EXPERIMENT 1 TOOLS FOR LAB MEASUREMENT

Transcription

1 EXPERIMENT 1 TOOLS FOR LAB MEASUREMENT INTRODUCTION A course in chemistry, one of the physical sciences, differs from a course in, say, literature or history. A main difference is that chemistry usually has a laboratory component. This reflects an important fact about the nature of scientific knowledge: the understanding of the world achieved by science is based on observation of how the natural world behaves. Various branches of science concentrate on particular aspects of nature. Geologists study the form and makeup of rocks in order to deduce the history of our planet. Biologists observe the structure of plants and animals in order to account for their functions. Chemists concentrate on the transformations of matter and the accompanying energy changes. The unique perspective of chemistry is to relate observations of natural events to the composition and structure of matter on the atomic/molecular level. All physical sciences require making controlled observations and interpreting them to learn how nature behaves. We call this process experimentation. The purpose of an introductory chemistry laboratory course is to help you understand how scientific knowledge is gained through careful observation and interpretation. In this course, we will do science in the way that practicing scientists actually do it. This involves a number of facets: acquiring the skills to make accurate, reproducible measurements; making quantitative measurements whenever possible; sharing data with each other in order to see the bigger picture that results from examining many systems rather than just one or two; using computers for data analysis and plotting; communicating your results and interpretations through written reports. This first lab experiment concentrates on tools you will need for lab work throughout this year. You will gain familiarity with electronic measuring devices, with balances, and with some scientific glassware. This experiment is in two parts: The MeasureNet System, and Measurement of Volume and Mass. A. THE MEASURENET SYSTEM Introduction Scientists need to make various kinds of measurements. Often they observe how a quantity such as temperature or pressure changes with time. Perhaps the voltage of an electrochemical cell or the ph of a solution may be needed. If you wished to measure how temperature varies with time, for example, you could use a thermometer and a stopwatch and write down the temperature of your sample at various times. You could then construct a plot of temperature vs. time from this information. However, data are unlikely to be collected and manipulated in this fashion by any academic or industrial scientist today. Electronic instruments are employed to automatically make, store, and plot the desired measurements. Unfortunately, such instruments are generally delicate and expensive. You will be using a measurement network system, MeasureNet, in this course. MeasureNet allows you to make many types of measurements simply by plugging in appropriate probes. Electrical signals from the probes are converted into corresponding values of the quantities of interest. Data can be collected over a time period and plotted on a display screen. They can also be downloaded to a computer where they can be further EXPERIMENT 1 1-1

2 manipulated, combined with data from other students, plotted, and printed out. The purpose of the first part of this experiment is to gain experience in using MeasureNet. The lab is equipped with MeasureNet workstations. Note that there may be more than one independent network in the lab. If so, it is important for you to be aware of the network to which your station belongs, particularly when you need to save or print data you have collected. FIGURE 1-1 shows a workstation face. (In the display in FIGURE 1-1, temperature vs. time data are plotted, and the final point collected is also shown numerically: 22.52C at sec). If the workstation is OFF when you arrive in the lab, nothing will be displayed on the LCD (liquid crystal display) screen. Simply press the ON/OFF button in the upper right corner. The display screen should then indicate that the station is online; if it does not, notify your instructor. With the station ON, press the MAIN MENU button. Along with other information, the station number should appear in the LCD. It is important to note the number of your station, since that number is used to identify any data you download to the lab computer. In this experiment you will learn to calibrate a probe, collect temperature vs. time data, and download the data to the computer to print out a plot. In the MeasureNet system, measurement probes are connected to a workstation and send electrical signals to it. The workstation converts each signal into a number and sends the numbers to the network controller. (The controller is the rectangular black box near the lab computer.) The controller converts these numbers into the corresponding values of the physical quantities being measured and sends these values back to the workstation to be displayed and plotted. In order for this to happen correctly, you tell the controller two things. First, by choosing from menus at the workstation, you indicate what quantity or quantities you will be measuring. Second, you may need to calibrate the probe or probes, by making a measurement on a system for which the value of the property to be measured is already known (perhaps by definition). Consider a simple calibration example not involving electronic devices. A mercury thermometer is conventionally used to measure temperature. It consists of a long glass tube with mercury (Hg) in a bulb at the bottom. If the thermometer bulb is placed in contact with a warm object, heat from the object warms the Hg, causing it to expand and rise up the tube. The temperature of the object determines the height to which the Hg rises. To be useful for measuring temperature, the height of the column of Hg must be correlated quantitatively with the temperature. This could be done in the following way. With the thermometer immersed in an ice-and-water bath, whose temperature is defined as 0 C, the height of the Hg column is marked with a 0 on the thermometer tube. The thermometer is then immersed in boiling water, whose temperature is defined as 100 C. The new height of the Hg column is marked with a 100, and the space between the two markings is divided evenly into 100 segments. Each segment corresponds to 1 C, and the thermometer can now be used to measure temperatures in the range of 0 100C. (The manufacturers of thermometers do something at least related to this when they fabricate new thermometers.) EXPERIMENT 1 1-2

3 Our temperature probes are constructed using electronic devices designed to produce electrical signals that vary with temperature in a known manner. For this type of probe, the change in signal caused by a change in temperature is reliably known from the manufacturer s specifications. For any particular probe, however, all signal values may be offset, or shifted, by a constant amount. This will lead to temperature values which are all shifted from the correct values by a constant amount, which could be a few tenths to as much as several FIGURE 1-1. MeasureNet workstation. degrees. Calibration of the MeasureNet temperature probe uses a measurement of one "known" temperature to measure and compensate for this offset. We generally use an ice-and-water bath, for which the temperature is 0C by definition. Armed with this information, we can easily decide when calibration is needed. Specifically, when we are only interested in determining the change in temperature accompanying some process, we need not go to the trouble of calibrating the probe. The observed change will be accurately indicated. On the other hand, when the best value of the temperature itself is required, such as a freezing point, for example, we must calibrate the probe. This is extremely important in cases where data from a number of different stations and probes are combined. Unless all probes are properly calibrated, it will be impossible to assume that all the measurements are on the same temperature scale, and the interpretation of the combined results will be difficult or impossible. OBJECTIVES to become familiar with the MeasureNet system to calibrate a temperature probe to collect measurements of temperature as a function of time to download collected data to a computer and print it EXPERIMENT 1 1-3

4 EQUIPMENT NEEDED 250-mL beaker small hot plate ring stand universal clamp cork stopper, notched temperature probe stirring rod Styrofoam cup ice Note: As with most experiments using the MeasureNet workstations, you will work in pairs on this part of the experiment. PROCEDURE Calibrating the Temperature Probe A1. Prepare an ice-and-water bath in a Styrofoam cup, filling the cup~3/4 of the way with ice and using only enough distilled water to just cover the ice. It is essential that there be ice all the way to the bottom of the cup. If the temperature probe is not already connected to the workstation, connect it now. (See the instructor if you need assistance.) A2. Press the MAIN MENU button on the workstation. A list of possible types of measurements will appear on the display. (Note the station number listed at the top of the screen. It will be needed when you transfer data to the computer.) Each menu choice is listed with a function key, something from F1 to F8. The eight function keys are arranged in two columns at the left edge of the workstation face (see FIGURE 1-1, above). Press the function key listed for TEMPERATURE, and a new menu will appear. Press the function key for TEMP V TIME. The display should confirm that you have selected temperature and list options now available to you. A3. To see the response of your probe before calibration, press the DISPLAY button. Stir the ice-andwater bath continuously and vigorously with the probe until the temperature reading is steady to ensure that the temperature of the bath is uniform throughout. (If too much of the ice melts, so that liquid water now covers most or all of it, pour out some of the water and add more ice.) The measured temperature will be displayed on the LCD of your station, updated twice per second. When the indicated temperature stops changing, record on the report sheet at the end of the experiment handout the value displayed. A4. Now press the CALIBRATE button and follow the instructions on the display. Remember that the temperature of the ice-and-water bath is 0.0C by definition, but this temperature is only achieved in a thoroughly stirred mixture filled with ice. In a calibration you are not waiting for the displayed temperature to reach 0.0. You are waiting for the value to stop changing, showing what is displayed when the temperature is 0.0C. Note that this value will not be converted to 0.0 unless you press ENTER at the end of the calibration procedure. When you have finished the calibration, while still stirring the ice-and-water bath with the probe, press the DISPLAY button again. The temperature on the workstation display will indicate what the system now measures as the bath temperature. It should read 0.0 C within ± 0.02 C while the temperature probe is still stirring the ice bath. If it does not, repeat the calibration procedure. When your calibration is satisfactory, record the temperature of the iceand-water bath indicated by the calibrated probe. EXPERIMENT 1 1-4

5 Measuring Water Temperature vs. Time A5. Next you will measure the temperature of a sample of water over time as it is heated and then cooled. Press the SETUP button on the workstation, then choose SET LIMITS FOR NEW ACQUISITION. This will allow you to set the temperature and time limits used for the graphical display on the workstation for the experiment. Follow the instructions on the display to set the temperature limits at 110 C maximum and 60 C minimum (y-axis) and the maximum time at 900 seconds (x-axis). Leave the minimum time setting at 0. Press the DISPLAY button. The display should now show an empty graph with the values 60 and 110 along the y-axis. If it does not, repeat the process above, making sure to press ENTER after inputting each value. A6. Plug in the hot plate, with the temperature control (if present) turned all the way down, and place a 250- ml beaker containing about 100 ml of distilled water on it. Set up a ring stand with a universal clamp and a split cork stopper to hold the temperature probe in a vertical position over the beaker. Adjust the height of the clamp on the ring stand so that the probe tip is immersed in the water, about 1 cm above the bottom of the beaker. Be careful to arrange things in such a way that the wire from the temperature probe to the station will not be close enough to the hot plate to be damaged. A7. Turn the temperature control on the hot plate to its highest setting to begin heating the water. The temperature of the water will be measured and shown on the workstation display, but no data will be collected and stored yet. When the temperature reaches about 60 C, begin to collect data by pressing the START/STOP button on the workstation. (We wait until 60 C has been attained just to limit the amount of data collected.) A plot of T vs. t will appear on the workstation LCD; new points are added as they are measured. At this point, begin stirring the water continuously with a glass stirring rod to be sure that the heat is distributed evenly. Keep heating and stirring until the water has reached a continuous vigorous boil for 2 or 3 minutes, then turn off the hot plate. Continue collecting data for a few minutes as the water begins to cool, then press the START/STOP button to quit collecting data. Since you set the time limit for this experiment at 900 seconds, the workstation will automatically stop collecting data at that time. If this occurs, you do not need to press the START/STOP button. A8. Now you will use the computer connected to the controller of your network to prepare a printed plot of your temperature vs. time data. You do this directly from your MeasureNet station by pressing the FILE OPTIONS button. Read the menu that appears, and press the function key listed for PRINT standard. Indicate how many copies should be printed (probably two one for you and one for your lab partner) and press ENTER. Your printed plots will appear in a little while at the printer connected to the computer. They will be labeled at the top with Station#, where # will be replaced with the number of your station. All stations on your network use the same computer and printer, so be careful to take only the printed plots labeled with your station number. Write your name and your lab partner's name at the top of the sheet. B. MEASUREMENT OF MASS AND VOLUME Introduction Not all measurements that are important in laboratory work can be made electronically using MeasureNet. Two such quantities that must often be measured are mass and volume. You must learn to use other measuring devices to accurately and reproducibly make these measurements. We will sometimes do EXPERIMENT 1 1-5

6 cooperative experiments, in which you will combine your results with other students on your network. Thus, it is essential that everyone obtain reliable data, so that all can have confidence in the shared data. It is also important for you to understand the accuracy and precision you can expect when using various measuring devices. The most accurate ones are often the most difficult to use, but you will not always need maximum accuracy. For example, in Part A of this Experiment, you obtained a sample of water in order to measure its temperature. It is obviously of no particular importance whether you took 150 or 160 or 140 ml; the temperature measurement would be the same in any case. There are circumstances, of course, when the exact amount does matter, and it may be important to know that you have taken 1.01 ml, for example, as opposed to 1.16 ml of a solution. This part of the experiment provides insight into what level of accuracy in measuring volumes is possible when using particular types of glassware. This is shown by examining repeated measurements to see how closely they agree, both with each other and with the true value. The level of agreement becomes more and more obvious as the number of measurements increases. By combining your data with those of the other students on your MeasureNet system, you will be able to see the results of many repetitions without having to do all of the work yourself. OBJECTIVES to learn to use laboratory and analytical balances to learn to use a volumetric pipet to learn to use a buret to compare the accuracy of several types of glassware for measuring volumes EQUIPMENT NEEDED balances-laboratory and analytical 150-mL beaker 250-mL plastic bottle with lid buret and buret clamp crucible tongs evaporating dish TECHNIQUE microspatula 100-mL graduated cylinder 10-mL volumetric pipet pipet pump temperature probe Your instructor will demonstrate how obtain volume measurements using a pipet and a buret. PROCEDURE Mass Measurement Analytical Balance Note: You are to work individually on this part of the experiment. The purpose of this exercise is for you to determine, for the analytical balance, (a) its sensitivity, that is, the smallest mass it can detect, and (b) its precision, that is, the agreement among repeated measurements of a mass. B1. Practice using tongs to pick up and put down a ceramic evaporating dish and a microspatula. When you can perform these activities without dropping anything, wipe the dish and spatula carefully with a paper towel. After they are clean, handle them only with tongs so that you do not leave fingerprints. Fingerprints have measurable masses. EXPERIMENT 1 1-6

7 B2. Take the dish and spatula to an analytical balance (these are the balances that read to four decimal places). Look at the readout on the balance. Is it all zeros? If the readout remains at some quantity other than zero, tap the control bar down gently to zero balance, making sure the door to the weighing chamber is closed. Repeat the zeroing process until the balance reads zeros to four decimal places. Carefully open the door of the analytical balance and place the crucible on the pan. Close the door to the weighing chamber. Analytical balances are very expensive about $2500 each and must be treated with care! Record the weight shown on the readout in your lab handout. B3. Carry out this sequence of weighings twice: dish, spatula, dish and spatula together. After each weighing, remove the object(s) from the balance pan and close the door of the weighing chamber. Then verify that the reading on the balance is exactly g. As you begin each new weighing, check the readout and re-zero the balance if necessary. When you complete the weighings, close the weighing chamber door. Volume Measurement Beaker and Graduated Cylinder Most beakers and flasks have approximate volume markings on them. Graduated cylinders are designed to be more accurate. Pipets and burets are even more carefully calibrated and are much more accurate (and harder to use). In this section you will compare experimentally the accuracy and precision of the volume measurements you make using several kinds of laboratory glassware. B4. Weigh a clean, dry 150-mL beaker on the laboratory balance. The laboratory balance is a top-loading balance with a pan on which to place the sample to be weighed. The mass is displayed to three decimal places. Go back to your desk and use the volume markings on the side of the beaker to place 50 ml of clear tap water into it as accurately as you can. (Any time you use tap water, collect a little of it and look at its color. If it has a reddish or brownish or yellowish color, there are probably iron deposits in it from the water pipes. If this occurs, let the water run until it is colorless. Use only colorless and clear tap water.) Whenever you are measuring the volume of a liquid, you make the reading at the bottom of the curved surface of the liquid (the meniscus); you should have the meniscus at eye level when you read the volume. Weigh the beaker and water on the laboratory balance and determine the mass of water in the beaker by difference. This mass can now be used to determine the actual volume of the water you measured out. B5. The Handbook of Chemistry and Physics has a table giving the density of water at 0.1 C intervals. Use the temperature probe you calibrated in Part A or a mercury thermometer to measure the water's temperature to the nearest 0.1 C and look up the density of water for that temperature. (If you wish to use the temperature probe and your MeasureNet station has been turned off since you calibrated the probe, you must re-calibrate it.) Use the density of water to determine the volume of this water sample from its mass. This may seem to be a strange and complicated way to determine a volume you already know. Remember that you are determining the volume from the mass, independent of the calibration marks on the beaker, so that you can make a comparison. Since the mass and the density will be known quite accurately, the volumes calculated from them will be the most accurate values for the actual volumes you measured out. B6. The absolute error of your measurement is the difference between the measured volume (the volume you actually did measure out, determined from the water's mass), and the true, or target, volume (the volume you were trying to measure out, using the beaker's markings): EXPERIMENT 1 1-7

8 absolute error = target value measured value (1-1) This might be a fairly large error. If you have used the beaker properly, aligning your eye with the meniscus, then the error is mainly due to the volume markings on the beaker. One purpose of this exercise is for you to determine how large this sort of error is for each type of glassware you use. Then you will know later what glassware is appropriate to use when a certain level of accuracy is required in a volume measurement. B7. Your error can also be expressed as a relative error. For this experiment, this is the absolute error divided by the target volume multiplied by 100% to convert it to percent error. absolute error relative error = 100% target value (1-2) Expressing errors as relative errors makes it much easier to compare the accuracy or precision of measurements of quantities differing greatly in magnitude. B8. Repeat the volume measurement procedure with a 100-mL graduated cylinder, again measuring 50 ml (not 100 ml) of clear tap water. Volume Measurement Pipet A pipet is used to measure volumes with high accuracy and precision, but some effort is required to learn to use a pipet correctly. One purpose of this part of the exercise is to learn to make accurate measurements with a pipet. You will probably have to practice several times before you are skilled enough. In contrast to the beaker and graduated cylinder measurements, the error in any volume measurement by pipet is likely to be mostly your error. That's why it's important to practice your technique. You will need this skill in various future experiments, so take the time now to become proficient. B9. Get about 30 ml of distilled water in a small, clean beaker. Obtain a 10-mL pipet and pipet pump. The pump is used to draw liquid into the pipet and transfer it into the weighing vessel. B10. Weigh the capped bottle using the analytical balance. As long as it is capped and dry on the outside, it doesn't matter if it is wet inside. You will determine the mass of water added from the pipet. Pipet 10 ml of distilled water into the bottle. You will notice that when you let the pipet drain, a small amount of water will remain in the tip. You should not try to force this water out of the pipet. Most volumetric pipets have a TD marking on them this stands for To Deliver, and means that the pipet has been calibrated to account for the water remaining in the tip. Weigh the capped bottle using the analytical balance. Use the density of water to calculate the actual volume of water you pipetted into the bottle. What is the error of your measurement? If it is greater than 0.10 ml, which might be expected if you are inexperienced in using a pipet, you need to improve your technique. Ask your instructor to watch to see whether you are using the pipet and pump correctly, and practice until you can reliably measure out ± 0.05 ml of water. For each attempt, empty the bottle into the beaker, weigh the capped bottle, pipet ten ml of water into the bottle, and reweigh the capped bottle with water. B11. After you have achieved an adequate level of proficiency in using the pipet, repeat the measurement two more times. You will copy these final two results into the data sheet for this part of the experiment later. When you have completed this work, return the pipet and pump. Empty the water from the bottle into the beaker, and keep the bottle and beaker of water to use with a buret. EXPERIMENT 1 1-8

9 Volume Measurement Buret B12. Set up a buret in a buret clamp. Close the stopcock of the buret and fill the buret to near the top with distilled water from the beaker. Open the stopcock and drain all the water back into the beaker. Look at the inside wall of the buret. Does any water cling in droplets? If so, the buret is not clean. If your buret needs to be cleaned, take it to a sink, close its stopcock, and scrub it with a few milliliters of detergent solution and a long-handled brush. Rinse it repeatedly with tap water to remove all the detergent, then rinse it three times with 5-mL portions of distilled water poured from the beaker or squeezed from a wash bottle. Tip and roll the buret so that each portion of water washes over the entire inside surface. B13. Set up the clean buret and fill it nearly to the top with distilled water. Open the stopcock and let water fill the tip of the buret. Look at the top of the tip (right next to the stopcock) for an air bubble. If you see one, open the stopcock all the way and briefly allow water to flow rapidly from the buret to remove this bubble. Adjust the water level to slightly below the zero mark on the buret barrel. (It is not necessary to have it exactly at the zero mark.) B14. Weigh the capped bottle on the analytical balance. Read the buret to the nearest 0.01 ml by estimating between the marks. Remember that you make your reading at the bottom of the meniscus. Drain as close to ml of water as you can into the bottle. (That is, adjust the level of water to a value of ml plus the initial reading the difference must be ml.) Allow the buret to stand for about 30 seconds before you read it again, so that water on the inside wall will have time to flow down into the remaining water. If necessary, make a small, final adjustment in the level of water, to get within 0.01 ml of the desired value. Weigh the capped bottle and water, then calculate the volume. If the difference between the volume you measured by reading the buret and the actual volume (calculated from mass and density) is more than 0.10 ml, you need to improve your technique. Practice until you can reliably measure out ± 0.10 ml of water. If the error is still more than 0.10 ml, you might be reading the buret incorrectly. Ask your instructor for assistance. B15. When you have achieved the required accuracy in using the buret, repeat the measurement two more times. You will copy these final two results in the data sheet for this experiment. B16. When you have finished the volume measurements for all four types of glassware, you are ready to enter your results into the MeasureNet system so that they can be combined with those of the others on your network. Press MAIN MENU on your workstation, press the function key for OTHER, and then press the function key for MANUAL ENTRY. Now you can enter your results in the spaces provided. The top line in the display will read # Column 1 Column 2, and five lines in the display will be numbered. As you enter a value, it will appear at the blinking cursor in the lower right corner of the display. When you press ENTER, that value will be placed in the list in the location marked by the two asterisks, and the asterisks will move to the next location. If you enter a value incorrectly, you can change it by using the left and right arrow keys to move the asterisks to the location of that value. Enter the new value and press ENTER to replace the incorrect entry, then use the arrow keys to move to the location for the next entry. B17. Enter in Column 1 a code for the type of glassware used (1=beaker, 2= graduated cylinder, 3= pipet, 4= buret), and enter in Column 2 the volume of water determined for that measurement. Each line will contain the glassware code and volume for one measurement, and there will be a total of six lines for your six measurements (beaker, graduated cylinder, pipet twice, buret twice). The order of the lines EXPERIMENT 1 1-9

10 doesn t matter. The following list is a sample showing a possible set of results (enter your own results, not these!) # Column 1 Column B18. When all six of your results have been entered, press FILE OPTIONS, press the function key for SAVE, and choose a three-digit number to identify the file to be saved. Be sure to make a note of the number you choose, so that your data can be retrieved and combined with those of the others on your network. (The number you choose will become part of a filename to use for storing your data on the computer attached to your network. That name will be STA#XXX.TXT, where # is the one- or twodigit number of your station, and XXX is the three-digit number you enter. Give this filename to your instructor.) B19. Although you are to work alone on this part of the experiment, your lab partner from the first part of this experiment will also need to use the station to enter his/her data. Be sure that you use different three-digit numbers to store your data. If you use the same number, the first file will simply be replaced by the second, so that the first file will be lost. B20. When all of the students on your network have entered their data, your instructor will run a program on the computer to combine the data from all the files you have stored. A histogram of the results obtained by the entire group for each type of glassware will be printed out. Be sure to obtain a copy of these results so that you can discuss them in your lab report. EXPERIMENT

11 EXPERIMENT 1 REPORT SHEET Name: Date: Partner: TEMPERATURE PROBE CALIBRATION Ice-and-water bath temperature prior to probe calibration Ice-and-water bath temperature after probe calibration MASS MEASUREMENT ANALYTICAL BALANCE TRIAL 1 TRIAL 2 Difference Mass of evaporating dish Mass of porcelain spatula Calculated mass of dish + spatula Measured mass of dish + spatula Difference between calculated and measured mass of dish + spatula EXPERIMENT

12 VOLUME MEASUREMENTS Station number Three-digit number used when saving file Water temperature Density of water at this temperature BEAKER Mass of 150-mL beaker Mass of 150-mL beaker + 50 ml water Mass of water Actual volume of water Absolute error of your measurement Relative error of your measurement Calculation of actual volume and error values GRADUATED CYLINDER Mass of 100-mL graduated cylinder Mass of grad. cylinder + 50 ml water Mass of water Actual volume of water Absolute error of your measurement Relative error of your measurement EXPERIMENT

13 EXPERIMENT 1 REPORT SHEET (CONT.) Name: Date: Partner: PIPET TRIAL 1 TRIAL 2 Mass of bottle + lid Mass of bottle + lid + 10 ml water Mass of water Actual volume of water Absolute error of your measurement Relative error of your measurement BURET TRIAL 1 TRIAL 2 Initial buret reading Final buret reading Measured volume of water Mass of bottle Mass of bottle + 10 ml water Mass of water Actual volume of water Absolute error of your measurement Relative error of your measurement EXPERIMENT

14 Notes for Experiment 1 (Part A) Procedure Considerations You may perform Part A in pairs. Be sure to use plenty of ice and stir continually when performing your calibration of the temperature probe. You must press ENTER after the temperature becomes steady, or the calibration will not be recorded. Be careful to not let the wire on the temperature probe touch the hot plate! Notes for Experiment 1 (Part B) Procedure Considerations You should perform Part B individually. The data for all of the students in the class will be combined and used to prepare several plots. These will be printed by your TA from the computer. You will need these plots to complete your lab report for Experiment 1. Helpful Hints: - There are two important aspects of making a measurement: the numerical value, and the unit. When making a measurement, you should record every decimal place given by the measuring instrument, and estimate an additional decimal place whenever possible (for example, when using a buret). Only when one is performing calculations using measured values should the number be rounded. The result of an addition of subtraction should have only as many decimal places as the measurement with the least decimal places, and the result of a multiplication or division should have only as many significant figures as the measurement with the least sig. figs. Remember that zeroes at the end of a number containing a decimal place are significant and should not be discarded. Be sure to also include a unit when recording a measurement. These aspects are an important learning objective of this experiment, which will be reflected in the grading of your lab report. EXPERIMENT