ChemTech - Introduction to Chemical Techniques and the...

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Introduction to Chemical Techniques and the Metric System Objective: The purpose of this laboratory exercise is to learn common chemical techniques practiced in the introductory chemistry laboratory and to use the metric system. Safety Tips: Wear safety goggles at all times in the laboratory. Background: Chemical Techniques Scientists in the laboratory use various pieces of laboratory equipment or measuring devices during an experiment to generate precise and accurate data. How precise and accurate the set of data is depends on 1) how careful the measurements are taken (laboratory technique); 2) how close the measurement is in determining the true value (accuracy); and 3) how reproducible the measurements are (precision). A measuring device usually contains a scale with subdivisions that limit the measuring device's precision. A measurement cannot be obtained that is more precise than your instrument is capable of reading. Consider the centimeter scale given in the ruler below. There are major divisions of 1 centimeter and subdivisions of 0.1 centimeter (1 millimeter). The precision of the ruler is to 0.1 centimeter; that is the measurement that is known for certain. It is possible, however, to read or estimate a value between the subdivisions. In the illustration above, the arrow is between 8.6 cm and 8.7 cm. A value of 8.65 cm could be assigned. Two digits are known with certainty, 8.6 and one digit, 0.05, is uncertain because it is the best estimate of the fractional part of the subdivision. The recorded value of 8.65 cm therefore contains three significant figures - two with certainty plus one uncertain (estimated). Remember when dealing with significant figures that the uncertainty is in the last recorded digit and the number of significant figures includes the number of digits known, plus one more that is estimated.
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Precision is the reproducibility of a measurement and gives an indication of how closely several measurements agree with each other. Measurements whose values are clustered together with little or no variation in value have high precision. Measurements whose values show a wide variation in value have low precision. Random errors affect precision, because these are errors that lead to differences in successive values of measurement. Random errors usually arise from sources that cannot be corrected. For example, a four-place balance may give the weight of the sample as 1.032g, when the actual weight is 1.03215 g. The 0.00015 g cannot be measured on this balance and is uncorrectable. The next time this same balance may give the weight of a different sample as 1.032 g but the actual weight is 1.03189 g. These are random errors. Accuracy is a measure of how closely the experimentally determined value agrees with the known or accepted value. Systematic errors affect accuracy. If the source of the error is found (determined) these types of errors can be corrected. Systematic errors result in measurements being higher or lower than the
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ChemTech - Introduction to Chemical Techniques and the...

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