Section 9.3: Chemical Analysis
Tutorial 1 Practice, page 439
1. (a) Step 1. According to the solubility table, sulfide ions precipitate both strontium and
manganese. However, sulfate ions precipitate strontium but not manganese.
Step 2. Strontium ions must be precipitated first by adding a solution of a soluble sulfate
compound such as sodium sulfate, Na2SO4(aq). If the test is positive, filter the mixture to remove
the strontium ions. Adding a solution of sodium sulfide should produce a precipitate of
manganese sulfide, if the manganese ions are present.
Step 3. The flow chart for the test sequence is shown.
(b) Step 1. According to the solubility table, sulfide ions precipitate both copper and iron.
However, chloride ions precipitate copper but not iron.
Step 2. Copper ions must be precipitated first by adding a solution of a soluble chloride
compound such as sodium chloride, NaCl(aq). If the test is positive, filter the mixture to remove
the copper ions. Adding a solution of sodium sulfide should produce a precipitate of iron sulfide,
if the iron ions are present.
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Chapter 9: Solutions and Their Reactions 9.3-1
Step 3. The flow chart for the test sequence is shown.
(c) Step 1. According to the solubility table, both acetate and hydroxide precipitate silver ions.
However, hydroxide ions precipitate copper but acetate does not.
Step 2. Hydroxide ions must be precipitated first by adding a solution of a copper compound
such as copper nitrate, Cu(NO3)2(aq). If the test is positive, filter the mixture to remove the
hydroxide ions. Adding a solution of silver nitrate should produce a precipitate of silver acetate if
the acetate ions are present.
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Chapter 9: Solutions and Their Reactions 9.3-2
Step 3. The flow chart for the test sequence is shown.
Section 9.3 Questions, page 441
1. (a) Testing an unknown gas with a burning splint is a qualitative analysis.
(b) Measuring the density of a pure metal is a quantitative analysis.
(c) Conducting a flame test to identify cations is a qualitative analysis.
(d) Determining the concentration of carbon dioxide in the atmosphere is a quantitative analysis.
(e) Testing the pH of rainwater and finding that it is 5.3 is a quantitative analysis.
2. The copper(II) ions are the limiting reagent since they are completely precipitated from the
solution. Therefore, the sodium hydroxide is the excess reagent.
3. It is necessary to perform the steps in the precipitation reactions in the correct order during a
qualitative analysis so that the appropriate ions are removed first from the solution so that they
do not interfere with later reactions.
4. Advancements in the design of the laboratory burner made spectroscopy possible as
spectroscopy required a stable, almost colourless flame. This is an example of how advances in
technology are necessary before advances in science can occur.
5. (a) When barium chloride (BaCl2) is placed in a Bunsen burner, the flame should turn
yellow-green.
(b) When zinc carbonate (ZnCO3) is placed in a Bunsen burner, the flame should turn
whitish-green.
(c) When copper(I) chloride (CuCl2) is placed in a Bunsen burner flame, the flame should
turn green.
(d) When sodium chloride (NaCl) is placed in a Bunsen burner flame, the flame should
turn yellow.
6. The cations responsible for the colours seen in the fireworks are copper (green), lithium or
strontium (red), and potassium (violet).
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Chapter 9: Solutions and Their Reactions 9.3-3
7. It is necessary to use distilled water when preparing solutions for a qualitative analysis to
make sure that no ions are added to interfere with the analysis. Distilled water contains no
dissolved ions while tap water does. The dissolved ions in tap water may form precipitates with
the ions in the chemicals used during the analysis and interfere with the analysis.
8. (a) According to the solubility table, iodide ions precipitate lead and acetate ions precipitate
silver. The addition of a sodium acetate solution should produce a precipitate if silver ions are
present. The mixture is then filtered. The addition of a sodium iodide solution should produce a
precipitate if lead(II) ions are present.
(b) According to the solubility table, phosphate ions precipitate calcium. Adding sodium
phosphate solution should produce a precipitate if calcium ions are present. The presence of
sodium ions is confirmed if the solution produces a yellow colour when it is added to a burner
flame.
(c) According to the solubility table, sulfate ions precipitate barium, chloride ions precipitate
copper(I), and hydroxide ions precipitate iron(III). Thus, the presence of barium ions is
confirmed if a precipitate forms when a solution of potassium sulfate is added. The mixture is
then filtered. The presence of copper(I) ions is confirmed in the remaining solution if a
precipitate forms when a solution of potassium chloride is added. The mixture is then filtered.
Iron(III) ions are confirmed in the remaining solution if a precipitate forms when a solution of
potassium hydroxide is added.
(d) According to the solubility table, barium ions precipitate sulfate and silver ions precipitate
chloride. The presence of sulfate ions is confirmed if a precipitate forms when a barium nitrate
solution is added. The mixture is then filtered. The presence of chloride ions is confirmed if a
precipitate forms when a silver nitrate solution is added.
9. Samples of each compound could be dissolved to form solutions. The addition of a sodium
hydroxide solution to each should produce a precipitate only in the magnesium chloride sample.
Potassium chloride and sodium chloride can be distinguished using flame tests. Placing a few
crystals of each compound into a flame should change the flame colour. The compound that
results in a violet flame colour is potassium chloride. The compound that results in a yellow
flame colour is sodium potassium chloride.
10. No, the analysis would not be effective if the reactions were reversed because the addition of
sulfate ions precipitates both lead and barium ions.
11. (a) A compound that precipitates silver, barium, and iron ions would be sodium phosphate.
(b) Step 1. According to the solubility table, acetate precipitates silver, but not barium or iron
ions. Sulfate precipitates both silver and barium ions but not iron. Hydroxide precipitates both
silver and iron ions but not barium.
Step 2. Silver ions must be precipitated first by adding a solution of a soluble acetate compound
such as sodium acetate, Na2C2H3O2(aq). If the test is positive, filter the mixture to remove the
silver ions. Adding a solution of sodium sulfate, Na2SO4(aq),should produce a precipitate of
barium sulfate, if the barium ions are present. If the test is positive, filter the mixture to remove
the barium ions. Adding a solution of sodium hydroxide, NaOH(aq), should produce a precipitate
of iron hydroxide if iron ions are present.
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Chapter 9: Solutions and Their Reactions 9.3-4
Step 3. The flow chart for the test sequence is shown.
12. A spectrophotometer uses light to determine the concentration of coloured solutions. A
sample of a solution is inserted into the spectrophotometer. A beam of light of a specific
wavelength is passed through the sample. The substances responsible for the colour of the
sample absorb a portion of the incident light. The amount of light absorbed is related to the
concentration of the sample. The spectrophotometer compares the intensity of the light entering
and exiting the sample. This difference is used to determine the concentration of the sample. A
spectrophotometer is commonly used to determine the concentration of metals in aqueous
solutions, measure the growth of microorganisms in a sample, food analysis, and matching
colours in textiles.
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Chapter 9: Solutions and Their Reactions 9.3-5
13. (a) A Breathalyzer determines blood alcohol by detecting the amount of alcohol in exhaled
air. The alcohol concentration in a person’s breath is related to the alcohol concentration in
blood. In one type of Breathalyzer, the breath sample is bubbled through a mixture of sulfuric
acid, potassium dichromate, silver nitrate, and water. The sulfuric acid removes the alcohol from
the air into a liquid solution. The alcohol then reacts with the potassium dichromate to produce
chromium sulfate, potassium sulfate, ethanoic acid (also called acetic acid), and water. During
the reaction, the potassium dichromate changes colour from reddish brown to the green
chromium ion. The amount of colour is related to the amount of alcohol in the breath.
Another type of Breathalyzer involves an oxidation reduction reaction. Ethanol in the
exhaled air is oxidized to ethanoic acid (also called acetic acid) at an electrical terminal of the
Breathalyzer unit called the anode. At another terminal called the cathode, oxygen molecules
from the atmosphere gain four electrons and react with hydrogen ions to form water. The process
of gaining electrons in a chemical reaction is called a reduction reaction. The flow of electrons
between the anode and cathode generates an electrical current that can be measured and
converted to give the blood alcohol content.
(b) An advantage of a pocket-sized Breathalyzer is that it provides a quick and convenient way
to test the BAC of a driver. It can also help the consumer better understand how the body
responds to the effects of alcohol consumption. However, a Breathalyzer reading below the
acceptable limit may give the consumer a false sense of security about his or her capability at
driving a vehicle safely.
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Chapter 9: Solutions and Their Reactions 9.3-6