Experiment Group MS: Mass Stoichiometry
(M. Briggs, P. Palko, July, 2009)
Revised 2014
Purpose
Chemical analysis - how much of which elements are present in a compound - plays a very important role
in many disciplines, such as geology, biology, biochemistry, and chemistry. Stoichiometry is the
technical word that describes the quantitative relationship between reactants and products in a chemical
reaction. Mass stoichiometry refers to that quantitative relationship based on, yes, mass. (Later in this
course, you will work on solution stoichiometry and gas stoichiometry.) In Experiment 1 of this series of
experiments, you will figure out quantitative relationships between reactant quantities and resulting
product quantities by studying the entertaining chemical reaction between calcium carbonate and
hydrochloric acid. Understanding the connection between reactants and products is a cornerstone of
quantitative chemical analysis. In Experiment 2 of this series, the excitement will go off the scale as you
will analyze a sample of fertilizer to determine its phosphorus content.
Goals of the Labs
Experiment 1: Concept Lab - How Much Reactant Do I Need ?
 Observe a reaction and predict which reactant limits a chemical reaction.
 Distinguish between different substances (reactants and products) based on their changes in
appearance.
 Distinguish between limiting and excess reagents in a reaction.
 Design an experiment to determine the reactant that is completely used up in a chemical reaction
and the point at which that occurs
Experiment 2: Application Lab- Gravimetric Determination of Phosphorus in Fertilizer
 Understand (and use in calculations) the mass percent composition and molecular formula of a
chemical compound
 Analyze a fertilizer unknown sample for percent mass phosphorus, and percent mass diphosphorus
pentoxide, which isn't even really present.
Scenario
You are a summer intern at the gardening company Plants Galore, which is exploring commercial
fertilizers. You job is to use gravimetric analysis to determine the phosphorus content of the fertilizer
samples the gardeners bring you, and report your findings.
Gravimetric analysis, as the name implies, is an analysis of composition based on masses.
Typically, a known mass of a compound of unknown percent composition is converted to another
compound of known composition, which is then weighed. The mass of the new compound of known
composition can be used to determine the percent chemical composition of the original unknown.
The starting point for gravimetric analysis depends on the form of the substance being analyzed
(we call it the "analyte"). If it's in a solid, we can find the sample mass directly. We then generally
dissolve it in a solvent, and react the analyte with an appropriate reagent to form a precipitate of known
composition. For the analysis to be quantitatively meaningful, this reaction must ensure that 100% of the
substance being analyzed appears in the precipitate. If the original analyte is already in solution, we
measure a known volume of solution, and again add an appropriate reagent to form the precipitate. The
precipitate is collected by filtration, washed, dried to remove traces of moisture from the solution, and
weighed. The drying is important because even trace amounts of water will distort the mass
MS-1
measurements. The amount of analyte in the original sample can then be calculated from the mass of the
precipitate and its chemical composition using reaction stoichiometry. (Reaction stoichiometry uses the
mole ratio of reactants and products in a chemical reaction as a basis for calculations. Composition
stoichiometry relates the amounts of elements in a particular compound.)
MS-2
Experiment MS-1: How Much Reactant Do I Need?
(MWB 22 July 09)
Revised by PJP Dec 2011
Pre-laboratory Assignment
(You can answers the questions by writing on this page)
Read Experiment MS-1. Answer these questions in the spaces below before coming to lab. They will
help you practice calculating masses of reactants and products in a balanced chemical reaction, and save
you time in the lab. Think of this question as you work through the following questions: “Can you find
the mass of a product directly from the mass of a reactant ?”
Concepts
1. What is a limiting reagent? Fire requires fuel, oxygen and a source ignition. Once a camp fire is
burning, which component is likely to be the limiting reagent?
2. Most people have observed the reaction of baking soda (NaHCO3) and vinegar which is 5% by mass
acetic acid (CH3COOH). Show why what is written represents a “balanced” chemical equation.
NaHCO3 (s) + CH3COOH (aq)
H2O (l) + CO2 (g) + CH3COONa (aq)
3. Look at the balanced chemical equation you have written. Based on what you know the reactants and
products, what observable changes do you expect to see to tell you this reaction is occurring?
4
a) Construct a model (drawing) of this reaction using symbols for the various atoms. Circles of
various colors work well. Use a box to represent the reactants before the reaction and another box to
represent the products after the reaction. Your model must obey the law of conservation of mass.
b) Using the same model, determine how many CO2 molecules would be produced if 4 units of
NaHCO3 and 8 units of CH3COOH are “reacted”.
c) Determine which reactant is left-over and how much remains.
MS-3
5. What is the relation, in moles, between a reactant in short supply and the amount of product formed in
a chemical reaction ?
Practice calculations of the kind you will use in this experiment
6. What is the molar mass of CaCO3? How many moles of CaCO3 are present in 1.00 g CaCO3?
7. What is the molar mass of CH3COOH? How many moles of CH3COOH are present in 100 grams of
vinegar that is 5.0 % CH3COOH by mass?
8. If you want 1.00 grams of CH3COOH and you have vinegar that is 5.0 % CH3COOH by mass, how
many grams of the solution should you obtain?
9. Examine this graph for trials where increasing amounts of acid where added to the same amount of
calcium carbonate. Identify the trials where the calcium carbonate is the limiting reactant and
calculate the mass of Calcium carbonate reacted for these trials.
Mass CO2 produced
Mass CO2 produced (g)
1
0.8
0.6
Mass CO2
produced (g)
0.4
0.2
0
0
20
40
Mass Acid Added
MS-4
Experiment MS-1: How Much Reactant Do I Need?
Background
When you are making a bicycle, you need two wheels, one frame, one seat, and one handle bar.
How many complete bicycles could you make with 20 wheels, 18 frames, 18 seats, and 18 handle bars ?
It seems clear that you would have too many frames, seats and handle bars but not enough wheels to use
up all of the raw materials. You are limited in the number of bicycles you can build because you only
have enough wheels to build 10 bicycles and the rest of the materials would be left over. The limitation
of enough materials to make things occurs in chemistry also. Sometimes we find we do not have enough
of one of the reactants in a chemical equation and the other reactants are in excess. Usually, we are
interested in the limiting reactant because it is the determining factor for how much product the chemical
reaction will produce.
The balanced assembly equation for one bicycle is this :
1 handlebar + 1 seat
+ 2 wheels
+
1 frame
=
1 bicycle
In the same way, in some reactions one reactant becomes entirely consumed and the other reactant
is left over. When one reagent is completely consumed, no more product can be formed (the reaction is
complete). Therefore, one reactant limits the amount of product that can be formed. In the bicycle
analogy above the handle bar, seat, wheels, and frame are not just analogies for the reactants; they also
represent the number of moles or particles available for reaction. As soon as one reactant is consumed,
the reaction stops because there are no longer particles of that reactant available for reaction. The reaction
is limited by the availability of this reactant: it's the limiting reagent.
In this experiment, you will design a method to collect data so that you can observe and predict a limiting
reactant during two reactions.
First you will examine the reaction of CaCO3 (s) and HCl (aq) to find the molar ratio of HCl to CaCO3
when the two are combined. You will then the reaction of acid + NaHCO3 (baking soda) to determine the
amount of baking soda in a tablet.
Safety
 As always, wear goggles to protect your eyes from the solutions.
 Place reaction vessels flat on a surface: sometimes reactions cause large temperature changes!
 Be careful not to spill any of the solutions. Wipe up spills with wet paper towels and immediately
wash off any solutions that slash on your skin.
 Dispose of the solids or solutions as directed by your instructor.
MS-5
Procedure
Part I: Formation of groups
Lab teams will be formed by your instructor. Assign tasks such as collecting materials, weighing
materials, recording data to each group member. You should do Part II as a group, but you may wish to
divide the work of Parts III and IV within your group, or with another group. All of you should participate
in the discussions of designing experimental procedures and explaining observations. When you weigh
solid crystalline substances, you should brush off the balance each time you finish. You should make one
group member responsible for checking that the mass balance is clean at the end of the experiment. If
your team fails to completely clean up at the end of a class, then each group member will receive a 1 point
deduction from the lab report grade.
Part II: Qualitative observation of the reaction of CaCO3 and HCl.
An acid-base indicator (bromophenol blue) has been added to the HCl(aq). The indicator is
yellow when the acid is present and blue when the acid is not present. CaCO3 is not appreciably soluble
in water. Obtain equal masses of the two reactants. Use no more than 1 gram of each reactant. This is a
little tricky because the HCl is in a solution! You’ll need to use the concentration on the reagent bottle to
calculate the mass of the solution to obtain in order to have the desired mass of HCl. Mix the reactants
and observe the reaction. What evidence is there of chemical change? Which reactant is the limiting
reactant? How do you know this?
Part III: Quantitative exploration of the reaction for different masses of reactants: Determining the
reaction mole ratio of CaCO3 to HCl
In science, we try whenever possible to collect measurements  numerical data that we can
tabulate and/or analyze graphically. When designing an experiment, we identify an "independent
variable"  something we control and change systematically, and a "dependent variable" - the outcome
that we measure or observe. Based on your determination of the limiting reactant in part II, design an
experiment where you use the same amount of limiting reactant (independent variable) but vary the mass
of the other reactant (dependent variable) from a few grams above what you used in part II to a few grams
less. Your goal is to perform a reaction for one of your combinations that will allow for both reactants to
be used up. (What will this look like?)
After your initial trials, you may need to conduct a second set of trials that narrows the range of
the amount of added reactant so that you can get more precise data closer to the masses where both
reactants run out. Ultimately, you want to run several reaction trials where both reactants are consumed
or at least have enough data to perform calculations to determine the moles of each reactant consumed in
several trials.
Part IV: Using stoichiometry to find the amount of baking soda in an “Alka-Seltzer” tablet.
An alka-seltzer tablet contains both an acid (Citric Acid) and a base (Baking Soda). Baking soda
is the excess reactant and the citric acid is present to react with the baking soda to create the “bubbles”. If
we add additional acid to the tablet we should be able to find the point where adding additional acid
produces no more product (the bubbles contain CO2). If we then calculate the amount of product
produced in the trials where the baking soda is known to run out, then we can calculate the amount of
baking soda in the tablet using stoichiomerty.
Determine the mass of the tablet. Use a tall reaction vessel to prevent bubbles from causing the
other reactants and water to escape the reaction vessel causing experimental error. Use trials where the
amount of acid and water added to the tablets always adds up to 60 grams. Do at least 3 trials where you
MS-6
are sure that the acetic acid is added in excess. Determine the total mass of reactants prior to reacting.
Mix the reactants and water in the reaction vessel. Allow the reaction to go to completion and carefully
shake the cup to remove any trapped bubbles. Find the final mass of the reaction (and now products)
mixture. Use this data to find the mass of CO2 produced by each reaction combination.
To determine the trials where baking soda is the limiting reactant, plot a graph of mass CO2 vs.
mass of acid added. It should be apparent in which trials the baking soda was the limiting reactant as no
more CO2 will be produced despite the addition of more acid.
Part V: Class results
Share your data for both part III and part IV with the class.
MS-7
Report Tutorial for Experiment MS-1: How Much Reactant Do I Need?
Complete the following report. Bulleted and italicized notes in this tutorial are guidelines to help you
complete the report.
Report Submitted by _________________________________
Date Submitted ______________________________________
Purpose: To develop a procedure to determine the limiting reagent in a chemical reaction
Procedure for Part I:
 Make a list of the students in your group and describe the major roles they played in the
experiment.
Data & Results for Part II: Qualitative observation of the reaction
Describe the appearance of the reactants before they were mixed.
CaCO3 solid
HCl(aq)
How did you know which reactant was used up by the reaction?
Identify what compounds are likely present at the end of the reaction. Cite your observations as
evidence of your identifications.
Why does the reaction stop?
MS-8
Data for Part III: Quantitative exploration of the reaction for different masses of reactants
Independent variable: Which reactant did you use the same mass of, for each trial?
Mass of that reactant used for each trial: _________ (g)
Table 1: Appearance of final solution for different masses of reactants
(There maybe more spaces in this table than you need!)
Trial Mass CaCO3(s)
Mass HCl (g)
Observations
At what combination of reactants does the appearance of the products "switch"? (Define what "switch"
means.)
Results for Part III:
Table 2: Class Results for Mole Ratios of CaCO3:HCl at point where both reactants are completely
consumed.
Group Mass CaCO3 Moles
Mass
Moles HCl
Ratio of moles
#
(g)
CaCO3
HCl (g)
HCl/moles CaCO3
AVE:
MS-9
Show the required calculations for your group’s data from Table 2.
Is the mole ratio of HCl:CaCO3 meaningful? Does the mole ratio change if different amounts of
reactants are used? Should the mole ratio be constant? Explain.
Results for Part IV: Mass Baking Soda in an Alka Seltzer Tablet
Mass tablet:_______________________
Table 3: Alka Seltzer and Water
Trial
Total mass of
Mass vinegar
reactants and
added (g)
container prior to
reaction (g)
Total mass of
reactants, products
and vessel after
reaction (g)
Mass CO2 produced
(g)
Plot a graph of mass CO2 (g) vs. mass of acetic acid solution added for each trial. Attach this graph to
the pages from this handout you turn in.
MS-10
Select trials where baking soda is the limiting reactant and complete this table. Show all relevant
calculations below the table:
Table 4: Analysis of Alka Seltzer data
Moles CO2 produced
Moles NaHCO3 reacted
Mass NaHCO3 reacted
Conclusions: Write your conclusion in your lab notebook, beginning with an opening sentence that
describes the concepts and ideas that you learned during this lab. Additionally, incorporate answers and
explanations of the following questions within your conclusion paragraph. Staple this page to the report
pages as part of your report.
1.
What is the mole ratio of CaCO3 to HCl when they react according to the class data. Compare
this result to the theoretical ratio you can obtain from a balanced chemical reaction. Discuss
possible reasons for the difference. Be specific when listing possible experimental errors. Link
the error to the result.
2.
What is the amount of baking soda in an Alka-Seltzer tablet? How reliable are your results? If
the results differ significantly from the value for baking soda listed on the box, offer concrete
reasons as to why there is a difference.
3.
Is the limiting reactant for a reaction always the reactant with the lower mass? Support your
answer with quantitative evidence.. (Refer to procedure Part II)
MS-11
Experiment MS-2: Gravimetric Analysis of Phosphorus in Fertilizer Prelab
Read the experiment. Answer the following questions in your lab notebook, showing the steps of your
calculations. (Selected answers are given in parentheses after the question.)
1. Molar masses Use the periodic table to calculate the molar masses of the following compounds:
(a) P
(b) P2O5
(c) MgSO4
(d) Mg(NH4)(PO4)(H2O)6
2. Preparing for the reaction:
(a) What does the labeling system tell you about the composition of a fertilizer or plant food that is
labeled 15-30-15, in terms of %N, % P2O5 and %K2O?
(b) Assume you have 100.0 g of P2O5. Use an appropriate ratio of the molar masses of P and P2O5 to
calculate the mass of phosphorus (P) in 100.0 g of P2O5. (43.64 g, Note this number is also the
mass % of P in P2O5.)
(c) Suppose you have 10.00 g of 15-30-15 fertilizer. How many grams of this fertilizer sample would
(allegedly) be P2O5? How many grams of this fertilizer sample would be P? (3.000 g P2O5, 1.309
g P)
(d) In the balanced chemical reaction of this experiment, the source of the Mg2+ is MgSO4 * 7H2O(s).
Use the flow chart below and dimensional analysis to calculate how many grams of MgSO4 *
7H2O(s) will be needed to react completely with the mass of phosphorus present in the 10.00 g
fertilizer sample. ( 10.39 g MgSO4 * 7H2O)
mass of P  mol P  mol PO43 mol Mg+2  mol MgSO4 * 7H2O(s).  g MgSO4 * 7H2O
(e) When analyzing the fertilizer for phosphorus, should you use a stoichiometric amount of MgSO4
solution or should one of the reactants be in excess? Explain.
3. Analyzing the results after the reaction:
(a) Using molar masses from the periodic table, calculate the mass percent of phosphorus (P) in the
precipitate, Mg(NH4)(PO4)(H2O)6. (12.62 % P)
(b) Using your answer from 3(b), if the mass of the precipitate, Mg(NH4)(PO4)(H2O)6, formed in this
experiment was 11.503 grams, what would be the mass P in the precipitate? (1.451 g P).
(c) Using your answer in 3(c), what is the percent mass P in the original fertilizer example of this prelab? Using the relationship between P and P2O5, what is the percent mass P2O5 in the original
fertilizer example of this pre-lab? (14.51 % P, 33.25 % P2O5)
MS-12
Experiment MS-2: Gravimetric Analysis of Phosphorus in Fertilizer1
Background
Gravimetric Analysis
Recall that gravimetric analysis is an important quantitative chemical technique for analyzing
"how much?" of a particular element is present in a compound. The typical procedure involves taking a
carefully measured amount of a compound or mixture, and reacting it with an excess amount of a
particular reagent - one that plucks out the element of interest, and forms a solid precipitate with it. The
precipitate is collected by filtration, dried and weighed. Using balanced reactions, the mass of the
precipitate can be related back to the mass of the element in the original compound. The mass of the
element compared to the mass of the original compound or mixture (times one hundresd) tells us percent
mass of the element that was present in the original compound. The analyzing reagent is always added in
excess to ensure 100% of the element in question is converted to the product precipitate.
Today, we are going to analyze the amount of phosphorus in fertilizer, and compare our results to
the labeling on the package.
Fertilizer labeling
The percent mass labeling (x % - y % - z% ) convention for fertilizer is % N - % P2O5 - % K2O.
However, the phophorus in fertilizer is present as the phosphate ion, PO43, not P2O5. Go figure.
(There's actually a historical reason for this labeling method, but we're not going to get into it.) We can
circumvent this labeling practice by tracking P, instead of PO43 or P2O5. We can extract the phosphorus
using magnesium cations in the presence of ammonia, NH3. The balanced chemical reaction that converts
the phosphorus P in the fertilizer to product, MgNH4PO46H2O(s), is:
PO43(aq) + Mg+2(aq) + NH3(aq) + 7 H2O(l)  MgNH4PO46H2O(s) + OH(aq)
We can interpret this balanced equation in a number of ways:
1. By counting units: There is one P for every one MgNH4PO46H2O.
2. By counting moles: There is one mole P for every one mole MgNH4PO46H2O.
3. By conservation of mass and using molar masses: Every 30.97 g of P will produce 245.45 g of
MgNH4PO46H2O.
Each one of the three statements above can be used to create a conversion factor for dimensional analysis.
To compare our results with the package labeling on the fertilizer (% P2O5), we will need to use the
relationship that one P2O5 contains two P, or that 141.94 g P2O5 contains 61.94 g P, or that P2O5 is
61.94x100/141.94 = 43.64 % P by mass. More dimensional analysis!
The reaction
Fertilizers contain both water soluble and water insolube components. The phosphorus component is
water soluble, so your first task will be to separate out the insoluble components. Next, you will add
magnesium ion in exces to react with 100 % of the phosphorus. Then, to force the precipitate out of
solution, you will add ammonia until the solution is very basic (pH = 9; pH is a logarithmic measure of
acidity. A solution with pH > 7 is basic.). Finally, you will filter, dry and weigh your product.
1
Adapted from the experiment Wink et al. Working With Chemistry, 2nd edition, W.H. Freeman, 2003
MS-13
Procedure
Part I: Formation of Groups
You will be working in groups of two or three people as assigned in the MS-1 experiment. Assign
tasks such as collecting materials and equipment, cleaning the balance after each use and cleaning up and
returning equipment at the end of the experiment. Your group will work together to analyze a single
fertilizer sample.
Part II: Determining the Phosphorus Content in a Fertilizer
Weigh a five gram sample of fertilizer. Record the mass of the fertilizer to 0.001 g. As a group,
discuss how you will separate the water soluble from the water insoluble components of the fertilizer.
Describe your procedure in your report form. Once you have separated the components, you can discard
the part that doesn't contain phosphorus.
Following the example in the pre-lab, but adjusting for your particular mass and your particular
fertilizer percent composition, calculate the mass of magnesium sulfate you should add to ensure 100%
reaction of the phosphorus present in your fertilizer. Each group member should conduct this calculation
independently, and then you should check that you all arrive at the same conclusion. Add 50% excess to
your answer. Show your sample calculation on the report form. Check your answer with your instructor,
and then add the MgSO4 to your dissolved fertilizer sample. Record your observations.
Test the pH of the solution using pH paper and the color guide. Slowly add ammonia until the pH
is at least nine. Record your observations. Set the mixture in an ice bath and leave it for at least 30
minutes. While waiting, prepare for vacuum filtration using the diagram below as a guide.
 to sidearm of tap
figure from /www.chem.wisc.edu
Weigh a piece of filter paper, place it in the funnel, and use water to dampen it and help it cling to
the funnel. Once your mixture has been on the ice bath for 30 minutes, turn on the water tap to create the
vacuum, and filter the mixture. Rinse the precipitate with a minimum amount of distilled water. Use a
spatula to fluff up the precipitate (be careful not to tear the filter paper) and let it sit on the vacuum to air
dry for at least ten minutes. Remove the filter paper and precipitate from the funnel and place it onto a
weighed watch glass. Set it aside to air dry.
When your sample is dry, determine the mass of just the precipitate. Calculate the mass of
phosphorus present. Calculate the mass of P2O5 that contains this mass of phosphorus. Calculate the %
mass P in the fertilizer, and the % mass P2O5 in the fertilizer. Compare this last result to the label on the
fertilizer box.
MS-14
Report Tutorial for Experiment MS-2:Gravimetric Analysis of Phosphorus in Fertilizer
(JCW/AEK, 5/30/2011)
In your report, use all the headings that appear below in BOLD. Italicized notes in brackets [ ] in this
tutorial are guidelines to help you complete the report, so do not include these notes in your report.
Create a new document to turn in as your report. This report should be an individual effort for each
member of the class. Not part of the report should be created by a single group member and shared.
Title of Experiment __________________________________
Report Submitted by _________________________________
Date Submitted ______________________________________
Purpose:
[Use the scenario of the experiment group to summarize the purpose of the experiment in 1-2
sentences. Focus on describing the problem your team is expected to solve and how they will solve it.]
Procedure:
Part I: Formation of Group
[Who did what? Be specific about each part of the procedure. Describe the sample type and sizes
analyzed by the members of your group.]
Part II: Determining the Phosphorus Content in a Fertilizer
[Use your answers to the pre-lab to help you complete this section. ]
(a) Summarize the procedure you used to separate the water-soluble from the water insoluble portions of
the fertilizer. Which part did you keep for further analysis?
(b) Show the calculation that you used to determine the mass of MgSO4 you used to react completely with
your fertilizer sample.
Data and Results
Part II: Collection and Analysis of Individual Data
Mass of the fertilizer sample:_________________
Percent composition of fertilizer, according to package label:______________
Description of solid fertilizer:
Description of fertilizer and water:
Description of mixture after addition of MgSO4:
Description of mixture after addition of NH3:
Description of product:
Mass of dried product:__________________
MS-15
Show sample calculations (word equations, numbers, units, dimensional analysis etc.) for the following:
(a) Use the mass of dried product, and an appropriate ratio of molar masses of your precipitate,
Mg(NH4)(PO4)(H2O)6, and P, to calculate the mass of phosphorus in your precipitate as the element,
P.
(b) Use an appropriate ratio of molar masses of P2O5 and P to convert the mass of phosphorus in your
precipitate to mass of P2O5.
(c) Use the mass of P2O5 and the mass of your fertilizer sample to calculate the percent of P2O5 in your
sample of the fertilizer.
(d) Calculate the % difference between your % P2O5 and the % P2O5 on the fertilizer label.
Conclusions: Report to the Plants Galore Gardeners
Write your conclusion as if you were explaining your findings in writing to the gardeners of the
Plants Galore Nursery. Your opening statement should state the final % P2O5, and should note whether
result matches that stated on the fertilizer packaging. Answer the question below, being sure to support
your conclusion statements by citing specific results from the lab experiment.
Question: Explain the factors that might affect the accuracy and precision of the results by answering
these three questions:
(a) What are sources of experimental error in your procedure? Would these errors result in a calculated
percentage P2O5 that is too high, too low, or unaffected? Explain.
(b) The large amount of potassium that is also present in fertilizers could cause formation of
MgKPO4(H2O)6, in addition to the desired precipitate. If some of the solid you obtained was actually
MgKPO4(H2O)6, instead of pure MgNH4PO4(H2O)6, would your calculated percentage be too high,
too low, or unaffected? Explain.
(c) Fertilizer companies sometimes over-formulate their fertilizers - i.e., they understate the percentages
of the key contents on the label. What difference would it make to your analysis if the fertilizer was
indeed over-formulated in phosphorus?
MS-16