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EXPERIMENT A: STOICHIOMETRY AND REACTIONS IN AQUEOUS SOLUTIONS
Textbook Reference: Zumdahl, pp. 55, 95, 100-104, 109, 905, 906, 926.
For Chem 10X Labs, your textbook, Chemistry ( 5th edition ) by S. S. Zumdahl and S. A.
Zumdahl, is the usual reference. Pages to some relevant materials are usually given as above for
each experiment. The index of your textbook might lead to other relevant materials. The margin
notes of the textbook reference other materials on the CD (Windows and Macs) that comes with the
textbook.
OBJECTIVE
Simple chemical reactions will be used to transform scrap aluminum from pop cans to alum,
and to form ammonium magnesium phosphate hexahydrate from a fertilizer sample in order to
determine the mass % of phosphorus in the fertilizer.
BACKGROUND
I. Formation of KAl(SO4)2•12H2O from aluminum scraps
Aluminum is the most abundant metal in the earth's crust and the third most abundant
element (behind oxygen and silicon). Aluminum is widely distributed in alumino-silicate minerals
and clays. The main ore of Al is bauxite, a hydrate of aluminum oxide Al2O3 , mixed with oxides
of iron, silicon and titanium. Because of the stability of Al2O3, Al was expensive to produce until
the development of the Hall-Heroult process in 1886. Even so, production of Al requires a lot of
energy. It has been estimated that recycling scrap aluminum by melting it (melting point=660°C)
takes only about 5% of the energy needed to produce new aluminum from bauxite.
Another way to recycle scrap aluminum is through chemical transformation. In this
experiment, you will transform scrap aluminum, from pop cans, into aluminum potassium sulfate
dodecahydrate, KAl(SO4)2•12H2O. This compound, commonly called alum, potassium alum or
kalinite, is used in a wide variety of processes such as the manufacture of dyes, explosives, and
porcelain cement, tanning, hardening gelatin and purifying water.
The relevant steps in the experiment are outlined below.
1. First, the scraps of aluminum from pop cans will be dissolved by reaction with hot
potassium hydroxide. ( Note that the following reactions are unbalanced. You will be
required to balance them in the lab assignments)
Al(s) + KOH(aq) + H2O → K+(aq) + Al(OH)4-(aq) + H2(g)
(1)
2. Next, sulfuric acid is added. Initially a white solid Al(OH)3(s) forms according to the
equation:
Al(OH)4-(aq) + H2SO4(aq) → Al(OH)3(s) + H2O + SO42-(aq)
(2)
3. However the Al(OH)3(s) then undergoes further reaction according to the equation:
Al(OH)3(s) + H2SO4(aq) → Al3+(aq) + SO42-(aq) + H2O
(3)
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Al(OH)3(s) is amphoteric as it can react either with acid or with base (Zumdahl, p. 905, 906). This
final solution will be left in your locker until the next lab period to slowly form crystals of
aluminum potassium sulfate dodecahydrate, KAl(SO4)2•12H2O(s).
II. Phosphorus in a Fertilizer
The label of fertilizers displays 3 numbers such as 10-20-10. The first is the guaranteed
minimum mass percent of total nitrogen, the second is the available phosphoric acid (cited as
"P2O5"), and the third is the soluble potash (cited as "K2O"). The usual phosphorous compound
in fertilizer is ammonium dihydrogen phosphate, (NH4)H2PO4. You will check the percent
available phosphoric acid by the following procedure:
1. A known mass of fertilizer will be mixed with water to dissolve the ammonium dihydrogen
phosphate which is the actual material present rather than "P2O5".
2. Excess magnesium sulfate solution is added (with ammonia to adjust the pH) to form
MgNH4PO4.6H2O(s).
3. This solid is isolated by filtration and weighed.
From the mass of MgNH4PO4.6H2O , you will calculate the equivalent mass of "P2O5" since this
is what the manufacturer cites. Once you have calculated the equivalent mass of "P2O5", you will
calculate what percent this mass of "P2O5" is of the original mass of fertilizer, and finally compare
that percentage to the manufacturer's specification.
So how would you convert your mass of MgNH4PO4.6H2O to the equivalent mass of
"P2O5"? The two substances are linked by
P 2O 5
cited by the manufacturer
→
(NH4)H2PO4
actual fertilizer
→
MgNH4PO4.6H2O(s)
your product
A general procedure for stoichiometric calculations is outlined on page 109 of Zumdahl. Steps 2 to
5 are illustrated in the following flow chart.
moles of substance A
1
molar mass of A
mass of substance A in (e.g.) grams
mol B
mol A
moles of substance B
molar mass of B
mass of substance B in (e.g.) grams
This procedure can be used to relate the mass of a reactant to that of a product, or to relate the mass
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of two reactants or of two products in a given reaction to each other.
Note that in your calculation of the equivalent mass of "P2O5" you do not have to balance
the complete reaction (step 1 of Zumdahl) to obtain the relevant mole ratio of P2O 5 to
MgNH4PO4.6H2O(s) as phosphorus will appear only as P2O5 on one side of the equation and as
MgNH4PO4.6H2O on the other side of the equation.
Observations Sheet and Report Sheet
In professional laboratories, notebooks provide permanent, legally-defensible
documentation of the work done: records must be in ink. As an ongoing record of your lab work,
the Observations Sheets in this Manual are to be treated similarly: entries must be in ink. It is not
acceptable to record your results in pencil and/or on other scrap paper, and then recopy the
"cleaned up" results onto the Observations Sheet. If a change is needed, "white-out" must not be
used. Instead, neatly draw a line through the old observation or data, and record the new
observation or data in unused space nearby.
The purpose of the Report Sheet is to explain your results, or to extract desired variables by
calculation from your measurements. Marks will be deducted if the Observations Sheet and Report
Sheet are not completed in ink. It is strongly recommended that you answer the Prelab and Report
Sheet questions on your own paper first, and then, after you are satisfied with your answer, copy
your answer onto the Prelab Sheet or Report Sheet supplied in this Manual.
Code of Student Behaviour: Plagiarism and Cheating
The University of Alberta General Faculties Council ( GFC ) mandates that every student be
told at the start of every class that the University and the Campus Law Review Committee of GFC
view Plagiarism and Cheating as "serious academic offenses" (see "Code of Student
Behaviour", UA Calendar, p. 713 or on the UA website for more details). Briefly, from the Code of
Student Behaviour:
(a) Plagiarism, sec 30.3.2(1). "No student shall submit the words, ideas, images or data of
another person as the Student's own..."
(b) Cheating, sec30.3.2(2)c and 30.3.2(2)d. "No Student shall represent another's substantial
editorial or compositional assistance... as the Student's own work.
(c) Misuse of confidential Materials, sec 30.3.2(3). No Student shall submit...all or a
substantial portion of any academic writing .... for which credit has previously been
obtained by the Student..."
Note also 30.3.6(5) Participation in an Offense:
No Student shall counsel or encourage or knowingly aid or assist, directly or indirectly,
another person in the commission of any offense under this Code.
This means for example that you MUST NOT bring completed observations sheets, or results in
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any form for the current experiment (even your own results if you had previously taken this course)
to the lab sessions for "reference" or "to check your results".
It is highly recommended that you visit the "Truth in Education" website maintained by the
Student OmbudServive at:
http://www.ualberta.ca/STUDENTSERVICES/tie/whatisit.htm
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EXPERIMENT A PRELAB ASSIGNMENT
STOICHIOMETRY AND REACTIONS IN AQUEOUS SOLUTIONS
Name ______________________________
TA ________________________________
Hint - If you do the prelab on ordinary paper first and then copy your solutions onto this form, it
will be neater and you can refer to the rough copy when doing the report form.
Submit this sheet with your solutions to the following problems as you enter the lab class.
Show your work - answers without supporting work will get zero marks. These problems are
worth a total of 7 marks.
Review the systematic naming of compounds in your textbook.
Bonus point (1 mark): Bring an aluminum soda can to use in Expt. A.
1. Read the safety regulations in the first part of this manual (pp. 10 - 12).
(a) When are safety glasses required in the lab? (0.5 mark)
(b)
What should you do immediately ( even before telling your TA ) if you get any lab
reagents in your eyes ? (0.5 mark)
(c)
What does the following WHMIS symbol mean?
(0.5 mark)
(d)
Where do you turn in lab report sheets?
(0.5 mark)
(Questions continue on the next page)
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(e)
EXPERIMENT A
STOICHIOMETRY AND REACTIONS IN AQUEOUS SOLUTIONS
PRELAB ASSIGNMENT (CONTINUED)
What are the room numbers for Lab and Lecture/Lab help?
(0.5 mark)
2. Differentiate between empirical formula, molecular formula and structural formula. Illustrate
your answer using the formulae for dinitrogen tetroxide. See p. 68 of Zumdahl. This
molecule is also on the textbook CD (you might want to change the Rasmol display to e.g.
"stick and ball" using the pull down menu). (1.5 marks)
3. Suppose you analyze a sample of fertilizer by the procedure of this experiment and obtain
1.000g of MgNH4PO4.6H2O(s) as the product.
(a) Calculate the molar mass of MgNH4PO4.6H2O(s). (1 mark)
(b)
Calculate the molar mass of P2O5. (1 mark)
(c)
Calculate the equivalent mass of "P2O5" initially in the sample. (Hint - You do not have to
balance the complete reaction to obtain the relevant mole ratio as phosphorus will appear only
as MgNH4PO4.6H2O on one side of the equation and as P2O5 on the other side of the
equation.). (1 mark)
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REAGENTS AND EQUIPMENT
hot plate
Al scraps
1.5 M KOH glass wool 6 M H2SO4
fertilizer ("plant food") containing P
0.4M MgSO4
6M NH3
9 cm filter paper
EXPERIMENTAL PROCEDURE
Put on your safety glasses.
I. Formation of KAl(SO4)2•12H2O from aluminum scraps
Obtain a piece of aluminum scrap. Cut up the scrap into small pieces with scissors from the
lab cart. Clean and dry a 250 mL beaker; label it with your initials. Weigh this beaker and record
its mass on the Observations sheet (reminder: record all weighings to 0.001 g ). Then weigh into it
(0.960 to 1.040) g of the Al scraps .
Caution: 1.5M KOH is corrosive. In the fumehood, carefully add about 50 mL of 1.5M
KOH solution to the beaker containing the Al scraps; then heat on a hot plate in the fumehood.
Check the liquid level occasionally; if the liquid level decreases to less than 20 mL, add distilled
water to return the volume to around 25 mL. While this solution is heating, go on to Part II. (You
will finish Part I after the product in Part II has been filtered and is drying).
After you've finished heating the solution, the final volume should be between 20 to 25 mL.
The solution will be filtered while hot. Warning: no makeup laboratory will be allowed in the
event that the beaker is dropped or your solution is otherwise spilled. So be careful.
Clean a 125 mL Erlenmeyer flask and label it with your initials. Then set up the apparatus
as follows:
(a) Attach a utility clamp to a lab stand (metal posts attached to your bench top).
(b) Press a small plug of glass wool into the bottom of a clean 65 mm short funnel.
(c) Then support the funnel with the utility clamp with its spout in a clean, dry 125 mL
Erlenmeyer flask as shown in Figure 1.
funnel
glass wool plug
utility clamp
Erlenmeyer flask
bench top
FIGURE 1
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Transport the hot beaker from the fumehood to your benchtop using a paper towel rolled up several
times (to increase the thickness of the paper). Pour the hot solution through the funnel. If there are
still particles in the filtrate, repeat the filtration. Then let the solution cool.
Bring the Erlenmeyer to the fumehood. In the fumehood, acidify the solution by
SLOWLY adding to the Erlenmeyer flask 20 mL of 6M H2SO4 while gently swirling the flask.
Heat the mixture on a hot plate gently ( while still stirring ) for about 2 min. until the solution is
clear. Transport the Erlenmeyer flask carefully from the fumehood to your benchtop using a paper
towel rolled up several times (to increase the thickness of the paper), and put it immediately
(uncovered) into your locker until next week.
Finish Part II.
II. Phosphorus in a Fertilizer
Start this part while the aluminum scraps are dissolving. Weigh a clean and dry 150 mL
beaker; then weigh 2 to 3 grams ( record masses to 0.001 g ) of plant fertilizer into it. Add about
40 mL of distilled water into the beaker, and stir to dissolve the fertilizer. Next, add approximately
30 mL of 0.4M MgSO4 solution to your beaker containing the fertilizer; gently stir.
In this next step you will be monitoring the pH of the solution with pH paper. Add 6M
NH3(aq) until the pH is 8-9; this requires about 6-10 mL of NH3(aq). Once the solution is at the
appropriate pH level, let the solution sit for 15 minutes (occasionally stir) to ensure that the reaction
goes to completion.
While you are waiting for this reaction to finish, set up a vacuum filtration apparatus (your
TA will demonstrate this) by the following steps:
(a) attach an utility clamp to the lab stand nearest to the bench top vacuum (yellow label).
(b) use this clamp to secure your filter flask.
(b) obtain a length of thick-walled vacuum hose, a black rubber adapter, and a large Buchner
funnel from the storage cupboard.
(c) use the vacuum hose to connect the line vacuum and the filter flask.
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Do not put the funnel on until you are ready to filter because it is heavy and can easily be knocked
over and break.
Weigh one piece of 9.0 cm filter paper on your 100 mm watch glass, and record the mass of
the watch glass and filter paper on the Observations sheet under "product" as the "mass of the
container". Put the rubber adapter on the filter flask, and put on the Buchner funnel (see following
figure - these are the large Buchner funnels).
FIGURE 2
Insert the weighed piece of filter paper in the Buchner funnel, wet the paper with distilled water, and
turn on the vacuum.
When the solution is ready for filtration, slowly transfer the solution and its precipitate onto
the filter paper in the Buchner funnel. Rinse the beaker three times with distilled water, pouring this
onto the precipitate. Rinse the precipitate with another two 5 mL portions of distilled water, and
then with two 5 mL portions of 95% ethanol. Leave the vacuum on for at least 20 minutes to
continue pulling air through the precipitate to dry it. Go back to Part I and finish it. The aluminum
scraps should now be dissolved.
After drying the precipitate, transfer the filter paper and precipitate to the watch glass
previously weighed with the filter paper. Weigh this ensemble and record its mass. The difference
of this mass to the mass of the initial mass of the watch glass and filter paper is your mass of
MgNH4PO4.6H2O(s).
Finally, scrape the MgNH4PO4.6H2O(s) into one of the liquid waste containers in the
fumehood. Rinse the filter paper with a bit of tap water and discard the filter paper into the garbage
container under the sink.
Part III. Crystals
In the second lab period, show your KAl(SO4)4•12H2O crystals to your Lab Instructor (do
not separate the crystals from the solution). Your Lab Instructor will visually assess them and
assign up to 2 marks based on the amount and quality of these crystals.
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Check that all your equipment is back in your locker, and that the lock is securely locked.
When you close the lock, spin the dial on the lock. Otherwise the lock might still open, and you
will have to pay for any missing items. If a Lab Instructor finds your locker unlocked, it will be put
on backwards to let you know that it had been found opened.
Before leaving the lab, wipe off your bench top with a damp towel and then wash your
hands. It's good practice to do this after each experiment.
ASSESSMENT
Turn in a copy of your Observations Sheet at the end of the lab period. Your mark for this
experiment will be based on the Prelab Assignment (7 marks), Observations Sheet (3 marks),
Report Sheet (8 marks) and your crystals (2 marks).
The Report Sheet is due according to the schedule on page 20. One mark will be deducted
if the Report sheet is not done in ink. You should answer the Report Sheet questions on ordinary
paper first and then copy your solutions onto the Report Sheet.
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REPORT: EXPERIMENT A
STOICHIOMETRY AND REACTIONS IN AQUEOUS SOLUTIONS
Name ______________________________
Lab Period _________ Lab Room _______
TA ________________________________
Show your work - answers without supporting work will get zero marks.
1. From your mass of MgNH4PO4.6H2O(s) collected, calculate your value
(a) for the mass of "P2O5 " in the initial mass of fertilizer analyzed. (1 mark)
(b) the % "P2O5 " in the fertilizer. Compare this with the manufacturer's specification.
(1 mark)
2. Balance the following equations by "inspection", that is, trial and error (recall that the charge
should be balanced first and then the number of each type of atom ).
(a) (1 mark) Al(s) + KOH(aq) + H2O → K+(aq) + Al(OH)4-(aq) + H2(g)
(b)(1 mark) Al(OH)4-(aq) + H2SO4(aq) → Al(OH)3(s) + H2O + SO42-(aq)
(Continued next page)
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(c)
(1 mark) Al(OH)3(s) + H2SO4(aq) → Al3+(aq) + SO42-(aq) + H2O
3. The atomic mass of Al is 26.98154 g/mol. Is it possible to have 5.0 x 10-25 g of Al?
Explain. (Hint - how many Al atoms would 5.0 x 10-25 g of Al correspond to?) (1 mark)
4. Assume that you carry out the following dilutions:
(i) You take 1mL of 10-6 M of AsCl3 and diluted it to 1 kiloliter (Solution A).
(ii) Then you take 1 µL of Solution A and dilute to 10 L to make Solution B.
(iii) Then you take 1 mL of Solution B and dilute to 1 L to make Solution C.
(a) Calculate the concentration of AsCl3 in Solution C. (1 mark)
(b) Calculate the number of molecules of AsCl3 that would be in 1 mL of Solution C at this
concentration. Look at your answer. What is the most likely number of molecules of AsCl3 in
that 1 mL? (1 mark)
ASSESSMENT
Prelab assignment
Observations sheet
Report form
Crystals
Other
TOTAL
(7 marks)
(3 marks)
(8 marks)
(2 marks)
(if any):
(max of 20 marks):
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OBSERVATIONS SHEET - EXPERIMENT A*
Stoichiometry and Reactions in Aqueous Solutions (3 marks)
Weighings
Al scraps
fertilizer
product
mass of container + solid
mass of container
mass of sample
Part I:
Note the formation/dissolution of any solids, colors of solutions, volume, formation of gases, etc.
Where you refer to the compounds, use the correct chemical formula or name.
2. After addition of KOH and heating (1 mark):
____________________________________________________________
________________________________________________________________________
________________________________________________________________________
3. After filtration (0.5 mark):
____________________________________________________________
________________________________________________________________________
4. After addition of 6M H2SO4 and before heating (1 mark):
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
After heating (0.5 mark):
________________________________________________________________________
________________________________________________________________________
Part II:
Record from the label of the fertilizer container, the guaranteed percent of available
"P2O5":
____________________________
__________________________
Signature of Lab Instructor
Name of Student
OBSERVATIONS SHEET - EXPERIMENT A*
*
Reminder - Observation Sheets (and Report Sheets) MUST be completed in ink.
_________
Lab Period
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Stoichiometry and Reactions in Aqueous Solutions (3 marks)
Weighings
Al scraps
fertilizer
product
mass of container + solid
mass of container
mass of sample
Part I:
Note the formation/dissolution of any solids, colors of solutions, volume, formation of gases, etc.
Where you refer to the compounds, use the correct chemical formula or name.
2. After addition of KOH and heating (1 mark):
____________________________________________________________
________________________________________________________________________
________________________________________________________________________
3. After filtration (0.5 mark):
____________________________________________________________
________________________________________________________________________
4. After addition of 6M H2SO4 and before heating (1 mark):
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
After heating (0.5 mark):
________________________________________________________________________
________________________________________________________________________
Part II:
Record from the label of the fertilizer container, the guaranteed percent of available
"P2O5":
____________________________
Signature of Lab Instructor
*
__________________________
Name of Student
Reminder - Observation Sheets (and Report Sheets) MUST be completed in ink.
_________
Lab Period