# lab2 EJM - Name _Erica J. Murray_ By Dr. Adams (05-03) Lab...

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Name ___ Erica J. Murray _______ By Dr. Adams (05-03) Lab 2-Precision, Accuracy and Significant Figures (10.0 points) While performing research chemists often perform measurements. The act of assigning mathematical values to measurements is called quantification . While assigning numbers scientists need to be realistic about how much confidence they have in the measurements they have made. Two relevant terms to be studied in this laboratory exercise are accuracy and precision . In chemistry, accuracy refers to how close a measurement is to its real value. For example, someone hands you a piece of lumber and asks you to guess the length of the piece of lumber. You guess 5 feet 6 inches. You then measure the lumber with a tape measure and determine the real length was 6 feet 1 inch. Your answer was a good estimation, but it was not accurate. An accurate answer would have been 6 feet 1 inch. Precision refers to how close repeated measurements of the same quantity are to each other. In our laboratory we had a spectrophotometer that was very precise. Spectrophotometers are instruments used to measure the amount of light that is absorbed by and/or transmitted through liquid solutions. This machine was used in our laboratory to measure the amount of light absorbed by a solution. When the same sample was measured five different times, absorption values of .410, .411, .411, .412 and , 410 were obtained. These repeated measurements of the same quantity were very close to each other. Since these repeated measurements of the same value were very close to each other, they were stated to have a “high amount of precision”. It was later determined that the spectrophotometer was broken. The accurate absorption for the sample should have been .700. Although each of the measured values of that sample were very close to each other, they were all very far from the real value. Thus they were precise, but not accurate. In the game of darts, it is advantageous to hit the center of the target. If you threw 5 darts and each hit the center of the target, the throws were accurate and precise. If all 5 darts hit in the same place on the dartboard but far away from the center, then the results were precise but not accurate. If there was no pattern to the throws and they did not hit near the center of the board, then they were neither accurate nor precise. List two examples from your daily life where it is important to be close to a measured value or a standard value. Use complete sentences in your answers. a) Cooking is very important to get your measurements precise. b) Injections from the doctors are very serious could be fatal if not accurately measured. When scientists measure the mass of an object, there is a limit to the confidence that they have in a measured value. Hospital scales used to measure the mass of a patient measure to the

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nearest 0.1 lb. A weight of 187.1 lbs is interpreted as the weight between 187.0 lbs and 187.2 lbs. Scales used to measure trucks loaded with crushed stone often measure to the nearest 10
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## This note was uploaded on 05/06/2010 for the course CHEM 101 taught by Professor Griffith during the Spring '08 term at Illinois Valley CC.

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lab2 EJM - Name _Erica J. Murray_ By Dr. Adams (05-03) Lab...

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