City Colleges of Chicago Harry S Truman College Gravimetric Analysis Paper

CHEM 222Exp. 2: Gravimetric Determination of Calcium as CaC2O4 ⋅ H2O
Page 1 of 4
Name: ______________________________________________ Date: ___________________
TA ́s Name: _________________________________________
PRE-LAB Answer all questions on one page to submit to gradescope:
Q1
Q2
Q3
Q4
Q5
CHEM 222
Exp. 2: Gravimetric Determination of Calcium as CaC2O4 ⋅ H2O
Page 2 of 4
Name: ______________________________________________ Date: ___________________
TA ́s Name: _________________________________________
UNKNOWN #: ___________
trial 1
empty funnel weight (g)
funnel + precipitate weight (g)
precipitate weight (g)
molarity of Ca2+
average molarity
std. dev. of molarity
relative std. dev. of molarity
g CaCO3/L
average g CaCO3/L
Sample Calculations
trial 2
trial 3
CHEM 222
Exp. 2: Gravimetric Determination of Calcium as CaC2O4 ⋅ H2O
Page 3 of 4
Name: ______________________________________________ Date: ___________________
TA ́s Name: _________________________________________
Post-LAB Answer all questions on one page to submit to gradescope (include
scan of notebook at end):
Q1
Q2
Q3
CHEM 222
Exp. 2: Gravimetric Determination of Calcium as CaC2O4 ⋅ H2O
Page 4 of 4
Name: ______________________________________________ Date: ___________________
TA ́s Name: _________________________________________
Submit notebook pages
3. Gravimetric Determination of Iron as Fe2O3
Quantitative Chemical Analysis
Gravimetric Determination of Calcium as CaC2CO4 × H2O1
2.
Calcium ion can be analyzed by precipitation with oxalate in basic solution
to form CaC2O4 × H2O. The precipitate is soluble in acidic solution because
the oxalate anion is a weak base. Large, easily filtered, relatively pure
crystals of product will be obtained if the precipitation is carried out slowly.
Slow precipitation is achieved by dissolving Ca2+ and C 2 O 24 – in acidic
solution and gradually raising the pH by thermal decomposition of urea:
O
C
H 2N
+ 3H2O
heat
CO2 + 2NH4+ + 2OH-
NH2
Research Questions:
•
How can you use gravimetric analysis to determine the concentration of an unknown.
Learning Objectives:
•
•
•
Relate steps in a laboratory procedure to chemical reactions and physical observations.
Develop strategies to use this information to determine the concentration of an unknown.
Perform a filtration using a sintered glass funnel.
Reagents
Ammonium oxalate solution: Make 1 L of solution containing 40 g of (NH4)2C2O4 plus 25 mL of
12 M HCl. Each student will need 80 mL of this solution.
Methyl red indicator: Dissolve 20 mg of the indicator in 60 mL of ethanol and add 40 mL H2O.
0.1 M HCl: (225 mL/student) Dilute 8.3 mL of 37% HCl up to 1 L.
Urea: 45 g/student.
Unknowns: Prepare 1 L of solution containing 15–18 g of CaCO3 plus 38 mL of 12 M HCl. Each
student will need 100 mL of this solution. Alternatively, solid unknowns are available from
Thorn Smith.
Pre-Lab: Reactions: we will use the pre-lab to understand what is in each solution and the
symbolic representations in the chemical equations for each reaction.
1. Ammonium oxalate solution (NH4)2C2O4: Ammonium oxalate is soluble in water, and forms
ions. When HCl is added, it reacts with the weak base C2O42- (aq). Complete the reaction
below, be sure to indicate the state of each species in solution:
2NH4+(aq) + C2O42-(aq) + 2H+(aq) +2Cl-(aq)
1.
C. H. Hendrickson and P. R. Robinson, J. Chem. Ed. 1979, 56, 341.
3. Gravimetric Determination of Iron as Fe2O3
Quantitative Chemical Analysis
2.
The unknown solution is prepared by mixing CaCO3(s) and HCl(aq). (This is the unknown
prepared by the stockroom.) The H2CO3 produced in this reaction decomposes to CO2 (g)
and H2O. Write the complete chemical equation for this reaction, indicating the state of
each species. If ionic compounds dissociate, separate the ions in the equation.
3.
What will be observed with the reaction in question 2? What chemical species will be
present in the solution?
As indicated in the introduction, the unknown calcium (II) solution (from question 2) is
mixed with ammonium oxalate solution (from question 1). Urea is added. The solution is
heated to decompose the urea and produce hydroxide ions (OH-). What do the hydroxide
ions react with? Show the chemical reaction below.
Explain why this procedure will produce CaC2O4, and why the reaction must proceed
slowly. Predict how long it will take for the methyl red indicator to turn yellow (see the
procedure below).
4.
5.
Procedure (note, procedure pieces that are not needed this term are greyed out)
1. Dry three medium-porosity, sintered-glass funnels for 2 h at 105°C. Use a paper towel or
tongs, not your fingers, to handle the funnels. Cool them in a desiccator for 30 min and weigh
them. Repeat the procedure with 30-min heating periods until successive weightings agree to
within 0.3 mg. You will need to experiment with your oven to find appropriate heating times.
2. Use a few small portions of unknown to rinse a 25-mL transfer pipet, and discard the
washings. Use a rubber bulb, not your mouth, to provide suction. Transfer exactly 25 mL of
unknown to each of three 250- to 400-mL beakers, and dilute each with ~75 mL of 0.1 M
HCl. Add 5 drops of methyl red indicator solution to each beaker. This indicator is red below
pH 4.8 and yellow above pH 6.0.
3. Add ~25 mL of ammonium oxalate solution to each beaker while stirring with a glass rod.
Remove the rod and rinse it into the beaker with small portions of distilled water. Add ~15 g
of solid urea to each sample, cover each with a watchglass, and boil gently for ~30 min until
the indicator turns yellow.
In you notebook, write or draw observations of any physical changes in the flask. Describe what
chemical changes correspond to the observations.
4. Filter each hot solution through a weighed funnel, using suction (Figure 2-17 in the
textbook). Add ~3 mL of ice-cold water to the beaker, and use a rubber policeman to help
transfer the remaining solid to the funnel. Repeat this procedure with small portions of icecold water until all of the precipitate has been transferred to the funnel. Finally, use two
3. Gravimetric Determination of Iron as Fe2O3
Quantitative Chemical Analysis
10-mL portions of ice-cold water to rinse each beaker, and pour the washings over the
precipitate.
5. Dry the precipitate, first with aspirator suction for 1 min, then store in a desiccator for a week
or more to dry. then in an oven at 105°C for 1–2 h. Bring each filter to constant mass. The
product is somewhat hygroscopic, so only one filter at a time should be removed from the
desiccator, and weighings should be done rapidly. Alternatively, the precipitate can be dried
in a microwave oven once for 4 min, followed by several 2-min periods, with cooling for 15
min before weighing. This drying procedure does not remove the water of crystallization.
6. Calculate the average molarity of Ca2+ in the unknown solution or the average weight
percent of Ca in the unknown solid. Report the standard deviation and relative standard
deviation ( s / x = standard deviation/average).
Post-Lab:
Using the equations in the pre-lab, the steps in the procedure, and observations made during lab,
develop a model for the experiment. The model should include a symbolic representation of
each reaction, a model of the contents of each solution, and a physical description of what is
happening at each stage.
1. Draw a molecular-level picture of the contents of the Ammonium oxalate solution
(NH4)2C2O4 after HCl is added. Hint: The beaker will include all the products shown in prelab question 1.
2. Draw a molecular-level picture of the contents of the unknown solution after HCl is added.
Hint: The beaker will include all the products shown in pre-lab question 2. Explain what
physical and chemical changes occur with the addition of HCl.
3.
Draw a molecular-level picture to describe what happens as the urea is decomposed.
Include a description of the physical and chemical changes that occur.
3. Gravimetric Determination of Iron as Fe2O3
Lab component: Description
Pre-lab, (10 pts) Graded on correctness.
Data table (10 pt) Complete, neat, single-line cross-outs.
Calculations properly performed.
Sample
Graded on correctness, must include units,
Calculations (5
labels and be clear.
pts)
Post-lab question Used evidence from the lab to improve or
(6 pts).
validate model.
Notebook pages Must include brief procedure, single-line
(9)
cross-outs, all data collected in notebook,
labels/dates/names on top of the pages.
Quantitative Chemical Analysis
Poor
OK
Good