EXPERIMENT 2. Laboratory Procedures INTRODUCTION

Transcription

1 EXPERIMENT 2 Laboratory Procedures INTRODUCTION Begin each experiment by taking the necessary safety precautions. All materials that will not be used in the lab should be placed out of the laboratory working area. This would include books, purses, lunches, water bottles, etc. For laboratory experiments you need to be wearing closed-toe shoes. Get in the "good habit" of always putting on your safety goggles, lab coat, and gloves. The best way to become familiar with chemical apparatus is to actually handle the pieces yourself in the laboratory. In this experiment you will learn to adjust the gas burner, make weighings, handle solids, measure liquids, filter a mixture, and assemble apparatus. TECHNIQUES 1. Decanting and Transferring Liquids The safest way of transferring a liquid from one test tube to another is shown in Figure 1. When transferring a liquid into either a test tube, beaker, or flask, the test tube, beaker, or flask should be on the lab bench. Sometimes liquids contain particles of insoluble solids that soon sink to the bottom of a test tube or beaker and you want to separate the two. (a) Figure 2 shows the proper method of decanting a supernatant liquid in a test tube. (b) Figure 3 shows the proper method of decanting a supernatant liquid in a beaker by using a stirring rod. The rod should touch the wall of the receiving vessel. Hold the stirring rod against the lip of the beaker containing supernatant liquid. As you pour, the liquid will run down the rod and drop off into the beaker resting below. In this way the liquid will not run down the side of the beaker from which you are pouring. 13

2 2. How to Pour Liquid from a Reagent Bottle When pouring a caustic or corrosive liquid into a beaker, use a stirring rod to avoid drips or spills. Hold the stirring rod against the lip of the reagent bottle. Estimate the amount of liquid you need and pour this along the rod into the beaker. See Figure 4. Extra precautions should be taken when handling a bottle of acid. Remember these two important rules: (a) Never add water to any concentrated acid, particularly sulfuric acid, because of spattering and heat generation. (b) To dilute any acid, add the acid to water in small quantities, stirring slowly and constantly. Remember to add alphabetically - Acid to Water. Never pour reagents back into stock bottles. At the end of the experiment, any excess chemicals should always be properly discarded under the direction of the instructor. 3. Testing an Odor Safely Test for the odor of gases by wafting your hand over the test tube and cautiously sniffing the fumes as shown in Figure 5. Do not inhale fumes directly. Use a fume hood whenever poisonous or irritating fumes are being evolved and do not waft in this case. 4. How to Heat Material in a Test Tube Check to see that the test tube is Pyrex or a similar heat-treated type, and always use a test tube holder or clamp when heating the test tube. Never point a heated test tube at anyone because the liquid may splash out of the test tube, and never look down into the test tube while heating it. Do not heat any one spot on the test tube. Heat the test tube from the upper portions of the tube downward and continuously move the test tube as shown in Figure 6. Heating a test tube at the bottom may cause vapor to meet a head of liquid above it, and this may cause the bottom of the test tube to blow out. 14

3 ACCURACY AND PRECISION Experimental determination of any quantity is subject to error because it is impossible to carry out any measurement with absolute certainty. The extent of error in any determination is a function of the quality of the instrument or the measuring device, and the skill and experience of the experimenter. Thus, discussion of errors is an essential part of experimental work in any quantitative science. The types of errors encountered in making measurements are classified into three groups: 1. GROSS ERRORS. Gross, careless errors are those due to mistakes that are not likely to be repeated in similar determinations. These include the spilling of a sample, reading the weight incorrectly, reading a buret volume incorrectly, etc. 2. RANDOM ERRORS. Random errors are due to the inherent limitations of the equipment or types of observations being made. Generally these can be minimized by using high-grade equipment and by careful work with this equipment, but can never be completely eliminated. It is customary to perform measurements in replicate in order to reduce the effect of random errors on the determination. 3. SYSTEMATIC ERRORS. Systematic errors are those that affect each individual result of replicate determinations in exactly the same way. They may be errors of the measuring instrument, of the observer, or of the method itself. Examples of systematic errors in chemical analyses include such things as the use of impure materials for standardization of solutions, or improperly calibrated volumetric glassware such as pipets, burets, and volumetric flasks. Two terms used to describe the reliability of experimental measurements are precision and accuracy. Accuracy refers to the agreement of an experimental measurement with the true value. Precision refers to the agreement among several experimental measurements of the same quantity, and reflects the reproducibility of a given measurement. The difference between these two terms is shown below by the results of four different dart throws. The different types of errors are illustrate on the right. A gross error or random error will produce a measurement that has an equal probability of being high or low, and large gross or random errors produce results that are not precise. The spread of the darts on the two top targets are a result of large gross or random errors, while the spread of the darts on the two bottom targets indicate small gross or random errors. The two top targets show throws that are not precise, while the two bottom targets show throws that are precise. A systematic error will produce a measurement that is off consistently in the same direction, and large systematic erros produce results that are not accurate. The spread of the darts on the two left targets are a result of a large systematic error, while the spread of the darts on the two right targets indicate a small systematic error. The two left targets show throws that are not accurate, while the two right targets show throws that are accurate. 15

4 In this experiment you will test both your precision and accuracy. You will perform multiple trials pipetting 5.00 ml of water, measure their masses, and the agreement between your masses will demonstrate your precision. You will then calculate the mean, or average, mass of 5.00 ml of water, and your mean will then be checked against its accepted value by the instructor, and the agreement between the two will demonstrate your accuracy. PROCEDURE 1. Students will work individually for this experiment. Except for the laboratory handout, remove all books, purses, and such items from the laboratory bench top, and placed them in the storage area by the front door. For laboratory experiments you should be wearing closed-toe shoes. Tie back long hair, and do not wear long, dangling jewelry or clothes with loose and baggy sleeves. Open you lab locker. Put on your safety goggles, your lab coat, and gloves. PART A - THE BURNER 2. The Bunsen burner or the Tirrell burner is commonly used as a source of heat in the laboratory. Larger Fisher burners are also used because the produce more heat than the smaller burners. While the details of construction of the burners vary, each has a gas inlet located in the base, a vertical tube or barrel in which the gas is mixed with air, and adjustable openings or ports in the base of the barrel. These ports admit air to the gas stream. The burner may have an adjustable needle valve to regulate the flow of gas. In some models the gas flow is regulated simply by adjusting the gas valve on the supply line. The burner is always turned off at the gas valve, never at the needle valve. See the figure on the next page and examine your burner by locating these parts. 16

5 3. Check the burner's rubber tubing for cracks or holes. If necessary, obtain new rubber tubing. Attach the rubber tubing to the gas outlet at your lab station. CAUTION: Before you light the burner, check to see that you have taken the following safety precautions against fires: you are wearing safety goggles, long hair is tied in back of the head and away from the front of the face, long sleeves on shirts, blouses, and sweaters are rolled up away from the wrists. Unscrewing the barrel of your Tirrell burner allows more air to enter the burner through an air inlet hole at the base. Screwing the barrel down reduces the amount of air that enters the burner. The barrel should be screwed down, but not too tightly. Turn the gas on full, hold the striker about 5 cm above the top of the burner and proceed to light it. If the flame quickly blows out, turn down the gas at the gas outlet and try again. The flame should be free of yellow color, nonluminous, and also free from the "roaring" sound caused by admitting too much air. Control the height of the flame so that it is about 8 cm high by using the gas valve, and allow in more air by slowly unscrewing the barrel to reduce any yellow color or "roaring". The flame should be quiet and steady with a sharply defined light blue inner cone. This gives the highest possible temperature with your burner. 4. Obtain a 10-cm piece of copper wire from the cart. Using crucible tongs, place the copper wire at the tip of the flame. Lower the wire slowly down through the flame. Do not put the crucible tongs in the flame. Answer question At the end of this part of the experiment, turn off the burner, checking to see that the gas valve is completely shut off. 17

6 PART B - THE BALANCE 6. When a balance is required for determining mass, you will use an analytical balance. Several analytical balances are found on the back counter of the laboratory room. The display reads to gram. This means that the balance is estimating the digit in the ten thousandths column, and all mass readings should be recorded to the one-ten thousandth of a gram. CAUTION: Whenever using a balance never place chemicals or hot objects directly on the balance pan. They can permanently damage the surface of the balance pan and affect the mass weighing. 7. Transfer a small amount of sodium chloride from its reagent bottle to a weighing cup or a glass or porcelain container by pouring. CAUTION: Never place your microspatula or scoopula into a reagent bottle. Take this container to your balance along with a 100-mL beaker, a clean microspatula, your Data Table, and a pen. Turn the analytical balance on by pressing the "on/tare" button. After a few seconds, the display should read " g". If it does not, press the "on/tare" button again to make sure the display reads "0.000 g". CAUTION: Carefully check the label on the reagent bottle or container before removing any of the contents. Never use more of a chemical than directed. 18

7 8. Obtain two spatulafuls of sodium chloride, using the following procedure to determine the mass of the sodium chloride: (a) Before weighing anything on the analytical balance, press the on/tare button and make sure the display reads " g". (b) Carefully open the side sliding door of the analytical balance. Watch for debris in the track of the sliding door and clean it out if it is present because the sliding door is one of the most fragile parts of the balance. Place a 100-mL beaker on the balance pan and close the door. When the display shows a mass with four places past the decimal point followed by a "g" for grams, this is the mass of the beaker. Record the mass of the beaker in your Data Table in pen. Record all digits shown. (c) Open the sliding door, remove the beaker, and place two spatulafuls of sodium chloride into the beaker. CAUTION: Never add chemicals to a container while it is in the balance. This increases the chance of corrosive chemicals spilling on the expensive balance. (d) Replace the beaker with its contents in the balance and close the sliding door. When the "g" appears, this will be the mass of the beaker and the sodium chloride. Record the mass of the beaker and sodium chloride in the Data Table in pen. NOTE: Calculations, graphs, and questions may be done in pencil, but all measured data must be entered into the lab report in pen. If an error is made, draw a single line through the incorrect entry (so that it can still be read), and write the correct entry next to it. Whiteout should never be used. (e) Remove the beaker with its contents and close the sliding door. The balance should be left clean for the next user. Clean off the microspatula and clean up any spilled chemicals on the balance or on the bench top immediately. Close all reagent jars to prevent contamination after using them. NOTE: If there are any crystals left on the balance or the lab bench at the end of the lab period, the instructor will deduct one point from everyone s lab score as a charge for cleaning up after you. CAUTION: As a precaution against contamination, never pour unused chemicals back into the reagent bottles. Any excess sodium chloride that is removed from its reagent jar should be disposed of properly. In this case, sodium chloride may be washed down the sink with water. 9. Bring the sodium chloride to your lab area. Calculate the mass of sodium chloride, and record the mass in your Data Table. The calculation may be done in pencil, and the calculated mass of sodium chloride may be recorded in your Data Table in pencil. Dispose of the sodium chloride in a trash can in the laboratory room. 19

8 PART C - THE THERMOMETER 10. Obtain a ring stand, attach an iron ring to the ring stand, and place a ceramic-centered wire gauze on the iron ring. Half fill a 250-mL beaker with deionized water, and place it on the ceramic-centered wire gauze. 11. Place a Fisher burner below the 250-mL beaker and adjust the beaker height so that it is 8-10 cm above the burner. Light the burner and heat the water. When the water starts to boil, turn down the burner so it provides just enough heat to produce a very gentle boil. When the water is gently boiling, attach a double buret clamp to the ring stand, and place the thermometer in the clamp so that the bulb of the thermometer is suspended in the water. Observe the reading on the thermometer until it is constant for one minute. 12. The tolerance of a graduated piece of equipment is the expected range of variation in a reading, or its expected deviation from the accepted value. When the tolerance of a graduated measuring device is not given, it can be estimated. To do this, first determine what the most accurate graduations represent on the device. Dividing the most accurate graduations by two gives the estimated tolerance of the device. Because the most accurate graduations on a student-grade thermometer are one Celsius degree, its tolerance is five-tenths of a Celsius degree. To make a proper a reading of a graduated device, the last digit recorded should be in the same decimal place as the tolerance. This means the temperature reading should be recorded to the tenths place. Record this value in the Data Table in pen, and then turn off the burner. NOTE: Calculations, graphs, and questions may be done in pencil, but all measured data must be entered into the lab report in pen. If an error is made, draw a single line through the incorrect entry (so that it can still be read), and write the correct entry next to it. Whiteout should never be used. In a proper reading of any graduated device, the last digit in the recorded reading should be the only digit estimated in the measurement. 20

9 PART D THE PIPET 13. A pipet is also used to accurately measure and deliver volumes of liquids. Two types of pipets are in common use: volumetric pipets and graduated pipets. A volumetric pipet has a single calibration mark and delivers the volume printed on the bulb of the pipet at the temperature specified (see the first pipet below). Volumetric pipets are to delivery, or TD. This means that the pipet has been calibrated to contain the last drop of liquid that adheres inside of its tip. Therefore, never blow out the last bit of liquid remaining inside the tip of a TD pipet. A graduated pipet has calibrations along the length of the pipet. Some graduated pipets have volume markings present all the way down to the tip so that they can be completely drained (see the second pipet below). Other graduated pipets do not have volume markings present all the way down to the tip so they cannot be completely drained (see the third pipet below). 14. Obtain a 5-mL volumetric pipet from the tray marked Clean Pipets next to the instructor desk. Always carry pipets in the vertical position. Obtain a pipet bulb from drawer 014 at the the back of the lab room. The 5-mL volumetric pipets each have a tolerance of two-hundredth of a milliliter. Because the tolerance shows accuracy to the hundredths place, any recorded volume measured with a 5-mL volumetric pipet should be written with the recorded volume s last digit in the hundredths place. 21

10 15. To use a volumetric pipet (a) Place the tip of the pipet below the surface of the liquid to be dispensed. (b) Compress a pipet bulb and press the opening of the bulb against the upper end of the pipet. Never force the pipet bulb over the end of the pipet, it does not fit and it is not supposed to fit. CAUTION: Always use the pipet bulb, never pipet by mouth. (c) Slowly release pressure on the bulb so that liquid is drawn into the pipet to a level about 3 cm above the calibration mark, but not into the bulb itself. (d) Remove the bulb and simultaneously place your index finger over the end of the pipet. If you are right-handed, you should hold the pipet in your right hand and the pipet bulb in your left. (e) Keep your index finger pressed firmly against the end. Withdraw the pipet from the liquid, and slowly reduce the pressure on your finger to allow excess liquid to drain back into its container until the bottom of the meniscus is at the calibration mark. (f) Now deliver the remaining liquid into the designated receiver. When releasing liquid from a volumetric pipet, let it drain completely. Wait five seconds, and then touch the pipet tip to the inside of the flask or to the surface of the liquid. This action will remove some, but not all, of the liquid in the tip. The pipet delivers the stated volume when this procedure is followed. A small amount of liquid will remain in the tip. Do not blow this out into your receiver. 22

11 16. Half fill a 100-mL beaker with tap water, and set an empty 125-mL Erlenmeyer flask next to it on your lab bench. The faucets labeled HW in red and CW in green are hot and cold tap water. Practice using the pipet to draw accurately 5 ml of water from the beaker, and emptying it into the Erlenmeyer flask. CAUTION: Do not get any liquid in the suction bulb. If it happens, immediately flush out the bulb with water several times and squeeze out the excess water. 17. When you have mastered the pipetting technique, write your name and lab station number on the board. The instructor will come to your station and ask you to pipet 5 ml of water into an Erlenmeyer flask using proper technique. When this is done correctly, the instructor will stamp your lab report on page 29. If there is a line of student names on the board after yours, you will have only 3 chances to perform the required operation before the instructor must move on to the next name on the list. You must then add your name to the bottom of the list. Continue with the experiment while you are waiting for the instructor. 18. Obtain a rubber stopper from the back of the lab room and place it in the mouth of a clean, dry 125- ml Erlenmeyer flask. Determine the mass of the flask and stopper on your analytical balance. NOTE: Calculations, graphs, and questions may be done in pencil, but all measured data must be entered into the lab report in pen. If an error is made, draw a single line through the incorrect entry (so that it can still be read), and write the correct entry next to it. Whiteout should never be used. Empty the tap water from your 100-mL beaker and rinse the beaker three times with deionized water. Half fill the beaker with deionized water. Remove the stopper from the 125-mL Erlenmeyer flask, carefully pipet 5.00 ml of deionized water into the Erlenmeyer flask, and then replace the stopper. Determine the mass of the flask, stopper and 5.00 ml of water on your balance, and then by subtraction, determine the mass of the 5.00 ml of water in the flask. 19. Remove the stopper, carefully pipet another 5.00 ml of deionized water into the Erlenmeyer flask, and replace the stopper. Determine the mass of the flask, stopper and ml of water on your balance, and then by subtraction, determine the mass of the second 5.00 ml of water that you added to the flask. 20. Remove the stopper, carefully pipet another 5.00 ml of deionized water into the Erlenmeyer flask, and replace the stopper. Determine the mass of the flask, stopper and ml of water on your balance, and then by subtraction, determine the mass of the third 5.00 ml of water that you added to the flask. 23

12 21. Precision is the degree of mutual agreement among a series of individual measurements. Look at the masses of the three 5.00 ml samples of deionized water you pipetted. If you pipetted exactly 5.00 ml of deionized water each time, their masses should all be the same! If they are not, you may not have done the second and third calculations correctly! Part of you lab grade will be based upon the precision of your work, and for pipetting you will be expected to pipet successive samples so that their masses are no further apart than g. If your three water samples do not agree within g of each other, and you wish to improve your grade on this experiment, you may repeat steps 16 to 18 as many times as necessary to produce three trials whose masses are are no further apart than g. 22. The faucet labeled DW in white is the deionized water. Fill your wash bottle with deionized water. Rinse the pipet out with deionized water from your wash bottle several times, and blow it out with the suction bulb. Wipe dry the outside of the pipet and return it to the stockroom window. Return the rubber stopper to the back of the lab room. Leave the deionized water in your wash bottle, and in the future only fill the wash bottle with deionized water. PART E - GRADUATED CYLINDERS AND BURETS 23. For approximate measurements of liquids, a graduated cylinder is generally used. These cylinders are usually graduated in milliliters, fifths of a milliliter, or tenths of a milliliter. See Graduated Cylinder A and Graduated Cylinder B below. The most accurate graduations on Graduated Cylinder A represent one-tenth of a milliliter. Dividing the most accurate graduations by two gives its estimated tolerance, which is five-hundredths of a milliliter. To make a proper a reading, the last digit recorded should be in the same decimal place as the tolerance. This means that volume readings using Graduated Cylinder A should be recorded to the hundredths place. The most accurate graduations on Graduated Cylinder B represent one milliliter. Dividing the most accurate graduations by two gives its estimated tolerance, which is five-tenths of a milliliter. To make a proper a reading, the last digit recorded should be in the same decimal place as the tolerance. This means that volume readings using Graduated Cylinder B should be recorded to the tenths place. 24

13 Look at Graduated Cylinders C and D. Because their most accurate graduations represent one-tenth of a milliliter, their tolerances are five-hundreths of a milliliter, therefore their volume readings should be recorded to the hundredths place. Notice that the last digit in each given reading is the only digit that is estimated. Always view the curved surface of the liquid in the cylinders, called the meniscus, at eye level. Determine where the bottom of the meniscus is relative to the graduations. Mentally make ten more calibration marks between the smallest intervals actually marked above and below the meniscus, then estimate the measurement where you think the meniscus is located. Verify the given measurements for Graduated Cylinders C and D. Notice that if a volume measure falls right on the 7 ml mark, the measurement is reported as 7.00 ml, not 7 ml. Look at Graduated Cylinders E and F. Because their most accurate graduations represent one milliliter, their tolerances are five-tenths of a milliliter, therefore their volume readings should be recorded to the tenths place. Notice that the last digit in each given reading is the only digit that is estimated. 25

14 24. A buret, fitted with a stopcock, is used to accurately measure and deliver volumes of liquids. When using a buret the buret should be held by a double buret clamp on a ring stand. See the figure below. The most accurate graduations on a 50-mL buret represents one-tenth of a milliliter. Because the most accurate graduations represent one-tenth of a milliliter, its tolerance is five-hundreths of a milliliter, therefore its volume readings should be recorded to the hundredths place. Notice in the reading that the last digit is the only digit that is estimated. Also, care should be taken when reading a buret, because the numerical values get larger as you go down the glassware. Verify the given measurement below. 25. Depending upon whether you lab station number is even or odd, find the location of the corresponding area in the lab room with three graduated cylinders and a buret. Bring a pen and your lab book to the area and record the volume of liquid in the three graduated cylinders and the buret. Have the instructor check your readings. If they have been done correctly, the instructor will stamp your lab report on page At the end of the experiment, all equipment you store should be cool, clean, and wiped dry, and then clean and wipe dry your laboratory work area. Pipets should be returned to the tray marked Used/Rinsed Pipets at the back of the lab room. When you have completed your lab report have the instructor inspect your working area. Once your working area has been checked your lab report can then be turned in to the instructor. Unless instructed otherwise, lab reports will always be due at the end of the lab period. 26

15 EXPERIMENT 2 LAB REPORT Name: Student Lab Score: Date/Lab Start Time: Lab Station Number: DATA TABLE Mass of Beaker. g Mass of Beaker and Sodium Chloride. g 1 Mass of Sodium Chloride. g Thermometer Reading for Boiling Water. C Mass of Flask, Stopper. g Mass of Flask, Stopper, 5.00 ml Water. g 2 Mass of First 5.00 ml of Water. g Mass of Flask, Stopper, ml Water. g 3 Mass of Second 5.00 ml of Water. g Mass of Flask, Stopper, ml Water. g 4 Mass of Third 5.00 ml of Water. g Mass of Flask, Stopper, ml Water. g 5 Mass of Fourth 5.00 ml of Water. g Mass of Flask, Stopper, ml Water. g 6 Mass of Fifth 5.00 ml of Water. g 7 Mean Mass of 5.00 ml of Water. g Volume of Water in First Graduated Cylinder. ml Volume of Water in Second Graduated Cylinder. ml Volume of Water in Third Graduated Cylinder. ml Buret Reading. ml 27

16 CALCULATIONS

17 INSTRUCTOR STAMPS 1. Have the instructor check off your technique for using a pipet. 2. Have the instructor check off your technique for reading graduated cylinders and burets. QUESTIONS 1. As soon as you begin your experiment, what equipment should you put on immediately? 2. Why is primary data writtin in pen, and corrections made by drawing a single line through errors? 3. What evidence is there that the hottest part of the flame is at the tip of the inner cone? 4. Why should you never touch chemicals with your hands? 29

18 5. Give the readings for each of the following. ml ml ml ml C C cm cm 30