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CHAPTER 15 The

These skills are usually tested on the SAT Subject Test in Chemistry. You should be able to . . . • Name, identify, and explain proper laboratory rules and procedures. • Identify and explain the proper use of laboratory equipment. • Use laboratory data and observations to make proper interpretations and conclusions. This chapter will review and strengthen these skills. Be sure to do the Practice Exercises at the end of the chapter.

Laboratory setups vary from school to school depending on whether the lab is equipped with macro- or microscale equipment. Microlabs use specialized equipment that allows lab work to be done on a much smaller scale. The basic principles are the same as when using full-sized equipment, but microscale equipment lowers the cost of materials, results in less waste, and poses less danger. The examples in this book are of macroscale experiments. Along with learning to use microscale equipment, most labs require a student to learn how to use technological tools to assist in experiments. The most common are: Gravimetric balance with direct readings to thousandths of a gram instead of a triple-beam balance pH meters that give pH readings directly instead of using indicators Spectrophotometer, which measures the percentage of light transmitted at specific frequencies so that the molarity of a sample can be determined without doing a titration Computer-assisted labs that use probes to take readings, e.g., temperature and , so that programs available for computers can print out a graph of the relationship of readings taken over time

LABORATORY SAFETY RULES

The Ten Commandments of Lab Safety The following is a summary of rules you should be well aware of in your own chemistry lab.

1. Dress appropriately for the lab. Wear safety goggles and a lab apron or coat. Tie back long hair. Do not wear open-toed shoes. 2. Know what safety equipment is available and how to use it. This includes the fountain, , fire extinguisher, and emergency shower. 3. Know the dangers of the chemicals in use, and read labels carefully. Do not taste or sniff chemicals. 4. Dispose of chemicals according to instructions. Use designated disposal sites, and follow the rules. Never return unneeded chemicals to the original containers. 5. Always add acids and bases to slowly to avoid splattering. This is especially important when using strong acids and bases that can generate significant heat, form steam, and splash out of the container. 6. Never point heating test tubes at yourself or others. Be aware of reactions that are occurring so that you can remove them from the heat if necessary before they “shoot” out of the . 7. Do not anything by mouth! Never use your mouth as a suction pump, not even at home with toxic or flammable liquids. 8. Use the when dealing with toxic fumes! If you can smell them, you are exposing yourself to a dose that can harm you. 9. Do not eat or drink in the lab! It is too easy to take in some dangerous substance accidentally. 10. Follow all directions. Never haphazardly mix chemicals. Pay attention to the order in which chemicals are to be added to each other, and do not deviate!

SOME BASIC SETUPS Throughout this book, drawings of laboratory setups that serve specific needs have been presented. You should be familiar with the assembly and use of each of these setups.

• Preparation of a gaseous product, nonsoluble in water, by water displacement from solid reactants

• Preparation of a gaseous product, nonsoluble in water, by water displacement from at least one reactant in solution

• of a liquid

• Titration

The following are additional laboratory setups with which you should be familiar:

1. PREPARATION OF A GASEOUS PRODUCT, SOLUBLE IN WATER AND LIGHTER THAN AIR, BY THE DOWNWARD DISPLACEMENT OF AIR. SEE FIGURE 42.

EXAMPLE: Preparation of (NH3). 2NH4Cl(s) + Ca(OH)2(s) → CaCl2(s) + 2H2O(g) 2NH3(g)

Figure 42. Preparation of Ammonia

2. SEPARATION OF A BY CHROMATOGRAPHY. SEE FIGURE 43.

Figure 43. Chromatography Setup

EXAMPLE: Chromatography is a process used to separate parts of a mixture. The component parts separate as the solvent carrier moves past the spot of material to be separated by capillary action. Because of variations in , attraction to the , and , each fraction moves at a different rate. Once separation occurs, the fractions are either identified by color or removed for other tests. A usual example is the use of Shaeffer Skrip Ink No. 32, which separates into yellow, red, and blue streaks of dyes.

3. MEASURING POTENTIALS IN ELECTROCHEMICAL CELLS. SEE FIGURE 44.

Figure 44. Potentiometer Setup for Measuring Potential

EXAMPLE: The in this zinc-silver electrochemical cell would read approximately 1.56 V. This means that the Ag to Ag+ half-cell has 1.56 V more electron-attracting ability than the Zn to Zn2+ half-cell. If the potential of the zinc half-cell were known, the potential of the silver half- cell could be determined by adding 1.56 V to the potential of the zinc half-cell. In a setup like this, only the difference in potential between two half-cells can be measured. Notice the use of the salt bridge instead of a porous barrier.

4. REPLACEMENT OF HYDROGEN BY A METAL. SEE FIGURE 45.

Figure 45. Eudiometer Apparatus

EXAMPLE: Measure the mass of a strip of magnesium with an to the nearest 0.001 g. Using a coiled strip with a mass of about 0.040 g produces about 40 mL of H2. Pour 5 mL of concentrated HCl into a eudiometer, and slowly fill the remainder with water. Try to minimize mixing. Lower the coil of Mg strip into the tube, invert it, and lower it to the bottom of the beaker. After the reaction is complete, you can measure the volume of the released and calculate the mass of hydrogen replaced by the magnesium. (Refer to Chapter 5 for a discussion of gas laws.)

SUMMARY OF QUALITATIVE TESTS

I. Identification of Some Common

Gas Test Result Ammonia 1. Smell cautiously. 1. Sharp odor.

NH3 2. Test with litmus. 2. Red litmus turns blue. 3. Expose to HCl fumes. 3. White fumes form, NH4Cl. Carbon dioxide 1. Pass through limewater, 1. White precipitate forms,

CO2 Ca(OH)2 CaCO3 Carbon monoxide 1. Burn it and pass product through 1. White precipitate forms, CaCO3 CO limewater, Ca(OH)2 Hydrogen 1. Allow it to mix with some air, then 1. Gas explodes. H2 ignite. 2. Burns with blue flame—product H2O 2. Burn it—trap product. turns cobalt chloride paper from blue to pink. 1. Smell cautiously. 1. Choking odor. HCl 2. Exhale over the gas. 2. Vapor fumes form. 3. Dissolve in water and test with litmus. 3. Blue litmus turns red.

4. Add AgNO3 to the solution. 4. White precipitate forms. Hydrogen sulfide 1. Smell cautiously. 1. Rotten egg odor.

H2S 2. Test with moist lead acetate paper. 2. Turns brown-black (PbS). 1. Insert glowing splint. 1. Bursts into flame.

O2 2. Add nitric oxide gas. 2. Turns reddish brown.

II. Identification of Some Negative Ions Ion Test Result

Acetate Add concentrated H2SO4 and warm Odor of vinegar released. − C2H3O2 gently. Carbonate Add HCl acid; pass released gas through − White, cloudy precipitate forms. CO3 limewater. Chloride 1. Add silver nitrate solution. 1. White precipitate forms.

2. Then add nitric acid, later followed by 2. Precipitate insoluble in HNO3 but Cl− ammonium hydroxide. dissolves in NH4OH. Hydroxide Test with red litmus paper. Turns blue. OH− White precipitate forms; insoluble in Sulfate Add solution of BaCl , then HCl. 2 HCl. − SO4 Sulfide Add HCl and test gas released with lead Gas, with rotten egg odor, turns paper S2− acetate paper. brown-black.

III. Identification of Some Positive Ions

Ion Test Result Ammonium Add strong base (NaOH); Odor of ammonia. + NH4 heat gently. Ferrous Add solution of potassium ferricyanide, Dark blue precipitate forms Fe2+ K3Fe(CN)6 (Turnball’s blue). Ferric Add solution of potassium ferrocyanide, Dark blue precipitate forms Fe3+ K4Fe(CN)6. (Prussian blue). Hydrogen Test with blue litmus paper. Turns red. H+

IV. Qualitative Tests of Some Metals

FLAME TESTS. Carefully clean a wire by dipping it into dilute HNO3 and heating in the Bunsen flame. Repeat until the flame is colorless. Dip heated wire into the substance being tested (either solid or solution), and then hold it in the hot outer part of the Bunsen flame. Compound of Color of Flame Sodium (Na) Yellow Potassium (K) Violet (use cobalt-blue glass to screen out Na impurities) Lithium (Li) Crimson Calcium (Ca) Orange-red Barium (Ba) Green Strontium (Sr) Bright red

HYDROGEN SULFIDE TESTS. Bubble hydrogen sulfide gas through the solution of a salt of the metal being tested. Check color of the precipitate formed. Compound of Color of Sulfide Precipitate Lead (Pb) Brown-black (PbS) Copper (Cu) Black (CuS)

Silver (Ag) Black (Ag2S) (Hg) Black (HgS) Nickel (Ni) Black (NiS) Iron (Fe) Black (FeS) Cadmium (Cd) Yellow (CdS)

Arsenic (As) Light yellow (As2S3) Antimony (Sb) Orange (Sb2S3) Zinc (Zn) White (ZnS)

Bismuth (Bi) Brown (Bi2S3)

Chapter Summary

The following terms summarize all the concepts and ideas that were introduced in this chapter. You should be able to explain their meaning and how you would use them in chemistry. They appear in boldface type in this chapter to draw your attention to them. The boldface type also makes it easier for you to look them up if you need to. You could also use the search or “Google” action on your computer to get a quick and expanded explanation of these terms, laws, and formulas. gravimetric balance with direct readings spectrophotometer pH meters computer-assisted labs salt bridge

Internet Resources

Online content that reinforces the major concepts discussed in this chapter can be found at the following Internet addresses if they are still available. Some may have been changed or deleted. Chemistry Safety Rules www.science.fau.edu/chemistry/chemlab/General/safety.html This site website gives a good review of laboratory safety rules.

Practice Exercises

1. In the reaction setup shown above, which of the following are true? I. This setup can be used to prepare a soluble gas by water displacement. II. This setup involves a decomposition reaction if the substance heated is potassium chlorate. III. This setup can be used to prepare an insoluble gas by water displacement. (A) I only (B) II only (C) I and III (D) II and III (E) I, II, and III Questions 2–4 refer to the following diagram:

(A) Around the (B) In the (C) In the circulating water (D) In the heated flask (E) In the distillate 2. In this laboratory setup for distillation, where does the vaporization take place? 3. If the liquid being distilled contains dissolved magnesium chloride, where will it be found after distillation is completed? 4. If the liquid being distilled contains dissolved ammonia gas, where will it be bound after distillation is completed? ______5. If the flame used to heat a flask is an orange color and blackens the bottom of the flask, what correction should you make to solve this problem? (A) Move the flask farther from the flame. (B) Move the flask closer to the flame. (C) Allow less air into the collar of the burner. (D) Allow more air into the collar of the burner. (E) The problem is in the supply of the gas, and you cannot fix it. Questions 6–8 refer to the following diagram:

6. In the above titration setup, if you introduce 15 mL of the NaOH with an unknown molarity into the flask and then add 5 drops of phenolphthalein indicator, what will you observe? (A) A pinkish color will appear throughout the solution. (B) A blue color will appear throughout the solution. (C) There will be a temporary pinkish color that will dissipate. (D) There will be a temporary blue color that will dissipate. (E) There will not be a color change. 7. If the HCl is 0.1 M standard solution and you must add 30 mL to reach the end point, what is the molarity of the NaOH? (A) 0.1 M (B) 0.2 M (C) 0.3 M (D) 1 M (E) 2 M 8. When is the end point reached and the volume of the HCl recorded in this reaction? I. When the color first disappears and returns in the flask. II. When equal amounts of HCl and NaOH are in the flask. III. When the color disappears and does not return in the flask.

(A) I only (B) III only (C) I and III (D) II and III (E) I, II, and III Questions 9–11 refer to the following diagram: In this setup, a clean strip of magnesium with a mass of 0.040 g was introduced into the bottom of the tube, which contained a dilute solution of HCl, and allowed to react completely. The hydrogen gas formed was collected and the following data recorded:

Air pressure in the room = 730 mm Hg Temperature of the water solution = 302 K Vapor pressure of water at 302 K = 30.0 mm Hg

The gas collected did not fill the eudiometer. The height of the meniscus above the level of the water was 40.8 mm. 9. What is the theoretical yield (in mL) at STP of hydrogen gas produced when the 0.040 g of Mg reacted completely? (A) 10 mL (B) 25 mL (C) 37 mL (D) 46 mL (E) 51 mL 10. What is the correction to the atmospheric pressure due to the 40.8 mm height of the solution in the tube and above the level in the beaker? (A) 3.0 mm Hg (B) 6.0 mm Hg (C) 13.6 mm Hg (D) 27.2 mm Hg (E) 40.8 mm Hg 11. What is the pressure of the collected gas once you have also corrected for the vapor pressure of the water? (A) 730 mm Hg (B) 727 mm Hg (C) 30.0 mm Hg (D) 697 mm Hg (E) 760 mm Hg Questions 12–14 (A) The rule is to add concentrated acid to water slowly. (B) The rule is to add water to the concentrated acid slowly. (C) Carefully replace unused or excess chemicals into their properly labeled containers from which they came. (D) Flush eyes with water at the eyewash fountain for at least 15 minutes, and then report the accident for further help. (E) Dispose of chemicals in the proper places and following posted procedures. Do not return them to their original containers. 12. Which of the above choices is the proper way to dilute a concentrated acid? 13. How do you properly dispose of chemicals not needed in the experiment? 14. What should you do if a chemical splatters into your eye? ______15. What instrument is used in chemistry labs to measure the molarity of a colored solution by measuring the light transmitted through it? (A) Electronic gravimetric balance (B) pH meter (C) Spectrophotometer (D) Computer assisted probes (E) Galvanometer Answers and Explanations 1. (D) This setup can be used to prepare an insoluble gas but not a soluble one. If the substance is potassium chlorate, it does decompose into potassium chloride and oxygen. 2. (D) Vaporization occurs in the heated flask. 3. (D) The magnesium chloride will be left behind in the heated flask since it is not volatile as the liquid boils off. 4. (E) Since the dissolved ammonia is volatile below the boiling point of water, it will be found in the distillate. Some will also escape as a gas. 5. (D) One of the basic adjustments to a burner is to assure enough air is mixing with the gas to form a blue cone-shaped flame. With insufficient air, carbon deposits will form on the flask and the flame will be orange. 6. (A) Phenolphthalein indicator is colorless below a pH of 8.3 but is red to pink in basic solutions above this pH. In this NaOH solution, it will be red to pink. 7. (B) Calculate the molarity by using the formula:

Macid × Vacid = Mbase × Vbase 0.1 M × 30 mL = Mbase × 15 mL Mbase = 0.2 M

Notice that as long as the units of volume are the same, they cancel out of the equation. 8. (B) The end point is reached when the color of the indicator disappears and does not return. The color will first disappear temporarily before the end point but finally will not return. 9. (C) The theoretical yield at STP can be found from the chemical equation of the reaction: 10. (A) The 40.8 mm of water being held up in the tube by atmospheric pressure can be changed to its equivalent height of mercury by dividing by 13.6, since 1 mm of Hg = 13.6 mm of water.

By correcting the atmospheric pressure, we get: 730 mm Hg - 3 mm Hg = 727 mm Hg 11. (D) Correcting the previous pressure by subtracting the given amount for the vapor pressure of water at 302 K gives you 727 mm Hg - 30 mm Hg vapor pressure = 697 mm Hg as the final pressure. 12. (A) The correct and safe way to dilute concentrated acids is to add water down the side of the beaker slowly and be aware of any heat buildup. 13. (E) You never return chemicals or solutions to their original containers for fear of contaminating the original source. 14. (D) It is essential to get your eyes washed of any chemicals. Know where the eyewash fountains are, and know how to use them. 15. (C) One of the new technological additions to chemistry labs is the spectron 20 that uses the absorption of light waves to do qualitative and quantitative investigations in the lab.

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