How is Lab Glassware Used For Students_revRS-5-15-18.pdf -...

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Arizona State University School of Molecular Sciences 1 How is Lab Glassware Used? BACKGROUND The glassware and equipment/apparatus used in our general chemistry labs to measure a quantity has inherent limitations. There are also differences in how people make the same measurement. Uncertainties always exist in measured quantities and we use the terms accuracy and precision to better understand them. Accuracy is how close a measured value is to the actual (true) value. The degree of uncertainty in any measured value will depend on the apparatus or glassware used. Precision is measured by repeating measurements under unchanged conditions and comparing how widely the results vary. The precision of the measurements may be expressed qualitatively or by calculations in terms of the standard deviation. When we perform a measurement in the laboratory, we should remember that there is always some uncertainty in the last digit reported for that measured quantity. All digits of a measured quantity, including the uncertain one, are called significant figures . A more precise measuring device allows for measurements to be made with more significant figures. The precision of an experimental result is no more precise than the measuring device used with the least precision. Precision Precision Precision Precision Accuracy Accuracy Accuracy Accuracy Percent Error A 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 University School of Molecular Sciences 2 Performing Measurements Using Common Glassware Lab equipment used to measure volumes varies in size, types, and purpose. Some common glassware used in lab include: beakers, burets, graduated pipets, graduated cylinders, 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 of glassware is used to either hold or deliver certain volumes of liquids, but they vary in the amounts 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 smallest division on the measuring device being used. For example, a buret or a 10mL graduated cylinder has gradations of a tenth of a milliliter (0.1 mL), therefore, volumes should be estimated to the hundredths place (0.01 mL). When measuring out a specific volume using graduated glassware, be sure to line up the desired volume line with the bottom of the meniscus.
Arizona State University School of Molecular Sciences 3 Figure 2. Graduated cylinder reading Some glassware such as volumetric flasks and volumetric pipettes only have a single line to indicate volume. This is because they are made to measure just one specific volume and will have two significant figures after the decimal point (i.e. 10.00 mL and 50.00 mL).

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