Exp 1 Accuracy, Precision and Beer_s Law

Exp 1 Accuracy, Precision and Beer_s Law - Chem 1312 Exp 1...

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Chem 1312 Exp 1 ± Accuracy, Precision and Beer’s Law Objective: The purpose of this lab is to familiarize students with the various instruments for making physical measurements, preparing analytical solutions and to familiarize them with the concepts of accuracy and precision. Introduction: Accuracy & Precision: Chemistry is a measurement science. Good observations, knowing the limitations of laboratory equipment and understanding when data is meaningful are the mark of an good scientist. Mass and volume measurements are perhaps the most often made measurements in chemistry. Many people use the words precision and accuracy interchangeably but to scientists they have separate meanings. Precision is the reproducibility of a measurement or how closely measurements agree with one another. The number of significant figures used in a measurement or statistical figures of merit such as average, standard deviation and confidence intervals are used to describe the precision of a measurement. Accuracy is how close a measurement comes to the real or actual value. Accuracy requires that a known standard or careful calibration of an instrument. A common depiction of these two concepts is a bull’s eye where the center circle is the “correct” or most accurate value. Figures of merit associated with accuracy are % error and % deviation (from true value) Experiment 1 Glassware and plastic ware used in a lab usually have volume measurements on them. It is very important to know the precision of the glassware used and when an approximate volume will do and when more precision is called for in measurements. Typically, graduation on beakers and Erlenmeyer flasks are not very precise and must only be used when very approximate volumes are needed. More precise glassware is called “analytical or volumetric glassware” and consists of graduated cylinders, pipets and burets. Volumetric glassware is labeled “to contain ( TC )” or “to deliver ( TD )”. A flask labeled “TC 20 200ml” would contain exactly 200 ml at 20 o C. Likewise a pipet labeled “TD 20 10ml” would deliver exactly 10 ml of liquid at 20 o C. - 1 -
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Accuracy, Precision and Beer’s Law Graduated cylinders are used for routine measurements of liquids. They are a glass or plastic tube with volume markings on the side of the cylinder. Volumes can be interpolated to one unit past the unit indicated by the markings. For instance, if a graduated cylinder is marked for each (1) milliliter (ml) of liquid, then the cylinder can be read to one unit past 1 ml. Perhaps not ± 0.1 ml but certainly ± 0.5 ml. When a liquid is contained in a narrow tube, the liquid surface may not be level. If the molecules of liquid are attracted to the sides of the container more than to themselves, the liquid will be curved upwards along the sides. This is called a meniscus. The meniscus must be read at the bottom of the meniscus and at eye level so that errors are not made when reading.
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Exp 1 Accuracy, Precision and Beer_s Law - Chem 1312 Exp 1...

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