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Page 1 of 15 INTRODUCTION Experiments in the chemistry laboratory involve measurement and observation. Any measurement taken must be made carefully and must always be representative of the equipment used. When using those measurements in calculations; the results, representing relationships between variables, must be written so that they are a reflection of the equipment used. This is why significant figures are important. Significant Figures (also called Significant Digits) are the number of known digits in a measurement or calculated relationship including one estimated digit. In this study, you will learn how to record measurements, read equipment, count significant digits, use scientific notation, and use significant digits when calculations are performed. REPRESENTING MEASUREMENTS There are two parts to every measurement: the number and its units. The Unit is an abbreviation which defines the measurement and immediately follows the number. For example, a child steps on a digital scale and the weight read is 20.5kg (read 20.5 kilograms). Had the unit, kilogram, not been included in the expression, the number 20.5, would be meaningless. REPORTING MEASURED QUANTITIES Every measurement made in the laboratory has an amount of uncertainty. In the previous example, the weight recorded for the child was 20.5kg. If the child steps back up again, the weight might be displayed as 20.4 kg or 20.6kg. The scales are not in error. The scales are displaying the estimation in the weight being reported. In other words, it is known with certainty that the child weighs at least 20kg and the equipment used will estimate the weight to the nearest 0.1kg. More sensitive balances may be able to determine weights estimated to the nearest 0.001kg. It is important to know the specific sensitivity of the balance being used so that the data reported are representative of the equipment used. If a balance is only able to weigh items to the nearest 0.1kg, then when this balance is used, every value reported must be recorded to the nearest 0.1kg (for example: 20.6kg, 10.8kg, 15.0kg). The example used in the previous paragraph involved the use of a digital readout. There are many examples of equipment used in the laboratory that are not digital and must be visually estimated. When using any equipment, examine it to determine the limit of its accuracy and then record all measurements through SIGNIFICANT FIGURES CHEM 1405 No measurement should ever be recorded without proper units.
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Page 2 of 15 the first estimated digit. For example, using a centimeter ruler requires a visual estimation. In Fig 1, the dimension of a floppy disc is being determined using a centimeter ruler. Fig 1. Determination of the Width of a Floppy Disc Reading the centimeter scale, the floppy is known to be between 8 and 9 centimeters in width. Each centimeter is further divided into 10 units each representing 0.1cm (0.1centimeter is equivalent to 1millimeter). Referencing the 0.1cm markings, the width of the floppy is known with certainty to be 8.9cm. In this case, the width falls precisely on the 8.9cm line.
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This note was uploaded on 02/09/2011 for the course CHEM 1405 taught by Professor Kaw during the Spring '11 term at Dallas Colleges.

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