Scientific Observations and Reaction Stoichiometry:
The Qualitative Analysis and Chemical Reactivity
of Five White Powders
Objectives
Part 1: To determine the limiting reagent and percent yield of CuCO3 when Na2CO3 (one
of the white powders used in Part 2 of this experiment) reacts with an equal mass of
CuCl2.
Part 2: (a) To determine some of the chemical and physical properties of five white solid
compounds, and (b) to use these properties to determine the composition of a binary
mixture of these compounds.
Background
In the first part of this experiment, the double displacement (or metathesis) reaction
between Na2CO3(aq) and CuCl2(aq) will be studied. As indicated in the reaction shown
below, one of the products of this reaction is a precipitate. A precipitate is an insoluble,
solid substance that separates from the solution in the course of a reaction.
Na2CO3 (aq) + CuCl2 (aq) → 2 NaCl (aq) + CuCO3 (s)
In this experiment, solutions containing equal masses of Na2CO3 and CuCl2 will be
reacted. The solid CuCO3 that precipitates out of solution (which represents the actual
product yield) will be collected and weighed. Using this information and the amount of
Na2CO3 and CuCl2 that were initially reacted, the limiting reagent, the theoretical yield of
CuCO3, and the percent yield of CuCO3 will be determined.
Chemical reactivity and physical properties are characteristic of a substance and may be
used for compound identification. Although one or two properties of compounds may be
similar, it is those properties which are different that may serve to distinguish one
substance from another. In the second part of this experiment, the solubility, solution pH,
and chemical reactivity [with HNO3, the sulfate ion (SO42-), and the silver ion (Ag+)] of
five white inorganic compounds, specifically BaCO3, BaCl2, BaSO4, Na2CO3, and NaCl,
will be examined. After completing these qualitative tests, each group will receive an
“unknown” containing only two of these white powders. The binary (2-component)
mixtures will be subjected to a battery of similar qualitative tests, and the observed data
will be used to identify the constituent compounds.
Pre-Lab Questions
1. Name the following compounds: (a) BaCO3; (b) BaCl2; (c) BaSO4; (d) Na2CO3;
(e) NaCl; (f) CuCl2; and (g) HNO3.
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2. When solutions containing 1.00 g of BaCl2 and 1.00 g of Na2SO4 were mixed, solid
BaSO4 formed. The BaSO4 precipitate was filtered out of the solution, allowed to
dry, and then weighed. The mass of BaSO4 that was isolated from solution was
1.04 g. Use this information to answer the following questions. Pay close attention
to the proper use and reporting of significant figures and units. You must show all
of your work (i.e., your calculations) for full credit.
(a) Write a balanced chemical equation that shows the reaction that occurs when
an aqueous solution of BaCl2 reacts with an aqueous solution of Na2SO4 to form
aqueous sodium chloride and solid BaSO4. In your equation, you should
indicate the physical states of each reactant and product. Note: Precipitation
reactions are discussed in Section 4.2 of your textbook.
(b) Calculate the moles of BaCl2 that are available for reaction.
(c) Calculate the grams of BaSO4 that should form if all of the available BaCl2
reacts.
(d) Calculate the moles of Na2SO4 that are available for reaction.
(e) Calculate the grams of BaSO4 that should form if all of the available Na2SO4
reacts.
(f) Which reactant (BaCl2 or Na2SO4) is the limiting reagent in this reaction?
Why?
(g) What is the theoretical yield of BaSO4?
(h) What is the percent yield of BaSO4? (See Section 3.10 of your textbook, pages
106-109, for information regarding the calculation of a percent yield.)
(i) If this reaction went to completion, then how many grams of the excess reagent
should remain after the reaction is complete?
Safety
Goggles must be worn at all times. Care must be exercised when using nitric acid.
Barium, silver, and copper compounds in solid or solution form (and the first washing of
the precipitates of these ions) must be collected and disposed of as directed by your TA.
Laboratory Procedures
Part 1
Reagents: Na2CO3 and CuCl2
Weigh accurately (correct to 0.0001 g) 1.0 g of Na2CO3 and 1.0 g of CuCl2. Record the
measured masses in your laboratory notebook. Place the Na2CO3 and the CuCl2 into
separate 100 mL beakers. Add approximately 25 mL of water to each beaker. Stir (or
agitate) the solutions until the solids fully dissolve. Record the color of each solution.
Pour approximately 5 mL of the CuCl2 solution into the beaker containing the dissolved
Na2CO3. Record your observations. Continue adding the CuCl2 solution a few mL at a
time. After pouring out all of the CuCl2 solution, rinse the emptied beaker with
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approximately 2-3 mL of water and add this rinse to the Na2CO3 beaker. If necessary,
repeat this a second time to ensure that all of the CuCl2 is transferred. Gently stir the
reaction mixture for several minutes with a stirring rod.
Assemble a vacuum filtration apparatus (see Figure 1 below) to separate the copper (II)
carbonate precipitate from the solution. While gravity filtration would allow isolation of
the insoluble product, vacuum filtration will be used to expedite the separation process
and to help dry the solid residue. Make sure that flexible vacuum tubing is used to
connect the filter flask to the water aspirator. DO NOT connect the tubing to the water
outlet. After assembling a vacuum filtration apparatus, have a TA verify the setup before
proceeding.
Figure 1. Different views of a vacuum filtration apparatus.
Weigh (correct to 0.0001 g) a piece of filter paper and record the measured mass. Place
the filter paper in the Buchner funnel, moisten it with a small amount of water, and then
turn on the water aspirator. The suction will seal the filter paper to the funnel. This will
prevent your precipitate from flowing underneath the filter paper when filtering your
reaction mixture. While the vacuum is on, slowly begin to pour your reaction mixture
into the funnel by decanting the liquid along the length of a stirring rod. Avoid pouring
the mixture along the edges of the filter paper. This could result in a loss of product. If
precipitate remains in the beaker after you have finished pouring, then rinse the beaker
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with a small amount of water and transfer the precipitate residue to the filter paper.
Repeat as necessary to make sure that all of the precipitate is collected onto the filter
paper. What color is the precipitate? What color is the filtrate (the fluid in the filter
flask)?
Continue to pull a vacuum until the precipitate is mostly dry. If the precipitate does not
dry sufficiently, then carefully transfer the filter paper and precipitate to a clean watch
glass and place the watch glass into an incubator oven for additional drying. You should
start Part 2 of this experiment while you wait for your solid CuCO3 to dry.
When the filter paper and precipitate are dry, weigh (correct to 0.0001 g) the filter paper
and precipitate. Record this mass. Use the mass of the filter paper and the combined
mass of the filter paper and precipitate to determine the mass of CuCO3 that was isolated.
Calculate the theoretical yield of CuCO3 based on the initial amounts of Na2CO3 and
CuCl2 that were allowed to react. Which reactant was the limiting reagent? Which
reactant was the excess reagent? Explain your reasoning. How many grams of the
excess reagent should remain in solution if the theoretical yield of CuCO3 is produced?
What is the percent yield of your reaction? If your calculated percent yield is not 100%,
explain possible reasons for this.
Part 2(a)
Reagents: white solids - BaCO3, BaCl2, BaSO4, Na2CO3, and NaCl; 3 M HNO3, 0.1 M
Na2SO4, 0.1 M AgNO3, Universal Indicator Solution
This part of the experiment involves the qualitative analysis of five white powders,
namely BaCO3, BaCl2, BaSO4, Na2CO3, and NaCl. Each of these solids will be tested for
solubility in water. The resulting aqueous mixtures will be tested for pH, treated with
nitric acid, and then subsequently treated with sodium sulfate and silver nitrate solutions.
Construct a table in your laboratory notebook like the one shown below.
Compound
Solubility in
water
Reaction
with HNO3
pH
Reaction
with Na2SO4
Reaction
with AgNO3
BaCO3
BaCl2
BaSO4
Na2CO3
NaCl
Record your observations in this table as you perform the five tests on each of the five
white solids.
A small amount (pea-sized or less is appropriate) of each of the white solids should be
placed in separate small test tubes. Add water to each test tube to dissolve (or attempt to
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dissolve) the solids. Record your observations. Add Universal Indicator to the "solution"
or mixture. Note the approximate pH of each. (A chart indicating the color of the
Universal Indicator solution at various pHs will be available in the laboratory.) Record
whether the measured pH indicates an acidic, neutral, or basic solution. Add nitric acid
to each test tube in a drop-wise manner until the solution becomes acidic. Record your
observations. The resulting solutions should be decanted (if necessary) and divided in
half. Add sodium sulfate solution to one portion and silver nitrate solution to the other.
Record your observations.
Part 2(b)
Reagents: 3 M HNO3, 0.1 M Na2SO4, 0.1 M AgNO3, Universal Indicator Solution, and
unknown binary mixtures of BaCO3, BaCl2, BaSO4, Na2CO3, and/or NaCl
A binary mixture of two of the white solids will be subjected to the same tests used in
Part 2(a) of this experiment, and the composition of the mixture will be determined.
Your TA will give you a small amount of an unknown mixture containing two of the
white solids you have studied. Record the number (#1, #2, #3, #4, or #6) assigned to
your unknown. Devise a method of analysis to determine the composition of your binary
mixture. Discuss this with your group and with your TA before you begin. You may
wish to develop a flow chart for your analysis. Perform your analysis. Be sure to
identify the components of your unknown binary mixture in your notebook. In your
laboratory report, you must justify how your data support your conclusions.
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Post-Lab Questions
Solubility tests, not unlike the ones that you performed in this lab, have been used to
formulate a list of rules that allow one to predict the solubility of common inorganic
compounds. The solubility rules that are given in Chapter 4 of your textbook are
summarized below.
Solubility Rules for Common Ionic Compounds in Water at 25 °C
Soluble Compounds
- Compounds containing alkali metal ions
or the ammonium ion (NH4+)
Exceptions
- Nitrates (NO3-), bicarbonates (HCO3-),
chlorates (ClO3-)
- Halides (Cl-, Br-, and I-)
- Halides of Ag+, Hg22+, Pb2+
- Sulfates (SO42-)
- Sulfates of Ag+, Ca2+, Sr2+, Ba2+, Hg22+, Pb2+
Insoluble Compounds
- Carbonates (CO32-), phosphates (PO43-),
chromates (CrO42-), sulfides (S2-)
Exceptions
- Compounds containing alkali metal ions or
the ammonium ion
- Hydroxides
- Compounds containing alkali metal ions or
the Ba2+ ion
Use these solubility rules to help you answer some of the following questions.
1. What is gravity filtration? How is this technique different from the filtration
method used in Part 1 of this experiment?
2. Explain how the porosity of the filter paper could affect the amount of precipitate
that is collected in Part 1 of this experiment.
3. Why is it important to dry your precipitate in Part 1 of this experiment?
4. In Part 2(a) of this experiment, you examined the solubility of each of the five white
solids in water. Do your recorded observations agree with the predictions given by
the solubility rules? Explain.
5. Acids react with carbonates and bicarbonates to produce a “salt” (which can be
described as an ionic compound made up of a cation other than H+ and an anion
other than OH-), H2O(l), and carbon dioxide gas (CO2). Did you see evidence of
gas formation when you added nitric acid to the two carbonates used in this lab
(BaCO3 and Na2CO3)? Explain. Write balanced chemical equations for these
chemical reactions (see below). In your equations, you should indicate the physical
states of each reactant and product. Use the solubility rules to help you predict
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whether the “salt” formed in each reaction is soluble or insoluble. Do your
experimental observations agree with your predictions? Explain.
BaCO3 (aq) + HNO3 (aq) → ?
Na2CO3 (aq) + HNO3 (aq) → ?
6. When an ionic compound dissolves in solution, the ionic substance dissociates into
its component ions. Based on solubility rules, which cation(s) in the five white
solids would be expected to react with the sulfate anion (SO42-) to form an insoluble
ionic compound? What is/are the precipitate(s) that should form? Explain how
your observations agree or disagree with your predictions.
7. Based on solubility rules, Ag+ should form silver carbonate (Ag2CO3), an insoluble
solid, when reacted with CO32-. Indeed, Ag2CO3(s) readily forms when aqueous
solutions of Na2CO3 and AgNO3 are mixed. Did you see a precipitate form when
AgNO3 was added to an acidified solution of Na2CO3? Explain why you would or
would not expect a precipitate to form under these conditions (consider solubility
rules and the concepts covered on Question 5).
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