chpt2 - Measurement Scientific measurement Number vs....

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Unformatted text preview: Measurement Scientific measurement Number vs. Quantity l Quantity - number + unit UNITS MATTER!! Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem Types of measurement l Quantitative- use numbers to describe l Qualitative- use description without numbers l 4 feet l extra large l Hot l 100F Scientists prefer l Quantitative- easy to check l Easy to agree upon, no personal bias l The measuring instrument limits how good the measurement is. How good are the measurements? l Scientists use two word to describe how good the measurements are l Accuracy- how close the measurement is to the actual value l Precision- how well can the measurement be repeated Accuracy vs. Precision Random errors: reduce precision Good accuracy Good precision Poor accuracy Good precision Poor accuracy Poor precision Systematic errors: reduce accuracy (person) (instrument) Differences l Accuracy can be true of an individual measurement or the average of several l Precision requires several measurements before anything can be said about it l examples Lets use a golf anaolgy Accurate? No Precise? Yes 10 Accurate? Yes Precise? Yes 12 Precise? No Accurate? Maybe? 13 Accurate? Yes Precise? We cant say! 18 Accurate? Yes Precise? We cant say! 18 Accurate? Yes Precise? We cant say! 18 Accuracy Precision Resolution subsequent samples time offset [arbitrary units] not accurate, not precise accurate, not precise not accurate, precise accurate and precise accurate, low resolution-2-3-1 1 2 3 In terms of measurement l Three students measure the room to be 10.2 m, 10.3 m and 10.4 m across. l Were they precise? l Were they accurate? Definitions: Precision Repeatability the precision for an analysis in which the only source of variability is the analysis of replicate samples. Reproducibility the precision when comparing results for several samples, for several analysts of several methods. Indeterminate error any random error that causes some measurements or results to be too high while others are too low. Human error is a type of indeterminate error David Harvery, Modern Analytical Chemistry , 2000, page 62 Precision is a measure of the spread of data about a central value and may be expressed as the range, the standard deviation, or the variance. Definitions: Error & Uncertainty Uncertainty the range of possible values for a measurement. David Harvery, Modern Analytical Chemistry , 2000, page 64 Error a measure of bias in a result or measurement. SI System for Measuring Length Unit Symbol Meter Equivalent kilometer km 1,000 m or 103 m meter m 1 m or 100 m decimeter dm 0.1 m or 10-1 m centimeter cm 0.01 m or 10-2 m millimeter mm 0.001 m or 10-3 m micrometer mm 0.000001 m or 10-6 m nanometer nm 0.000000001 m or 10-9 m Comparison of English and SI Units 1 inch 2.54 cm 1 inch = 2.54 cm Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 119 Reporting Measurements l Using significant figures l Report what is known with certainty l Add ONE digit of uncertainty (estimation) Davis, Metcalfe, Williams, Castka, Modern Chemistry , 1999, page 46 Practice Measuring 4.5 cm 4.54 cm 3.0 cm Timberlake, Chemistry 7th Edition, page 7 cm 1 2 3 4 5 cm 1 2 3 4 5 cm 1 2 3 4 5 Reading a Vernier...
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This note was uploaded on 11/09/2011 for the course CHEM 110 taught by Professor Sullivan during the Fall '10 term at BYU.

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chpt2 - Measurement Scientific measurement Number vs....

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