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This 7 page Class Notes was uploaded by Antonetta Hermann on Tuesday September 8, 2015. The Class Notes belongs to CH 227 at University of Oregon taught by Deborah Exton in Fall. Since its upload, it has received 14 views. For similar materials see /class/187294/ch-227-university-of-oregon in Chemistry at University of Oregon.
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Date Created: 09/08/15
General Chemistry Lab CH227 Due Today reading questions from the web Exp 2 Introduction to Excel Spreadsheets and Statistics Prelab reading Pages 21 25 and Appendix C of lab text This week in lab Turn in completed prelab questions from text pg 54 these do not need to be written in your lab notebook The lab assignment will be distributed in lab Take lab text to lab lab reports Prior to submitting in lab all lab reports must be submitted to SafeAssign on Blackboard for a plagiarism check TAs will provide more information in lab Goals gt Working with quantitative data assess limitations and reliability reporting methods gt Introduction to data analysis spreadsheets Microsoft Excel Reporting numerical values The number of figures reported indicates the precision in a given measurement What is the uncertainty how well do we know the value gt Fewer sig figs less precision 140 3 More sig figs more precision 1445 Quantitative measurements have three parts Numerical value Units 0 Precision Important to record all three properly Example mass measurements Analytical balance quot 7 50002 g a Implies 1 00001 g 3quot Toploader balance 500 9 Ci mpies 1001 g It is wrong to report fewer or more digits than the measuring device provides Uncertainty 3E Error Uncertainty how reliable is the value Precision Indicated with sig figs Lfno other information is available Error distance from the true value Accuracy Fundamentally unknowable Better to take a series of measurements Some a little high some a little low With enough measurements errors cancel out Report the mean or average It should approach the true value Ex newborn birth weights N 25 392 N 1oo N500 N 2000 How confident are we in the mean Quantify the spread or uncertainty in the mean with the standard deviation s Mean 205 1 Curve A s 005 Curve B s 002 Smaller standard 7 deviation indicates 95 weygmgl 395 less uncertainty more confidence in the mean for curve B To calculate the standard deVIatIon for three or more measurements Individual value Mean Number of measurements see page 25 of text Or use MS Excel or scientific calculator 1 Reporting the mean and standard deviation Calculate mean and standard deviation Example For a given series of measurements Mean 13 4673290 9 Standard deviation 00732965 g Reporting the mean and standard deviation 2 Round standard deviation to one significant digit 00732965 007 3 Round mean so it has same number of digits after the decimal point as the standard deviation 13 4673290 quot39t 1347 Reporting the mean and standard deviation To summar39ze39 If there is only one measurement the 439 Report mean and Standard deV39atlon uncertainty is indicated by the number of as sig figs mean i one standard deviation For a series of measurements calculate Mass 1347 1 007 9 the mean and standard deviation uncertainty is indicated by the standard deviation What does the standard Experimental error When deviation tell us making repeated measurments of the same thing why do we have a range of values Two categories of experimental error Systematic determinate error a consistent error that may be detected and corrected every measurement will be off in the same direction Example miscalibrated or 7 1pm risl njaolf f uz can be eliminated theoretically Random indeterminate error Range of values due to limitations of physical measurement Some too high some too low Normal scatter in the data Random indeterminate error can never be eliminated but a better measuring device will minimize magnitude indicated by the magnitude of the standard deviation good precision poor accuracy poor precision poor accuracy good precision good accuracy To summarize 0 Precision represented by standard deviation or significant figures associated with random error 0 Accuracy correctness of the value associated with systematic error And nally A blunder means you start over sigh
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