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average, average derivations

# average, average derivations - Most exercises in CHE 133...

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Most exercises in CHE 133 - 134 involve the replicate determination of some measurable quantity. The number of repetitive measurements is usually small (3 - 5 determinations). We invoke simple statistical concepts in the reporting of the results of such measurements. SUPL-001 , Analysis of Experimental Reliability, discusses these concepts in detail. This page presents supplementary information about three of the most common quantitites. We use a concrete example based on a typical exercise: You are instructed to determine the concentration (in grams/100 mL) of a solution of sodium hypochlorite (NaOCl) by titration with sodium thiosulfate of given concentration. The titrations are performed on "precisely equal" 25.00 mL volumes of the unknown solution (aliquots delivered by a 25 mL transfer pipet). Barring errors introduced by the investigator, the volumes are "precisely equal" within the intrinsic error of the pipet. The instructions state that you should " report three determinations of the concentration that have a percent deviation less than 1% ." This translates operationally into the statement: "If the percent deviation of the first three determinations is greater than 1%, you should peform a fourth determination." and so on, until the required percent deviation is achieved. Your first titration requires 21.37 mL of sodium thiosulfate. The second titration requires 21.97 mL. While the third titration has not yet been performed, it is useful to examine the results obtained thus far.

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