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April 2021


The Degree Of Agreement Among Several Measurements

3 QUESTION The conformity of a given value to the actual value is a) precision b) error c) accuracy (e) accuracy e) the safety of the accuracy of a measurement at fair value for that measurement. The accuracy of a measurement system refers to the proximity of the concordance between repeated measurements (repeated under the same conditions). Measurements can be both accurate and precise, accurate, but not precise, accurate, but not accurate, but not accurate or not. 32 MIXTURE and COMOUNDS MIXTURES PURE COMPOUNDSA mixture can be physically separated into compounds or pure elements. A pure assemblage has a constant composition with solid ratios of elements. Mixes can have a set of changing physical properties. For example, mixing alcohol and water cooked over a temperature range. Physical properties such as the point of ingion or the melting point of pure substances are invariant. For example, pure water is boiling at 100 degrees 1. Technical errors can be divided into two categories: accidental errors and systematic errors. As the name suggests, random errors occur at regular intervals, with no apparent motive. Systematic errors occur when there is a problem with the instrument.

For example, a scale could be poorly calibrated and read 0.5 g with nothing on it. All measures would therefore be overestimated by 0.5 g. If you don`t take this into account to your extent, your measurement contains a few errors. As mentioned above, the more measures there are, the closer we can get to the knowledge of the actual value of a quantity. For several measurements (replications), we can evaluate the accuracy of the results, and then use simple statistics to estimate how close the average value would be to the actual value if there was no systematic error in the system. The average value deviates less from the “real value” as the number of measures increases. 42 Exercise a. TC – TK 273 – 233 x 273 – -40 C56. Convert the following Kelvin temperatures to Degrees Celsius and Fahrenheit. a. the temperature that records the same value on both the fahrenheit and Celsius scales, 233 K b.

the helium graduation point, 4 K c. the temperature at which many chemical quantities are determined, 298K d. tungsten melting point, 3680 K a. TC – TK n 273 – 233 x 273 – -40 C TF – 9/5 × TC – 32 – 9/5 × (-40.) – 32 – -40 -F b.TC – C; TF – 9/5 × (-269) – 32 – -452 -F TC – 25 degrees Fahrenheit; TF – × – 77 FTC – 341 0 degrees Fahrenheit; TF – 9/5 × – 6170 F 31 MIXTURE and COMOUNDS A MIXTURE is a combination of two or more substances that are not chemically combined and do not exist in solid proportions. Most natural substances are mixtures. An example of air is the mixture of several gases, oil, etc. All measures are subject to error, which contributes to the uncertainty of the result. Errors can be classified as human or technical error. Maybe you pass a small volume from one tube to another and you don`t get all the fullness in the second tube because you knocked it over: it`s a human error.

Accidental error is reduced with a more precise instrument (measurements become thinner) and with more repeatability or reproducibility (precision). Consider a common laboratory experiment in which you must determine the acid content in a vinegar sample by observing the volume of sodium hydroxide solution needed to neutralize a certain volume of vinegar. You experience it and you get value. Just to be on the safe side, repeat the process on another identical sample of the same bottle of vinegar. If you have actually done this in the laboratory, you will know that it is very unlikely that the second study will produce the same result as the first. In fact, if you do a series of replication tests (i.e. identical in all respects), you will probably get scattered results. 4 QUESTION Uncertainty in a measured quantity is determined by: a) the dexterity of the observer and the limitations of the measuring instrument.