Laboratory: Stoichiometry & % Yield
Purpose: To predict a theoretical yield for a chemical reaction using stoichiometry, then to calculate the
% yield after measuring the actual yield of the reaction.
Background: By using the coefficients of a balanced equation, it is possible to predict the ratios in
which reactants and products are consumed and/or are made. The principal tool for such calculations is
the mole ratio, which is a conversion factor has the format X moles A/Y moles B, assuming the chemical
reaction xA + yB  zAB. As an example, the mole ratio between hydrogen and oxygen in the reaction
2 H2 + O2  2 H2O would be 2 mol H2/1 mol O2.
In this lab, you will be studying the reaction between calcium chloride (Here found in the form
CaCl22H2O) and sodium carbonate (Na2CO3). One of the products of this reaction is a precipitate: By
filtering and drying out this precipitate, we will be able to compare the theoretical and actual masses.
Materials:
CaCl22H2O powder
Filter paper
Squeeze bottle of DI water
Na2CO3 powder
Funnel
250 mL beakers (2)
Electronic balance
Safety: Wear safety goggles and aprons during the performance of this lab. Avoid inhalation of the solid
chemicals.
Procedure:
Place a 250-mL beaker on the balance and press “zero” (or “tare”) to turn the mass to zero.
Slowly add solid CaCl22H2O until you have between 1.00 and 2.25 g. Record this in the appropriate
place in your data table. Repeat, using your other beaker and the Na2CO3.
Obtain a piece of filter paper and record its mass in your data table. Prepare your filtering
apparatus as was demonstrated in class (fold filter paper in ¼, put 3 sheets one way, 1 the other). Place
the cone of paper in the filter and spray it with a bit of deionized water to make it stay in the filter.
Add about 20 mL of deionized water to each beaker, and carefully swirl until all solid has
dissolved. If some solid remains, add a bit more water to dissolve it. Mix the two solutions by pouring
from one beaker into the other. Swirl the beaker for 10-15 seconds to ensure complete reaction.
Filter the reaction mixture by pouring from the beaker into the filter paper. Be sure not to overfill
the capacity of your filter paper/funnel! Rinse the reaction beaker with small portions of DI water and
add to the filter, until all product is in the filter. Allow most of the liquid to go through the filter, then
rinse the filtrate with two additional portions of 20 mL of DI water, one after the other.
Wash/rinse/dry out the beaker you’re not using as the catch-vessel for your filtration and mark it
with your initials and class period with permanent marker on the clear part of the glass. After the
dripping from the filter has slowed to a drop every 2 seconds or so, carefully remove the filter (without
losing your product) and place it, point down, into the cleaned/labeled beaker. Place the beaker into the
drying oven until the next lab period to dry.
Dispose of the liquid that went through the filter into the sink, and wash out your funnel and the
2nd beaker, putting them back where you got them.
After the filter and product has dried, remove measure the mass of the filter and product. Record
this into your data table. Once you have recorded this, throw out the filter/product and wash out the
beaker—Scrub the marker off the side with a green scrubby-pad. Put beaker back where it was obtained
the previous day.
Data:
Mass of CaCl22H2O (g)*
Mass of Na2CO3
(g)
Mass of dry, empty filter paper (g)
Mass of filter paper + dried product (g)
Mass of dried product (g)
*Note: The calcium chloride is a hydrate, which means there are water molecules locked into the solid
crystals. You need to include (add) the mass of two water molecules when getting the molar mass of the
compound. Once the hydrate is dissolved in water, it acts just like CaCl2; you can use “CaCl2” in writing
your chemical equations.
Data analysis: Do these on your lab report, showing all work and writing complete sentences. When
doing a calculation, briefly explain what you are calculating before showing work (“Theoretical mass of
product”). Written answers must either have the question written, followed by the answer, or you must
write complete sentences that make clear what the original question was.
1. Using your solubility table from our last unit, predict products and write the balanced equation
for the reaction. Clearly label the reaction’s precipitate.
2. Write the balanced net ionic equation for the reaction above.
3. Calculate the theoretical yield of your product, using your data from lab. Separately, clearly
indicate the theoretical yield, the identity of the limiting reactant, and the identity of the excess reactant.
4. Using your lab data, calculate the actual yield of your product. Show the work done.
5. Calculate the % Yield for your reaction—Don’t forget to write the bare-bones equation first!
Post-lab questions:
1. What are the spectator ions in the reaction you performed?
2. What was the formula of the product you filtered?
3. What would happen to the value of the percent yield if you forgot to wash all the precipitate
out of the beaker and into the filter? Explain.
4. What would happen to the value of the percent yield if the product was not totally dry when
you got your final mass? Explain.
5. Explain in your own words why filtration is the ideal method for determining the yield of a
precipitation reaction. (Hint: Look at your products, and think about where each product
ended up after the filtration)
6. What is the purpose of the extra washings of the precipitate once you have transferred the
wet precipitate to the filter?
Conclusion: Paragraph one: Summarize your findings: re-report the results of your post-lab analysis,
using gloriously well-crafted complete sentences.
Paragraph two: Talk about your % yield: How did it compare to 100%? If you were over
100%, how can you explain this? If you were under 100%, how could you explain this? When discussing
error, don’t just lob “maybe this, maybe that” grenades; rather, use intelligent thought to explain the
directionality of the error. If you made an error performing the procedure, discuss it here. DO NOT
“imagine” errors of procedure! (“Maybe I spilled some”—there is no maybe! If you did, record that in
your observations during the procedure. If you did not, don’t say maybe I did!!!) Explain whether you
feel your yield was satisfactory and why/why not.
Paragraph three: Discuss how this lab helps your understanding of chemistry. If you feel
that you now better understand precipitation reactions, or LR problems, or the mole ratio, or ???; here’s
where you get to state it. This is a good place to reflect back on the background: You can now use this
lab as an example to illustrate how the concepts brought up in the background were used.
Grading Rubric: Stoich/% Yield lab
Honors Chemistry
Name
Period 1
3
Date
*Your lab report is YOUR lab report. I will award ZERO credit to labs that have copied from each other!
Discussion of ideas is fine and great, but this needs to be your work, entirely! DO NOT lend someone
your finished report, or you risk receiving a zero!
* I reserve the right to award fewer points for a section (0), or if you really wow me, I may award a
bonus point on any section.
* This rubric is to be turned in with your finished report, stapled to the top. You will need to have it
during the first lab period so that I can check your prelab onto the rubric.
1
2
Title/purpose/background/purpose/
Safety/prodcedure/empty data
tables not completed before lab.
Background is mostly missing, or is
filled with incorrect statements; no
example of concepts is given, no
tie-in is made between topics of
study and lab procedure.
Prelab is missing either
background, procedure,
or data table on lab day
Background is missing
more than one of main
topics of study covered
by the lab, or has errors
of fact in this
discussion, or is largely
incomplete.
Data
Data is missing, or values are not
legible
Data is sloppy, or values
have incorrect SF, or
units are missing
Data
analysis
(format)
Work is sloppy, unclear, untitled;
units are not used.
Titles are not used to
introduce calcs, or units
are not used
throughout, or answers
are difficult to locate.
Postlab Q’s
Answers not anwered in Q/A
format or in complete sentences
that make the original Q clear.
Number of questions correct/acceptable out of 5:
Conclusion
(Paragraph 1)
Conclusion
(Paragraph 2)
Results from lab not restated, or
some results are missing.
Error discussion is incomplete, or
discussion of error does not
correspond with the actual
data/data analysis
Paragraph is largely missing
discussion of lab concepts, your lab
data/analysis is missing from the
discussion. No personal growth is
discussed.
Answers either done in
Q/A format or in
complete sentences that
make original Q clear.
Results are completely
restated.
Error discussion is
complete, but has one
flaw in its logic
Paragraph shows a
lesser amount of
thought, or link to
background is unclear
or less than convincing.
Lab data/analysis not
used to illustrate
convincingly.
Report has minor errors
of grammar, style,
and/or spelling; or
appears rushed in its
production.
Paragraph includes thoughtful
discussion of concepts of lab,
using your own data/analysis to
provide examples. Link to
background section is clear and
convincing. Discussion of the
growth of your understanding is
present and thoughtful
Report is free of errors of
grammar, style, spelling; and
shows the care and thought that
went into its creation.
Prelab
Background
Data analysis
(correctness)
Conclusion
(Paragraph 3)
Writing
Report has major or too many
errors of grammar, style, and
spelling. Little or no apparent
effort has gone in to
writing/editing.
3
4
Title/purpose/background/purpose/
prodcedure/empty data tables are
completed before lab period.
Background thoughtfully
summarizes the main topics of
study that are important to
understanding the lab. An original
example(s) is used to help
illustrate the concept(s). A brief
statement of how this lab will show
those concepts in action is made.
Background is missing
an original example, or
a reference to tie
concepts to lab
procedure, or is
missing one part of the
main topics of study
covered by the lab.
Data table is neat
(straight lines),
complete, and values
have correct SF and
units.
Each calculation is
titled to show what is
being calculated. Units
are used in the work
shown. Answers boxed.
Number of analysis calculations
correct out of 7: (3 points for the
three parts of #3
Error discussion is complete,
thoughtful, and intelligent (does
not sound like guessing)