Absolute Error Burette
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Treatments MSDS Resources Applets General FAQ Uncertainty ChemLab Home Computing Uncertainties in Laboratory Data and Result This section considers the error and uncertainty in experimental measurements and
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calculated results. First, here are some fundamental things you should realize accuracy of burette about uncertainty: • Every measurement has an uncertainty associated with it, unless it is an exact, counted
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integer, such as the number of trials performed. • Every calculated result also has an uncertainty, related to the uncertainty in the measured data used to calculate it. This percentage error of equipment uncertainty should be reported either as an explicit ± value or as an implicit uncertainty, by using the appropriate number of significant figures. • The numerical value of a "plus or minus" (±) uncertainty value tells you the range of the result. For example a result reported as 1.23 ± 0.05 means that the experimenter has some accuracy of burette pipette and measuring cylinder degree of confidence that the true value falls in between 1.18 and 1.28. • When significant figures are used as an implicit way of indicating uncertainty, the last digit is considered uncertain. For example, a result reported as 1.23 implies a minimum uncertainty of ±0.01 and a range of 1.22 to 1.24. • For the purposes of General Chemistry lab, uncertainty values should only have one significant figure. It generally doesn't make sense to state an uncertainty any more precisely. To consider error and uncertainty in more detail, we begin with definitions of accuracy and precision. Then we will consider the types of errors possible in raw data, estimating the precision of raw data, and three different methods to determine the uncertainty in calculated results. Accuracy and Precision The accuracy of a set of observations is the difference between the average of the measured values and the true value of the observed quantity. The precision of a set of measurements is a measure of the range of values found,
very nature involves errors and inaccuracies in the course of experimental work. The important issue here is that the inaccuracies are minimised and errors recognised as part of the results and conclusions process. Experimentation and measurement Apparatus
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and instrumentation Inaccuracy Instrumental tolerance Error recording Percentage error calculation Multi-stage procedures Experimentation and measurement percentage error of measuring cylinder Chemistry is an experimental science. All of the laws, rules and principles of chemistry have been elaborated by experiment and observation over
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many years. This process is known as the experimental method and involves the following stages: 1 Observation of a fact pattern or principle. 2 Hypothesis as to the causal factors 3 Experiment to support the hypothesis https://www.dartmouth.edu/~chemlab/info/resources/uncertain.html 4 Repetition and duplication of the experimental results by other research groups. 5 General acceptance of the hypothesis. top Experimental science in schools In principle, there are few actual measuring devices in common use in the laboratory of a normal school. Direct measurements may usually be made of the following quantities: Temperature Liquid volume Gas volume Time Mass Length A more specialised laboratory also may have devices for measuring: Voltage Current pH http://ibchem.com/IB16/03.63.htm Light absorbance top Apparatus and instrumentation The common laboratory apparatus used to take direct measurements: Concept Instrument units abbreviation Temperature Thermometer degrees Celsius ºC Mass Electronic balance grams / kilograms g / kg Time Stopwatch seconds s Length Ruler / Micrometer metres m Liquid volume Measuring cylinder / pipette / burette centimetres cubed / litres cm3 / dm3 Gas volume Gas syringe centimetres cubed / litres cm3 / dm3 top Inaccuracy Any experiment has inherent inaccuracies that must be considered when analysing results. These inaccuracies, or errors, derive from three general sources. Instrumental tolerance Experimental design Human limitations The reliability of any experimental data must take these factors into consideration. In many cases it is possible to estimate the degree of accuracy quantitatively by consideration of the percentage error in the measurements at each stage of a procedure. top Instrument tolerance The instrumental tolerance is the degree of accuracy of a specific instrument, or piece of apparatus, being used to take a measurement. The instrument or apparatus may have the tolerance written on it, or a judgement must be made regarding the accuracy of any measurement. For example, a thermometer may have an inherent inaccuracy of ± 0.25 ºC. This means that its accuracy lies within this range. However, it is also pos
simple piece of laboratory equipment, for example a burette or a thermometer, one would expect the number of variables contributing to uncertainties in that measurement to be fewer http://www.csudh.edu/oliver/che230/textbook/ch05.htm than a measurement which is the result of a multi-step process consisting of http://www.thestudentroom.co.uk/showthread.php?t=97664 two or more weight measurements, a titration and the use of a variety of reagents. It is important to be able to estimate the uncertainty in any measurement because not doing so leaves the investigator as ignorant as though there were no measurement at all. The phrase "not doing so" perpetuates the measuring cylinder myth that somehow a person can make a measurement and not know anything about the variability of the measurement. That doesn't happen very often. A needle swings back and forth or a digital output shows a slight instability, so the investigator can estimate the uncertainty, but what if a gross error is made in judgment, leading one to estimate an unrealistic "safe" envelope of uncertainty accuracy of burette in the measurement? Consider the anecdote offered by Richard Feynman about one of his experiences while working on the Manhattan Project during World War II. Although this example doesn't address the uncertainty of a particular measurement it touches on problems which can arise when there is complete ignorance of parameter boundaries: Some of the special problems I had at Los Alamos were rather interesting. One thing had to do with the safety of the plant at Oak Ridge, Tennessee. Los Alamos was going to make the [atomic] bomb, but at Oak Ridge they were trying to separate the isotopes of uranium -- uranium 238 and uranium 235, the explosive one. They were just beginning to get infinitesimal amounts from an experimental thing [isotope separation] of 235, and at the same time they were practicing the chemistry. There was going to be a big plant, they were going to have vats of the stuff, and then they were going to take the purified stuff and repurify and get it ready for the next stage. (You have to purify it in several stages.) So they were practicing on the one hand, and they were just gett
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