Na 2 CO 3 (s)
Filter paper
Mass of CuCO 3 (s)
and filter paper
Mass of CuCO 3 (s)
1. You will need two clean 250 mL beakers or Erlenmeyer flasks.
2. Measure out approximately 1 g each of CuCl 2 and Na 2 CO 3 solids. The actual amount can be
between 0.9 g and 1.1 g; it does not have to be exactly one. Record the exact amount that you
measured in your data table.
3. Place each solid in a beaker and add 25–30 mL of distilled water. Gently stir the mixture to
dissolve the salts. Be sure to use a different stirring rod for each, so that you do not cross–
contaminate your solutions.
4. Record the color of each of your solutions.
Materials
• Two 250 mL Erlenmeyer flasks or beakers
• Two stirring rods
• One medium funnel
• One piece of fast-flow filter paper
• Distilled water bottle
• Copper(II) chloride (CuCl2 )
• Sodium carbonate (Na2CO3 )
• Mass balance
Table 1. Data table for Part 1
Mass (g) Observations
A NATURAL APPROACH TO CHEMISTRY 87

Investigation 11B: Stoichiometry: Quantitative Precipitate
Part 2: The precipitate reaction
1. Slowly add a little CuCl 2 solution to the Na 2 CO 3 .
Swirl to mix and record your observations.
2. Slowly continue to add the CuCl2 to the Na2CO3 and
swirl to mix. The slow addition of CuCl2 along with
continuous swirling allows for the greatest amount of
product to be formed, because more ions are able to
contact each other.
3. When all of the solution is added, rinse the empty
flask with 3–5 mL of distilled water and add this to the
mixture. This removes any remaining CuCl2 ions.
Part 3: Isolating the product
1. Obtain a piece of filter paper and fold it according to
your instructor’s demonstration. (Fold it in half and
then in half again.) Write your name in pencil near the
top outside edge of the filter paper.
2. Weigh the filter paper.
3. Place the filter paper in the funnel, and place the
funnel inside the neck of an empty Erlenmeyer flask.
To make the funnel more secure, it helps to use a ring
stand and place the funnel inside the ring with the
flask below it.
4. Wet the filter paper with a little distilled water so that three folds of paper are on one side and one
paper thickness is on the other. This helps it stick to the funnel and keeps it in place.
5. Swirl your mixture to suspend the solid in the solution. Slowly pour the mixture into the filter
paper. As you pour the mixture, be sure that the solution does not come above the filter paper.
Check to see that the “filtrate” (the solution draining into the flask below the funnel) is clear. If it
is not clear, you may have a tear in your filter paper, or some mixture may have escaped over the
top. You will need to refilter this to ensure that you capture all of your solid.
6. Remove your filter paper containing the CuCO3 solid and place it in the area designated by your
instructor, where it will dry overnight.
7. Clean and rinse all equipment. You may need to use a test tube brush to remove solid particles
stuck to the sides of glassware.
8. During the next class period, weigh your filter paper and solid. Record the color and mass of the
CuCO3 in your data table.
88 A NATURAL APPROACH TO CHEMISTRY

Investigation 11B: Stoichiometry: Quantitative Precipitate
Part 4: Determining the percentage yield of your reaction
1. Write out the balanced equation for this reaction.
2. Complete the data table below.
CuCl 2 (s)
Na 2 CO 3 (s)
CuCO 3 (s)
(actual yield)
Limiting reactant
Theoretical yield
of CuCO 3 (s)
3. Calculate your percentage yield of CuCO 3 in this reaction.
4. Was your percentage yield 100%? If not, give some reasons that you think may have caused this
difference.
5. List two sources of error that could have caused your actual yield to be low.
6. List one source of error that could have caused your actual yield to be falsely high. Explain.
Part 5: What did you learn?
Table 2. Data table for Part 4
Mass (g) Molar mass
(g/mole)
a. Describe what you learned about precipitate reactions from this investigation in a couple of
sentences.
b. Write a couple of sentences that explain how the actual yield and theoretical yield of CuCO 3 compare
for your experiment.
c. Compare your results with those of another group. How do your results compare?
Moles
d. What do you think was the largest source of error in obtaining a high percentage yield? Explain.
e. In this experiment, what is the mole relationship between your reactants and the product CuCO 3 ?
f. Did the limiting reactant need to be calculated in this investigation, given the fact that you used
almost the same amounts of reactants? Briefly explain your answer.
g. If you could do this experiment again, what would you do differently that might improve your
percentage yield of CuCO 3 ?
A NATURAL APPROACH TO CHEMISTRY 89

Investigation 11B: Stoichiometry: Quantitative Precipitate
Part 6: Practicing what you learned
Suppose you carry out another similar experiment, this time with a different precipitate according to the
following equation:
In this experiment you obtain the following data:
2NaOH(aq) + CuSO 4 (aq) → Cu(OH) 2 (s) + Na 2 SO 4 (aq)
CuCl 2 (s) 1.0
NaOH(s) 1.8
Cu(OH) 2 (s)
actual yield
Limiting reactant
Theoretical yield
of Cu(OH) 2 (s)
Table 3. Data table for Part 6
Mass (g) Molar mass
(g/mole)
a. Calculate the molar mass of your reactants and solid product [Cu(OH) 2 ], and list them in the table
above.
b. Calculate the moles of your reactants and solid product [Cu(OH) 2 ], and list them in the table above.
c. Determine which reactant was limiting. Support your answer with a calculation.
d. Using your limiting reactant, determine how much solid Cu(OH) 2 you could have made if all of your
reactants reacted and you were able to collect 100% of your solid product. What is this called?
e. Determine the percentage yield for this reaction. Support your answer with a calculation.
f. Give at least one source of error that could account for your result. Explain.
Moles
90 A NATURAL APPROACH TO CHEMISTRY