Lectures 3 and 4 CHE 1316

Lectures 3 and 4 CHE 1316 - Treatmentofexperimental data

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Treatment of experimental  data
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Errors in chemical analysis Impossible to perform a chemical  analysis totally free of errors Every measurement is influenced by  many uncertainties Produces a scatter of results
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Errors in chemical analysis Graphical representation of results from six  replicates for the determination of iron (III)
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Definitions Replicates--results from analytical procedure  carried out on multiple representative portions  of a sample to be analyzed Accuracy—measure of how close  experimental results are to an accepted value Precision—a measure of how close     experimental values are to each other
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Precision and accuracy
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Methods of expressing  precision and accuracy
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Methods of expressing  precision and accuracy
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Methods of expressing  precision and accuracy Mean—synonymous with arithmetic  mean and average  (x),  ( sample mean ) Population mean (μ), the true mean of  the population as N approaches infinity
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Definitions Error—difference between the “true”  value and the measured  value Deviation—difference between an  observed value and the mean Relative deviation—              deviation/mean X 1000 ppt x
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Definitions Average relative deviation    Average of deviations for replicates  divided by the mean X 1000 ppt  for titrations a value of 4 ppt for this value  is considered good
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Significant figures Number of significant figures is all of the  certain digits plus the first uncertain Express data in scientific notation to  avoid confusion in determining whether  terminal zeros are significant
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Significant figures-rules Disregard all initial zeros Disregard all final zeros  unless they  follow a decimal point All remaining digits including zeros  between nonzero digits are significant
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Significant figures Sums and differences—can be found by  visual inspection  3.4 + 0.020 + 7.31 = 10.73 = 10.7
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Significant figures Products and quotients—rule of thumb    same number of significant figures as  the original number with least significant  figures           24 X 4.02/ 100.0 = 0.965 = 0.96
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Significant figures Logarithms—keep as many digits to the right of the  decimal point as there are significant figures in the  original number Antilogarithms—keep as  many digits as there are  digits to the right of the decimal point in the original  number Log 4.000 X 10 -5  = -4.3979400 = -4.3979 Antilog 12.5 =3.162277 X 10 12  = 3 X 10 12
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Significant figures Round computed results of a chemical  analysis in an appropriate way   When rounding 5, round to the nearest  even number Round so result contains only  significant figures Postpone rounding until the calculation  has been completed
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Types of errors in  experimental data
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This note was uploaded on 06/08/2010 for the course CHEM 1316 taught by Professor Rabbe during the Spring '08 term at Baylor.

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Lectures 3 and 4 CHE 1316 - Treatmentofexperimental data

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