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
to determinethe areasof the shapes.
measurements
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-beambalanceand 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 chemicalprocesseslargely dependson our ability to obtain correct
informationaboutmatter. dft"n this information is quantitative,in the form of measurements.In this
lab, studentswill be introduced to some common measuringinstrumentsso that they can practice
and to learn aboutinstrumentprecision. In Part A of this lab, a metric ruler will
making measurements,
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 electronicbalanceand a triple-beambalance
will be to measuremassin grams(g). In Part D, a thermometerwill be usedto measuretemperaturein
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,suchas an
electronicbalance,ale designedto limit themselvesto the correctnumberof 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. Theseinstrumentsareproperly readto oneplacebeyondthe 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
insteadestimatedit as 1.68cm.
Example2: Measuringthe Volume of a Liquid
Ato
When measuringliquid volumes,the graduatedscalemust be read from the
lowestpoint of the curvedsurfaceof the liquid - the liquid meniscus.
30
The graduatedcylindermarkingsare every 1-milliliter. The correctreadingis
30.0 mL. The first 2 digits $.0 are known exactly. The last digit 30.Qis
uncertain. Even thoughit is a zero,it is significantand must be recorded.
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Page I of5
Example3: MeasuringTemPerature
Here, the thermometermarkingsare every l-degree. The correctreadingis 33'6
oC. The first 2 digits 33.6 areknown exactly. The last digit 33.$ is uncertain.
'C.
You may haveinsteadestimatedit as 33.5
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.
it is important to be as accurateand preciseas possible. Accuracy is a
When making measurements,
measureof how close an experimentalmeasurementis to the true, acceptedvalue. Precisionrefersto
the degreeof uncertainty in a measurement. For example;a mass measurementof 48.26 g has an
of 48.3 g has an uncertaintyof 10.1 g. Since the
uncertaintyof 10.01 g, while a measurement
nt of 48.26g ttu. less uncertainty,it is the more precisemeasurement.In general,the more
measureme
will be.
decimalplacesprovidedby a device,the moreprecisethe measurement
are often used in calculationsto obtain other values of interest,it is importantto
Sincemeasurements
considerthe numberof significantfiguresthat 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 subfractingmeasuredvalues,the resultshouldbe
reportedwith the lowestnumberof decimalplaces tsed in the calculation.
Example4: SignificantFiguresin CalculatedValues
(a) A studentruns l8.752metersin 54.2seconds.Calculatehis 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
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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.Sinceall pure substanceshaveuniquemelting
points,a measuredmelting point can be usedto identiff an unknown substanceby comparingit with a
and their accepted,true melting points.
list of known substances
The accuracyof a measuredvalue, suchas a melting point, may be evaluatedby a calculationof percent
error. percenterror is a commonway of reportinghow closea measuredexperimentalvalue (El) is to
the hue value(TI):
I EV -TV
il Error: !-:- :-:-txl00
Percer
will typically havelow percenteffors of <5Yo.
Accuratemeasurements
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 pleaserememberthat the
heateditemswill 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,
beaker,standand ring clamp, small watch glass,wire gauze,capillary tube, latex
thermometei+OO-111l
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
on your reportform.
circle. Recordthesemeasurements
3. Whenfinished,retum the ruler to the stockroom.
to calculatethe areasof the assignedgeometricalshapes.
4. Use your measurements
.
Areaofarectangle: I xw
.
Area of a ckcle: n/
(wherer: radius= r/zthediameter)
Page3 of5
Part B: Measuringthe Volumeof a Sampleof Water
1. Obtaina largetesttubefrom your instructor.Fill this test-tubeto the brim with tapwater,then
carefullytransferit to a 100-mLbeaker(obtainfrom your locker).Notethat if your 100-mLbeaker
hasno scalemarkingson it, you will needto takeit to the stockroomandswapit for onethat does.
Measureandrecordthe volumeof waterin thebeaker.
2. Again,fill the sametest-tubeto the brim with tapwater,thencarefullytransferit to a 100-mL
graduated
cylinder(obtainfrom your locker).Measureandrecordthe volumeof waterin the
volumeshavethe samenumberof significantfigures?
graduated
cylinder.Do thesemeasured
Part C: Measuringthe Massof Solids
Comparingthe Precisionof rwo Qpesof Balances
1. Usea triple-beambalanceto obtainthe massof a 250-mLErlenmeyerflask (obtainfrom your
locker).
2. Now usean electronicbalanceto obtainthe massof the sameErlenmeyerflask.Do thesemeasured
masseson your
masseshavethe samenumberof significantfigures?Be sureto recordyour measured
reportform.
Weighingby Dffirence
3. Usingthe electronicbalanceagain,obtainthe massof a 100.mLbeaker.If you alreadyusedthis same
beakerin PartB, makesurethat you carefullydry it beforeweighingit.
4. Add two spoonfulsof sugarto this beaker,usingyour scoopula.Do not do this over the balance!
Thenobtainthe new combinedmassof boththe beakerandthe sugar.Be sureto usethe same
electronicbalanceasbefore.
5. Whenfinished,disposeof the sugarusedin the sink.
to determinethemassof sugar(only) weighedout.
6. Useyour two measurements
Part D: Measuringthe Temperatureof Water
in
distilledwater. Placea thermometer
1. Usea large300-mLbeakerhalf-filled with roomtemperature,
of the water.
the waterfor 3-5 minutesthenrecordthe temperature
2. Usethebeakerfrom (l). Pourthe waterin the sink until approximately50 mL areleft. Add iceto
in the beakerfor 10minutesthentakethe
the beakeruntil it is two-thirdsfull. Placethethermometer
reading.
temperature
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