How is Lab Glassware Used For Students_113_revised S19v3-1.pdf

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Arizona State UniversityCHM 113 S19School of Molecular Sciences1How is Lab Glassware Used?BACKGROUNDThe glassware and equipment/apparatus used in our general chemistry labs to measurea quantity has inherent limitations. There are also differences in how people make thesame measurement. Uncertainties always exist in measured quantities and we use theterms accuracy and precision to better understand them.Accuracyis how close a measured value is to the actual (true) value. The degree ofuncertainty in any measured value will depend on the apparatus or glassware used.Precisionis measured by repeating measurements under unchanged conditions andcomparing how widely the results vary. The precision of the measurements may beexpressed qualitatively or by calculations in terms of the standard deviation.When we perform a measurement in the laboratory, we should remember that there isalways some uncertainty in the last digit reported for that measured quantity. All digits ofa measured quantity, including the uncertain one, are calledsignificant figures.A more precise measuring device allows for measurements to be made with moresignificant figures.The precision of an experimental result is no more precise than themeasuring device used with the least precision.PrecisionPrecisionPrecisionPrecisionAccuracyAccuracyAccuracyAccuracyPercent ErrorA percent error calculation tells us about the accuracy of an experimental measurement(or an experimental result).%????? =|?????𝑖????𝑎? 𝑉𝑎??? − 𝐴??????? 𝑉𝑎???|𝐴??????? 𝑉𝑎???×100
Arizona State UniversityCHM 113 S19School of Molecular Sciences2Performing Measurements Using Common GlasswareLab equipment used to measure volumes varies in size, types, and purpose. Somecommon glassware used in lab include: beakers, burets, graduated pipets, graduatedcylinders, and volumetric flask.Figure 1.Some common glassware equipment used in lab.Each of these pieces of glassware are commonly used in the laboratory. Each type ofglassware is used to eitherholdordelivercertain volumes of liquids, but they vary in theamounts and accuracy with which they can measure these volumes.The glassware used to measure volumes has some degree of uncertainty, therefore,when reading a measurement you should estimate one more digit past the smallestdivision on the measuring device being used. For example, a buret or a 10mL graduatedcylinder has gradations of a tenth of a milliliter (0.1 mL), therefore, volumes should beestimated to the hundredths place (0.01 mL).When measuring out a specific volume using graduated glassware, be sure to line up thedesired volume line with the bottom of the meniscus.
Arizona State UniversityCHM 113 S19School of Molecular Sciences3Figure 2.Graduated cylinder readingSome glassware such as volumetric flasks and volumetric pipettes only have a single lineto indicate volume. This is because they are made to measure just one specific volumeand will have two significant figures after the decimal point (i.e. 10.00 mL and 50.00 mL).

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Term
Fall
Professor
Birk
Tags
Chemistry, Accuracy and precision, Laboratory glassware, School of Molecular Sciences

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