Lab Experiment 1- Laboratory Techniques - Data Analysis PDF

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Laboratory Report Experiment 1. Laboratory Techniques and Data Analysis Chem 134 Laboratory Section # ____________

Name: ______________________________ Date: ____________________

Chemistry 134

Experiment 1

Laboratory Techniques and Data Analysis Introduction In order for any experimental data to be of value, the data must have been obtained using reliable methods. This includes the proper use of any equipment and apparatus involved as well as correct evaluation and interpretation of the data. This is true regardless of the complexity or the simplicity of the method. In this experiment, a number of relatively simple but fundamental methods of collecting data, such as measuring mass and volume, will be covered both in terms of operation of the equipment as well as recording the data correctly. From these data, values will be derived and then evaluated with respect to accuracy. The accuracy of a given measurement indicates how close the measured value is to the accepted or “true” value. A highly accurate measurement shows close correspondence to the “true” value. When the true value is known, the accuracy may be evaluated by calculating the absolute error (E) or relative error (RE). Topics relevant to this experiment are presented in Appendix D, which should be read thoroughly in addition to this procedure.

Techniques Mass Measurements The determination of the mass of an object is accomplished using a balance. Currently, most balances are electronic with digital readouts and require very little training to use. An electronic balance utilizes a servomotor that translates linear motion (placing an object on the balance pan causes the pan to move downward) into an electromagnetic force. The balance pan starts out in a “null” or zero position relative to a light sensor. Any mass placed on the pan deflects the pan from the zero position. This initiates an electromagnetic current to return the pan to the zero position. Because a greater mass requires a greater electric current to accomplish this task, the current can be read directly as mass once the balance is calibrated. Since this zero position can be established with a mass on the pan, taring and/or recalibration can be accomplished by simply pressing a bar or button. Even the easiest to operate most sensitive balance will not give correct results if it is used incorrectly or is not properly maintained. Two types of balances will be used in this experiment. This first type is the top-loading balance, which is sensitive to either 0.01 g or 0.001 g. The sensitivity can be determined by looking at the number of decimal places shown on the digital readout. The second type, the analytical balance, is located on an isolated table to eliminate any vibrations that can cause fluctuations in the readings. With this type of balance, the object to be weighed is placed on the balance pan in an enclosed case. The doors are closed to prevent air currents from affecting the readings, the mass is then University of Michigan-Dearborn

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measured. Analytical balances are sensitive to 0.0001 g. Guidelines for the proper use of balances are given on page 7. The operating instructions for the two types of balances used in this laboratory are found in Appendix B. Volume Measurements The measurement of liquids is conveniently carried out by measuring volume. The measurement of volume utilizes various types of laboratory glassware that has been calibrated either to contain (TC) or to deliver (TD) specific volumes. As with balances, the sensitivity of this glassware varies. The user must be conscious of this in order to determine which apparatus should be used for a specific measurement. A sampling of laboratory glassware in shown in Figure 1. The appropriate number of decimal places that should be recorded for each size and type of glassware is given on page 9. In general, the measurement of volume relies of the ability to either read or adjust the liquid level (the bottom of the (a) (b) (c) (d) meniscus) in relation to one of the calibration marks on the glassware. This Figure 1. Volumetric Glassware: (a) graduated measurement is usually done within the cylinder ± 1%; (b) volumetric flask ± 0.1%; narrowest cylindrical portion of the (c) buret ± 0.02 mL; (d) transfer pipet ± 0.2%. glassware. The narrower this portion of the glassware, the more accurate the measurement can be. As with balances, proper procedures exist for using each glassware type. Before using the glassware note the following: x

Permanent markings that indicate if the glassware is TC or TD.

x

Presence of a single calibration mark or a continuous scale of markings, the latter allowing for measurement of more than one specified volume.

x

For TD glassware, rinsing initially with small portions of the solution to be used should be carried out to prevent dilution due to water or other liquids in the glassware.

x

To keep glassware clean, it should be immediately rinsed with distilled water after use, cleaned, and then stored.

Specific details regarding the use of the pieces of glassware to be utilized in this experiment are covered in the following sections. University of Michigan-Dearborn

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Volumetric Pipets The procedure for rinsing and filling pipets by means of a pipet bulb is shown in Figure 2. Note that the pipet is held near the top, with the index finger in position to be easily moved to cover the open end. By varying the pressure of your finger against the pipet the liquid can be held in the pipet indefinitely or dispensed at a controlled rate. The pipet bulb should be held firmly against the end of the pipet when drawing up the liquid. Do not force the bulb over the end of the pipet as this enlarges the hole in the Teflon end of the bulb and renders it unusable. Releasing the pressure and allowing the pipet bulb to expand creates a suction that causes the liquid to be drawn up into the pipet. Rinse the pipet with two or three small aliquots of the liquid or solution to be measured by pulling enough of the liquid into the pipet to bring the liquid into the glass bulb. Remove the pipet bulb and cover the upper end with your finger. Then tip and roll the pipet to coat the entire upper 90% of the pipet. Drain the rinse aliquot from the pipet and discard in appropriate waste container.

Figure 2. Use of a Volumetric Pipet. (a) filling, (b) wiping outer surface, (c) adjusting liquid level, (d) draining.

When the pipet has been rinsed, pull the liquid up to fill the entire pipet up to about 1 inch above the calibration mark. Cover the upper end with your finger, wipe the liquid from the outer surface using a Kimwipe, and adjust the meniscus at eye level so that the bottom rests on the calibration mark. With the tip of the pipet in the container to be used for the sample, allow the liquid to drain out. Do not blow out the residual liquid in the tip. Touch the tip to the side of the container for 10 seconds and the volume specified by the markings will have been delivered into the container being used. Burets Burets are calibrated with a continuous marking scale and can be used to measure more than one volume. Before using a buret, rinse it three times with small samples of the solution by adding a small aliquot to the top of the buret and allowing the liquid to coat the entire inner surface. Turn the stopcock and allow this liquid to run out through the tip. In this manner, the entire buret from top to tip has been rinsed. Fill the buret above the top calibration mark, then drain some of the liquid through the tip to refill the tip. By allowing liquid to run out of the buret, adjust the meniscus University of Michigan-Dearborn

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at eye level so that its position can be read. It is important that your eye level is not appreciably above or below the meniscus or parallax error will result. The volume should be read to one decimal place beyond the smallest scale division marked. Always record two decimal places, even if the hundredths place is zero. Note that the volume increases downward. This is illustrated in Figure 3. Always be sure to record both the initial and final buret volumes since the difference between the two values gives the volume delivered from the buret. Volumetric Flasks Volumetric flasks, as shown in Figure 1 (b), vary in volume from Figure 3. Reading a Buret. Arrow represents eye level, 1 mL to 5 L and are calibrated TC (or to contain) a specified 18.28 mL is volume shown. single volume. They are used for the accurate preparation of solutions. Volumetric flasks are marked with a single calibration mark on the narrow neck of the flask. The solute used in solution preparation can be measured in a number of ways. Weighing for solids or using a pipet for liquids are two of the most common methods. Once the solute has been delivered into the flask, the solvent (commonly water) is added to the flask to about half fill the flask. The flask is the swirled to mix the liquid or completely dissolve the solid. More solvent is added with mixing until the bottom of the meniscus rests on the single calibration mark. The last small portions of the solvent should be added slowly and carefully with a dropper to ensure that the liquid level does not go above the specified volume. The stopper is placed in the flask, turned slightly to seat it and then the flask is inverted 15 – 20 times to thoroughly mix the contents of the flask. The solution should now be homogenous and ready to use.

Derived Results In this experiment, the types of measurements described above will be used to determine the density of solids and solutions. Graphical display of these quantities will also be used to determine the density of an unknown solution. In order to determine the density of any substance, two quantities must be known, the mass and the corresponding volume. The density is then calculated as shown in Equation 1. density

mass volume

eqn 1

Usually for liquids and solids, the mass is measured in grams (g) and the volume in milliliters (mL). Appendix D covers the necessary information on preparing graphs from experimental data.

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Procedure In the following procedure, check the data sheets for any calculations that need to be completed and/or posted before leaving the lab. There is class data that needs to be copied and a graph constructed before you leave the laboratory. Part 1. Density of a Solid The instructor will assign you a metal whose density will be determined as specified in the following. The metal will be identified by its density. Obtain two slugs of the same metal. Be sure that the metal slugs are dried thoroughly before any mass measurements are performed. 1. Wear gloves when handling the metal slugs. Weigh the assigned metal slug to ±0.01 g. 2. Place about 25 mL of distilled water in a 50 mL graduated cylinder. Record the exact volume of water shown on the graduated cylinder. Carefully slide the metal slug into the graduated cylinder. Be sure that it is fully submerged in the water. (If it is not, start with a larger volume of water in the graduated cylinder.) Check that there are no trapped air bubbles. Record the volume registered on the graduated cylinder. 3. Repeat the determination with a different slug of the same metal to obtain two density measurements. Return the metal slugs to the instructor’s desk when done. Part 2. Density of a Sugar Solution The instructor will assign you a sugar solution of a given concentration expressed as a weight percent (%w/w) and a sugar solution of unknown density. Using the following procedure, determine the density of each solution. Finally, class data for the sugar solutions of known concentration will be collected and used to generate a calibration curve. The regression line of the calibration curve is used to determine the concentration of the sugar solution of unknown density. 1. Obtain about 40 mL of the assigned standard sugar solution in a labeled 150 mL beaker. Record its concentration, which is given as weight percent (%w/w) of sugar on the bottle label. 2. Rinse and fill the buret at your station with the solution. Be sure that the tip of the buret is filled and that here are no trapped air bubbles in the tip of the buret. Adjust the liquid level and record the exact value. Use the meniscus reader to maximize accuracy in the reading. 3. Clean and dry a 100 mL beaker and determine its mass to ±0.01 g. 4. Using the buret, deliver 10 mL ±0.02 mL into the beaker. This volume does not have to be exactly 10.00 mL, just as long as you record it exactly; for example, 9.96 mL is an acceptable University of Michigan-Dearborn

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value. Measure the mass of the beaker and the sample to ±0.01 g. 5. Repeat the measurements for a new 10 mL aliquot of assigned standard sugar solution, for a total of two determinations. 6. Obtain about 40 mL of an unknown solution from the instructor in a labeled 150 mL beaker. Record its code. 7. Using the procedure above, determine the density of this unknown solution. 8. Dispose of the solutions in the appropriate waste container and wash all glassware. Invert glassware to dry. 9. Calculate the densities determined for the known solution and post these values as directed by the instructor. Part 3. Graphical Analysis of Sugar Solution Density as a Function of Weight Percent, %(w/w), of Sugar 1. Using Excel 2007, construct a plot of density as a function of weight percent, %(w/w), sugar in the solution, where the plot type is Scatter with only Markers. Use the class average densities for the dependent variable and the posted %(w/w) for the independent variable. Be sure to label the data columns and the axes on the graph with correct measurement quantities and units. Add a linear Trendline and make sure to check the boxes for Display Equation on chart and Display R-squared value on chart. The regression equation and square of the correlation coefficient must be shown on the graph. Refer to Appendix C and Appendix D for software instructions, graphing guidelines, and regression analysis concepts. The graph must include a text box in the upper right-hand corner of the page with your name, “Chem 134”, lab section number, and date. The graph must be submitted with the report. 2. Record the regression analysis (Trendline) results in Table 5 on the Data and Results sheets. Important! When using the laboratory computer, remember… x to remove gloves. The keyboards must not become contaminated with chemicals. x to not place chemicals on the computer carts. x to send the print job only once. If you have trouble obtaining the printed document, consult with the instructor. Do not press any buttons on the laser printer. x to shut down the computer when you are done using it. Do not save files on the computer.

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Report The report for the experiment should consist of at least the following items. 1. Cover Page. Complete and attach the cover page that is provided in Appendix E. 2. Enter all data in the places provided on the Data and Results sheets. 3. Perform the calculations specified on the Data and Results sheets. Calculations should be done according to the guidelines provided in the Introduction to the Laboratory. Refer to Appendix D for mathematical concepts used. 4. Prepare the graph as specified in Part 3 of the Procedure. Attach the graph to the report. 5. Include answers to the questions in the spaces provided.

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Data and Results Part 1. Density of a Solid Code of Metal Unknown: __________

Identity of Metal: _____ _ _ _____

Table 1. Density Determination for a Metal Slug. Trial

1

2

mass of slug (g) final volume reading (mL) initial volume reading (mL) volume of slug (mL) density of metal (g/mL)

Density calculation for trial 1. Show all work.

Calculation of average value. Show all work.

Look up accepted density in one of the references available in the lab. Calculate the relative error(%) of the average experimental value. Show all work.

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Part 2. Density of a Sugar Solution Concentration of Assigned Known Solution: ______________ Table 2. Density Determination for Known Sugar Solution Trial

1

2

mass of beaker+sample (g) mass of beaker (g) mass of sample (g) final volume reading (mL) initial volume reading (mL) volume of solution (mL) density of solution (g/mL) Calculation of density for trial 1. Show all work.

Calculation of average value. Show all work.

Unknown Solution Code: ________________ Table 3. Density Determination for Unknown Sugar Solution Trial

1

2

mass of beaker+sample (g) mass of beaker (g) mass of sample (g) final volume reading (mL) initial volume reading (mL) volume of solution (mL) density of solution (g/mL) Calculation of density for trial 1. Show all work.

Calculation of average value. Show all work.

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Part 3. Graphical Analysis of Sugar Density as a Function of Weight Percent, %(w/w), of Sugar Table 4. Class Density Values for section ________ %(w/w) Concentration

Average Density (g/mL)

%(w/w) Concentration

Average Density (g/mL)

*Assigned standard, enter your calculated average density Construct a graph of density versus %(w/w) Concentration using the data in Table 4. Add a Trendline. Be sure to display the equation of the line and square of the correlation coefficient, R2, on the graph. Attach a copy of the graph to the report. Refer to Appendix C for software instructions and Appendix D for notes regarding regression analysis. Table 5. Linear Regression Results. Quantity

slope

y-intercept

R2

value units Write the exact equation for the line in terms of the experiment.

Use the equation to determine the %(w/w) Concentration of the Unknown solution. Show all work.

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Questions 1. Below each figure, record the reading for the (i) top-loading balance in grams and the (ii) aneroid barometer in mmHg with the correct number of significant figures and units.

(i)

(ii)

70

80

90

2. Below each figure, give the reading for the (i) 50 mL graduated cylinder, (ii) thermometer, and (iii) 25 mL buret with the correct number of significant figures and units. Important. The liquid volume in the graduated cylinder and buret must be read from the bottom of the meniscus. Notice that the continuous scale on the graduated cylinder increases upward, while that of the buret increases downward.

(i)

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(ii)

(iii)

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3. Specify the equipment that is needed to carry out the following operations. A. Prepare 100.00 mL of a 0.50 M calcium nitrate solution

B. Weigh 0.8256 g of potassium sulfate

C. Add 5.00 mL of acetic acid to a volumetric flask

D. Add 12.84 mL of sodium hydroxide solution to an acid

4. A student performed an exp...