BASIC LABORATORY TECHNIQUES Revised 1-2-16
BASIC LABORATORY TECHNIQUES(Revised 1-2-16)(See Appendix II: Summary for making Spreadsheets and Graphs with Excel andAppendix III parts C, C1 and C2: Significant figures, scientific notation and rounding)A. WEIGHINGThe determination of the quantity of matter in a sample is most directly determined by measuringits mass. The process by which we determine the mass of an object is commonly referred to, incorrectly,as weighing. There are two closely related quantities, weight and mass. The weight of an object is theforce which the earth's gravitational field exerts on the object. This force diminishes as the object ismoved to a greater distance from the center of the earth (e.g., the weight of an object is actually less onthe top of a mountain than at sea level.) The weight of an object, therefore, depends upon gravity andconsequently upon the object's location. This can be expressed mathematically as:w mgwherew the force of attraction of the earth for the object, or weightm mass of the objectg gravitational acceleration (varies with distance from the earth’s center)In this equation we have defined mass. Mass is a measure of quantity of matter and does not depend onother variables as does weight. In carrying out quantitative experiments, we wish to determine the massrather than the weight of samples. This can be accomplished using a set of objects of accurately knownmass—these are called weights. We compare the weight (force of attraction) of our unknown mass to theweight of the known masses. When these two forces are equal, they have the same mass.1. Laboratory BalancesWe will carry out mass measurements on electronic balances that rapidly compare the mass of anobject placed on the top pan with calibrated mass objects inside the instrument and read out the mass indigital form.Since balances are precise and delicate instruments, every precaution to prevent damage must betaken. The following rules are necessary if the balances are to remain accurate scientific instruments.a) Never move the balances. Some contain knife-edges which may be damaged if the balance isjarred. To work properly a balance must be level. Moving the balance may change the level.b) Never place any chemicals directly on the balance pan. Place materials to be weighed onweighing paper or in a suitable container.c) If your balance isn’t working properly, do not try to fix it. Call the instructor.d) If anything is spilled on the balance (solid or liquid) remove it immediately.e) Never weigh anything hot on the balances. There is the possibility of damage to the balance, aswell as the likelihood of weighing errors due to buoyancy effects from the warmer air surrounding theobject.1
There are simple-to-use electronic, digital balances in the laboratory. The balances require a30 minute warm-up and therefore will be left on continuously. The maximum amount that can beweighed on these balances is either 110 or 150 grams, depending on which type of balance you areusing. When that weight has been exceeded the readout will be "H."These balances can weigh in different modes. You will be using only the grams mode, whichhas the following symbol on the readout: "g." If the readout is different see your instructor!!All of these balances will give you the mass to the nearest milligram, or 0.001 g. Make surethat you record every mass to three digits after the decimal point! That is, you should always havenumbers (even if they are zeros) in the tenths, hundredths, and thousands of a gram places. The unitfor all of your masses should be grams, not milligrams, however.2. Direct Weighinga) Place either a piece of weighing paper or weighing container (lightweight plastic beaker) onthe balance pan and tare or zero the balance by pressing "TARE" on the front (left or right side) of thebalance. All 0’s will appear on the readout, indicating that the mass of the container has beensubtracted.b) Now place the material to be weighed on the paper or into the weighing container and thereadout will be the mass in grams. Remember to record three digits after the decimal point. If thebalance does not show three places there, see your instructor.3. Weighing by Differencea) Make sure balance is in grams mode.b) Press the TARE key to zero the balance with nothing on the balance pan.c) Weigh the container and record its mass.d) Add the material to the container and reweigh.e) Find the mass of material alone by subtracting the two weights.f) Use the same balance for the two weighings.B. USING VOLUMETRIC GLASSWAREMost of the glassware in your laboratory locker has been marked by the manufacturer to indicatethe volume of liquid contained by the glassware when filled to a certain level. The graduations etchedor painted onto the glassware by the manufacturer differ greatly in the precision/accuracy theyindicate, depending on the type of glassware and its intended use. For example, beakers andErlenmeyer flasks are marked with very approximate volumes, which serve merely as a rough guide tothe volume of liquid in the container. Other pieces of glassware, notably burets, pipets, and graduatedcylinders, are marked much more precisely by the manufacturer to indicate volumes. It is important toknow when a precise volume determination is necessary and appropriate for an experiment, and whenonly a rough determination of volume is needed.Glassware that is intended to contain or to deliver specific precise volumes is generally markedby the manufacturer with the letters "TC" (to contain) or "TD" (to deliver). For example, a flask thathas been calibrated by the manufacturer to contain exactly 500 mL of liquid at 20 o C would have thelegend "TC 20 o C 500 mL" stamped on the flask. A pipet that is intended to deliver a 10.00 mLsample of liquid at 20 o C would be stamped with "TD 20 o C 10 mL." It is important not to confuse2
"TC" and "TD" glassware. Such glassware may not be used interchangeably. The temperature(usually 20 o C) is specified with volumetric glassware since the volume of a liquid changes withtemperature, which causes the density of the liquid to change. While a given pipet will contain ordeliver the same volume at any temperature, the amount of substance present in that volume will varywith temperature.1. Graduated CylindersThe most common apparatus for routine determination of liquid volumes is the graduatedcylinder. Although a graduated cylinder does not permit as precise a determination of volume as doother volumetric devices, for many applications the precision/accuracy of the graduated cylinder issufficient. Examine the graduated cylinders in your lab locker and determine the smallest graduationof volume that can be determined with each cylinder.When water (or an aqueous solution) is contained in a narrow glass container, such as agraduated cylinder, the liquid surface is not flat as might be expected. Rather, the liquid surfacecurves upward where it meets the container walls (see figure below). This curved surface is called ameniscus, and is caused by an interaction between the water molecules and the molecules of the glasscontainer wall. When reading the volume of a liquid that makes a meniscus, hold the cylinder so themeniscus is at eye level, and read the liquid level at the bottom of the curved surface.Reading a meniscus. Read the bottom of the meniscus whileholding at eye level.Always record the volumes of 100 mL graduated cylinderto /- 0.1 mL (one place after the decimal).2. PipetsWhen a more precise determination of liquid volume is neededthan can be provided by a graduated cylinder, a pipet may be used. Pipets are especially useful ifseveral measurements of the same volume are needed (such as in preparing similar-sized samples of aliquid unknown). Two types of pipet are commonly available. The Mohr pipet is calibrated at eachmilliliter and can be used to deliver any size sample (up to the capacity of the pipet). The volumetricpipet can deliver only one size sample (asstamped on the barrel of the pipet), butgenerally it is easier to use and is morereproducible. Always record the volumeof a pipet to /- 0.01 mL (two places afterthe decimal).Pipets are filled using a rubber bulb tosupply the suction needed to draw liquid into the pipet. IT IS ABSOLUTELY FORBIDDEN TOPIPET BY MOUTH IN CHEMISTRY LAB!!!The safety bulb should not actually be placed around the barrel of the pipet. This would mostlikely cause the liquid being measured to be sucked into the bulb. Rather, squeeze the bulb, andmerely press the opening of the bulb against the opening in the barrel of the pipet to apply thesuction force, keeping the tip of the pipet under the surface of the liquid being sampled. Yourinstructor will show the class how to use a pipet at the start of the period.3
Here is a description of the technique:Allow the suction to draw liquid into the pipet until the liquidlevel is 1 or 2 inches above the calibration mark of the barrel of thepipet. At this point, quickly place your index finger over theopening at the top of the pipet to prevent the liquid level fromfalling. By gently releasing the pressure of your index finger fromthe pipet opening, the liquid level can be allowed to fall until itreaches the calibration mark of the pipet. The tip of the pipet maythen be inserted into the container which is to receive the sample,and the pressure of the finger removed to allow the liquid to flowfrom the pipet.When using a pipet, observe the following rules:a) The pipet must be scrupulously clean before use: wash with soap and water, rinse with tapwater, followed by distilled water. If the pipet is clean enough for use, water will not bead upanywhere on the inside of the barrel.b) To remove rinse water from the pipet (which would dilute the solution to be measured), rinsethe pipet with several small portions of the solution to be measured, discarding the rinsings in a wastebeaker for disposal.c) The tip of the pipet must be kept under the surface of the liquid being drawn up during theentire time suction is being applied, or air will be sucked into the pipet.d) Allow sufficient time for the pipet to drain when emptying, to make certain the full capacityof the pipet has been delivered. Remove any droplets of liquid adhering to the tip of the pipet bytouching the tip of the pipet to the side of the vessel that is receiving the sample. Do not shake.e) If you are using the same pipet to measure out several different liquids, you should rinse thepipet with distilled water between liquids, and follow with a rinse of several small portions of the nextliquid to be measured.C.RECORDING DATA CORRECTLY USING SIGNIFICANT FIGURESMany operations in the chemistry laboratory involve measurements of some kind. Examples areweighing a compound or measuring the volume of a liquid. It is important to record these dataproperly so that the number recorded correctly represents the certainty of the measurement. Thefollowing discussion is intended to serve as a guide for calculating and reporting numerical results.Every measurement that is made is really an approximation. For example, the length of theobject below is between 1.5 and 1.6 units. Its length is seen to be approximately 1.56 units. There isuncertainty in the last digit, 6; it is estimated. If you had an object with the recorded weight of 2.6grams, this would mean that the object was weighed to the nearesttenth (0.1) of a gram and that its exact weight was between 2.5 gand 2.7 g. In recording a result, it is the last digit that represents adegree of uncertainty; for example, in 2.6 g it is the number 6 thatrepresents some degree of uncertainty. We say the number 2.6 gcontains two significant figures, the numbers 2 and 6 being thesignificant figures. If the recorded weight of the4
object were 2.634 g, there would be four significant figures (2, 6, 3, and 4), and this would mean thatthe object was weighed to the nearest thousandth (0.001 g). [Note: The 2 is in the ones place, the 6 inthe tenths place, the 3 in the hundredths place and the 4 in the thousandths place.] Thus, it is the 4(bolded) that has been estimated. Significant figures refer to those digits we know with certainty plusthe first doubtful or estimated digit.It is important that you be aware that every measuring device, regardless of what it may be, haslimitations in its accuracy. Moreover, to take full advantage of a given measuring instrument, youshould be familiar with or evaluate its accuracy. Careful examination of the subdivisions on thedevice will indicate the maximum accuracy which you can expect of that particular tool. In thisexperiment you will determine the accuracy of your 10 mL pipet, 10 mL graduated cylinder, and adevice called a RePipet. This device is useful for delivering a fixed volume of liquid relativelyquickly. The approximate accuracy of some of the equipment you will use in CHEM 109 is in Table 1below.Not only should you obtain a measurement to the highest degree of accuracy that the device orinstrument permits, but you should also record the reading or measurement in a manner that reflectsthe accuracy of the instrument. For example, a mass obtained from a “top loader” balance should beobserved and recorded to the nearest 0.001 g. The final digit of each “uncertainty” in Table 1represents the estimated or uncertain digits and must be recorded. Table 2 gives examples of correctlyand incorrectly recorded data for the thermometer and top-loading balance.Table 1. Equipment AccuracyEquipmentUncertaintytop loading balance /-0.001 g ( /-1 mg)meter stick /-0 .01 cm ( /-0.1 mm)thermometer /-0.2 o C10-mL graduated cylinder /-0.01 mL100-mL graduated cylinder /-0.1 mL10-mL volumetric pipet /-0.02 mL50-mL buret /-0.02 mLTable 2. Correct and Incorrect DataThermometerTop Loading Balanceo86 C (incorrect)85.9 g (incorrect)o85.9 C (correct) 85.93 g (incorrect)85.93 o C (incorrect)85.932 g (correct)o85.932 C (incorrect)86 g(incorrect)D. EVALUATING EXPERIMENTAL DATAIf any piece of experimental data is to be of much use, some idea of the reliability of itsvalues is important. The measurement of any physical quantity is subject to some degree ofuncertainty, depending on the equipment or instruments used to make the measurement as well as onthe skill of the experimenter. There are two components to this uncertainty: accuracy and precision.Precision refers to how closely several determinations of one quantity agree with eachother. In other words, are measurements made on the same sample under the same conditionsrelatively close together (high precision), or spread out (low precision?) Precision can be expressed interms of "deviation," the absolute value of the difference between each measured value and the meanor average of that set of measurements. Taking the absolute value of a number means making thenumber positive. Therefore the deviation will always be a positive number.5
Example of calculating deviation: For the following set of numbers (1.452, 1.211, 1.389 and 1.198)whose average is 1.313, the deviation of each number would be found by subtracting that number fromthe average (then taking the absolute value ie. making it positive.) 1.452 1.313 0.139 1.389 1.313 0.076 1.211 1.313 0.102 1.198 1.313 0.115Accuracy refers to how closely a measured value approaches the true or accepted value. Itcan be expressed experimentally by the percentage of error:% Error (Experimental Value - True Value) x 100True ValueNote that the % error may be negative or positive, and that this sign ( or ) clearly hasinformation content.Measurements showing a high degree of precision do not always show a high degree ofaccuracy and vice versa. See figure below.xyyxwwz zzwxyw good precision, poor accuracyx poor precision, poor accuracyy poor precision, good accuracy? (Find the mean [center] of these points to understand.)z good precision, good accuracyYour BLT report includes:(1) Pages 31-33(2) Your spreadsheet. (Remember to include row and column headings on yourspreadsheet and formula page.)(3) Formula page. (To view the formulas, press “Ctrl” and “ ” simultaneously.Note: The pre-lab, which is due at the beginning of the recitation, is located on page 29.6
PROCEDURE:A. Significant figures and measurementsFollow the instructions for part 1. For part 2a, make sure you measure the length in cm (not inches). Ifyour ruler is labeled in mm, you must convert to cm. For part 2b, measure the temperatures in o C. To make anice bath, fill a beaker with ice from the ice machine. Then add water and swirl the beaker to cool the water. Letit sit for a couple of minutes with the thermometer resting inside, then swirl the beaker again before measuringthe temperature. To record the temperature of tap water, fill a 400 mL beaker with tap water and let thethermometer sit in the water for several minutes before making the reading. To correct your room temperatureand tap water temperature readings, determine how far your ice-water reading deviate from 0.0 o C (the truevalue for the temperature of ice-water.) This is a measure of how far “off” your thermometer was. Subtract/addthis deviation from/to your thermometer readings to get the corrected temperature values.B. Laboratory BalancesYou are to weigh one of the given objects by the “direct method” on two different balances. (See pp.21-22 for instructions.) Record the number of each balance you use on the data sheet. You are then to weighthe same object on one of the balances using the “weighing by difference” technique. Record weights on thedata sheet to three places after the decimal! Small plastic beakers are to be used as weighing containers.C. Reading a Graduated CylinderYour instructor will set up a display of graduated cylinders filled with different amounts of water. Readthe volume of liquid contained in a 10 mL and 100 mL cylinder. (See p. 23) Record your readings using thecorrect number of significant figures after the decimal point as permitted by the precision of each cylinder.D. Evaluation of the Precision and Accuracy of various volumetric measuring devices1. The Graduated Cylinder. This exercise will demonstrate the accuracy with which a 10 mL graduatedcylinder is able to deliver 10 mL of water. You will be recording your data on page 31. Clean and dry a 50 mLplastic beaker and weigh to the nearest milligram (three places after the decimal point) once. Record the sameweight on the data sheet (page 31) for all three trials. Use the 10 mL graduated cylinder to transfer 10 mL ofwater to the beaker. Reweigh the beaker containing the water and subtract to determine the mass of watertransferred by the graduated cylinder. Do three trials, in total, thoroughly drying the beaker after each trial.Enter your data on the copy of the spreadsheet on page 28 and on the spreadsheet that you can downloadfrom http://faculty.uscupstate.edu/jkrueger. Enter appropriate formulas to perform the calculations. Do not putunits on the spreadsheet! Refer to “Summary for Making & Printing Spreadsheets & Graphs” in Appendix IIin your lab manual. You will need to determine the mass of the water transferred, and then calculate theaverage value for the mass of water transferred. Calculate the deviation of each individual measurement fromthe average by subtracting each measurement from the average. Your instructor will give you the true value forthe mass of 10 mL of water at the current room temperature. Record the value on your data sheet and use it tocalculate the percent error for your three measurements of the mass of water. Find your average % error. Usecorrect sig. fig. On page 31, you also need to show three hand calculations for trial 1. Remember to use correctsig. fig and units!2. The Pipet. This procedure demonstrates the accuracy of a pipet in delivering 10 mL of water. Cleanand dry a 50 mL plastic beaker and weigh to the nearest milligram. Record the weight on your data sheet. Halffill an Erlenmeyer flask with water. Pipet 10 mL of water f
BASIC LABORATORY TECHNIQUES (Revised 1-2-16) (See Appendix II: Summary for making Spreadsheets and Graphs with Excel and Appendix III parts C, C1 and C2: Significant figures, scientific notation and rounding) A. WEIGHING The determination of the quantity of matter in a sample is most directly determined by measuring
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