Introduci Measurements in the Laborato

Introduci Measurementsin the Laborato

Objectives

The objectivesof this laboratoryare: a) To use a metric ruler to measurethe dimensionsof regular geometric shapes,and to use these measurementsto determinethe areasof the shapes. to b) To measurethe volume of a sampleof water using a graduatedcylinder and a beakerin order comparetheir precision. in c) To measurethi massof an item using a triple-beambalance and an analytical(electronic) balance orderto comparetheir precision;also, to determinethe massof a powderby weighing by difference. d) To measurethe melting point of an unknownsolid and identiff it using this measuredvalue.

Background

Our knowledgeof chemistry and chemical processeslargely dependson our ability to obtain correct informationabout matter. dft"n this information is quantitative,in the form of measurements.In this lab, studentswill be introduced to some common measuringinstruments so that they can practice making measurements,and to learn about instrumentprecision. In Part A of this lab, a metric ruler will be usedto measurelength in centimeters(cm). In Part B, a beakerand a graduatedcylinder will be used to measureliquid volume in milliliters (rnl). In Part C, an electronicbalance and a triple-beambalance will be to measuremass in grams(g). In Part D, a thermometerwill be usedto measuretemperature in degreesCelsius ('C).

Since all measuringdevices are subject to some error, it is impossibleto make exact measurements. Scientistsrecord all the digits of a measurementthat are known exactly, plus the first one that is uncertain. Thesedigits are iollectively referredto as significantdigits. Digital instruments,such as an electronicbalance, ale designedto limit themselvesto the correctnumber of significantdigits, and their readingsare properly recoicledas given. However, when using analog instrumentssuch as rulers and thermometerr,tne experimentalistis responsiblefor determining the correct number of significant figures. Theseinstruments are properly readto oneplace beyond the graduationsof the scale.

Example1: MeasuringLength The ruler markingsare every 0.1-centimeter.The correct reading is 1.67 cm. The first 2 digits !!7 are known exactly. The last digit 1.6f is uncertain. You may have insteadestimated it as 1.68cm.

Example2: Measuringthe Volume of a Liquid

A- When measuringliquid volumes,the graduatedscale must be read from the to lowest point of the curvedsurface of the liquid - the liquid meniscus.

30 The graduatedcylinder markingsare every 1-milliliter. The correctreading is 30.0 mL. The first 2 digits are known exactly. The last digit 30.Qis 25 $.0 uncertain. Even thoughit is a zero,it is significantand must be recorded.

Page I of5 Example3: MeasuringTemPerature

Here, the thermometermarkings are every l-degree. The correctreading is 33'6 oC. The first 2 digits 33.6 areknown exactly. The last digit 33.$ is uncertain. 'C. You may haveinstead estimated it as33.5

Note that the measuringdevices used in this lab may have different scale graduationsthan the ones shownin theseexamplei. Thus, be sureto makeit a regularhabit to checkthe scaleson all equipment.

When making measurements,it is important to be as accurateand preciseas possible. Accuracy is a measureof how close an experimentalmeasurement is to the true, acceptedvalue. Precisionrefers to the degreeof uncertainty in a measurement. For example;a mass measurementof 48.26 g has an uncertaintyof 10.01 g, while a measurementof 48.3 g has an uncertaintyof 10.1 g. Since the measurement of 48.26g ttu. less uncertainty,it is the more precisemeasurement. In general,the more decimalplaces provided by a device,the moreprecise the measurementwill be.

Sincemeasurements are often used in calculationsto obtain other values of interest,it is importantto considerthe numberof significant figuresthat shouldbe recordedfor the resultsof suchcalculations. If multiplying or dividing measuredvalues, the result should be reported with the lowest number of signifiianlfigzres usedln the calculation. If addingor subfractingmeasured values, the resultshould be reportedwith the lowestnumber of decimalplaces tsed in the calculation.

Example4: SignificantFigures in CalculatedValues

(a) A studentruns l8.752metersin 54.2seconds. Calculate his averagevelocity (or speed). velocity : distance/time : 18.752m / 54.2s : 0.345978m/s from calculator = 0.346m/s to 3 significantfigures

(b) The massof a glass is measuredtobe 12.466grams. If 10.33 grams of water are addedto this glass,what is the total combinedmass? totalmass= 12.466g+ 10.33g = 22.796g from calculator = 22.80E to 2 decimalPlaces

Page2 of 5 The temperaturethat will be measuredin this lab is the melting point of an unknown solid. Melting point is a physical property. When a solid is heatedcontinuously, a point will eventuallybe reached *hr." it undergoeJu pttyri.ul changeand becomesa liquid. The temperatureat which liquid first appearsis definid as the melting point of that substance.Since all pure substanceshave unique melting points,a measuredmelting point can be usedto identiff an unknown substanceby comparingit with a list of known substancesand their accepted,true melting points.

The accuracyof a measuredvalue, suchas a melting point, may be evaluatedby a calculationof percent error. percenterror is a commonway of reportinghow close a measuredexperimental value (El) is to the hue value(TI):

I EV -TV Perceril Error: !-:- :-:-txl00

Accuratemeasurements will typically havelow percenteffors of <5Yo.

Procedure

Safety

In part D you will be heating a solid powder and severalpieces of equipmentwith an open Bunsen burner flame. Exercise extra caution while using the Bunseh burner, and pleaseremember that the heateditems will be very hot to the touch.

Materials and Equipment

Metric ruler*, shape sheet, electronic balance, large test tube, 100-mL beaker, 100-mL graduated cylinder, triple-beam balance, 250-mL Erlenmeyer flask, electronic balance, sugar, Bunsen burner, thermometei +OO-111lbeaker, stand and ring clamp, small watch glass,wire gauze,capillary tube, latex tubing, scoopulaand unknown solids.

Part A: Measuringthe Dimensionsof Regular GeometricShapes

1. Checkout a ruler from the stockroom. 2. Obtaina "shapesheet" from your instructor,and then usethe ruler to measurethe dimensionsof the two geometrii shapeson it. Measurethe length and width of the rectangle,and the diameterof the circle. Recordthese measurements on your report form. 3. When finished,retum the ruler to the stockroom. 4. Use your measurementsto calculatethe areasof the assignedgeometrical shapes. . Areaofarectangle: I xw . Area of a ckcle: n/ (wherer: radius= r/zthediameter)

Page3 of5 Part B: Measuringthe Volumeof a Sampleof Water

1. Obtaina largetest tube from your instructor.Fill this test-tubeto the brim with tap water,then carefullytransfer it to a 100-mLbeaker (obtain from your locker).Note that if your 100-mLbeaker hasno scalemarkings on it, you will needto takeit to the stockroomand swap it for onethat does. Measureand record the volumeof waterin the beaker. 2. Again,fill the sametest-tube to the brim with tap water,then carefully transfer it to a 100-mL graduatedcylinder (obtain from your locker).Measure and record the volumeof waterin the graduatedcylinder. Do thesemeasured volumes have the samenumber of significantfigures?

Part C: Measuringthe Massof Solids

Comparingthe Precision of rwo Qpesof Balances 1. Usea triple-beambalance to obtainthe massof a 250-mLErlenmeyer flask (obtainfrom your locker). 2. Now usean electronicbalance to obtainthe massof the sameErlenmeyer flask. Do thesemeasured masseshave the samenumber of significantfigures? Be sureto recordyour measuredmasses on your reportform. Weighingby Dffirence 3. Usingthe electronicbalance again, obtain the massof a 100.mLbeaker. If you alreadyused this same beakerin PartB, makesure that you carefullydry it beforeweighing it. 4. Add two spoonfulsof sugarto this beaker,using your scoopula.Do not do this over the balance! Thenobtain the new combinedmass of boththe beakerand the sugar.Be sureto usethe same electronicbalance as before. 5. Whenfinished, dispose of the sugarused in the sink. 6. Useyour two measurementsto determinethe massof sugar(only) weighedout.

Part D: Measuringthe Temperatureof Water

1. Usea large300-mL beaker half-filled with roomtemperature, distilled water. Placea thermometerin the waterfor 3-5 minutesthen record the temperatureof the water. 2. Usethe beakerfrom (l). Pourthe waterin the sink until approximately50 mL areleft. Add iceto the beakeruntil it is two-thirdsfull. Placethe thermometerin the beakerfor 10 minutesthen take the temperaturereading.