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Unformatted text preview: 2-1Experiment 2SCIENTIFIC MEASUREMENT AND ERRORI. Learning Objectives To learn the use of common scientific glassware.To understand the importance of precisionand accuracyin scientific measurement. II. Background Information Chemistry is an experimental science that depends upon both careful observation and the use of good laboratory technique. One of the goals of this experiment is to become familiar with scientific glassware (see Figure 2.1) and to learn some basic operations essential to all chemistry labs. Mastering these fundamental operations will ensure both success in the course and a safe working environment in the lab. 2-2Precision and AccuracyMany operations in the chemistry laboratory, such as weighing a compound or measuring the volume of a liquid, involve measurements of some kind. Because every measurement made in the laboratory is really an approximation, it is important that the numbers recorded reflect the accuracyand precisionof the measuring device used. Some laboratory errors occur more frequently than others, but one type of error, experimental error, is associated with every measurement. At best this type of error can only be minimized by carefully following proper laboratory technique. Experimental error is classified as either systematicor random. A systematicor determinate errorcan in principle be discovered and corrected. Using a pH meter that has been standardized incorrectly produces a systematic error in the results. If the pH of the buffer used to standardize the meter is labeled 7.00 but is really 7.08, a systematic error will occur. If the meter is otherwise working properly, all of the pH readings will be 0.08 pH units too low. In other words, when the incorrectly standardized pH meter displays a reading of 5.60, the actual pH of the sample is 5.68. This systematic error could be discovered by using a buffer of known pH to test the meter. Systematic error may be positive in some regions and negative in others. The key feature of systematic error is that with care and cleverness, it can be detected and corrected. Random orindeterminate errorarises from limitations in the ability to make physical measurements. Random error has an equal chance of being positive or negative. It is always present and cannot be corrected. One random error commonly encountered in the laboratory is associated with reading a scale. For example, the length of the object in Figure 2.2 is between 1.4 and 1.5 units. Its length is "read" to be approximately 1.46 units. There is uncertainty in the last digit, 6, and therefore it is an estimated value. Different people reading the scale might report a range of values representing their subjective interpolation between the markings. In fact, one person reading the same instrument 2-2Precision and AccuracyMany operations in the chemistry laboratory, such as weighing a compound or measuring the volume of a liquid, involve measurements of some kind. Because every measurement made in the laboratory is really an approximation, it is important that the...
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- Spring '08