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, Na 2 SO 4 (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. Copyright 2011 Nelson Education Ltd. 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(NO 3 ) 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. Copyright 2011 Nelson Education Ltd. 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 (BaCl 2 ) is placed in a Bunsen burner, the flame should turn yellow-green. (b) When zinc carbonate (ZnCO 3 ) is placed in a Bunsen burner, the flame should turn whitish-green. (c) When copper(i) chloride (CuCl 2 ) 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). Copyright 2011 Nelson Education Ltd. 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, Na 2 C 2 H 3 O 2 (aq). If the test is positive, filter the mixture to remove the silver ions. Adding a solution of sodium sulfate, Na 2 SO 4 (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. Copyright 2011 Nelson Education Ltd. 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. Copyright 2011 Nelson Education Ltd. 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. Copyright 2011 Nelson Education Ltd. Chapter 9: Solutions and Their Reactions 9.3-6