CHE_106_Lecture3_2009

CHE_106_Lecture3_2009 - Chemistry 106 Lecture 3 Topic...

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Chemistry 106 Lecture 3 Topic: Measurement Chapter 1.4 –1.6
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Measurement and Significant Figures Measurement is the comparison of a physical quantity to be measured with a unit of measurement --that is, with a fixed standard of measurement. Length (meters, kilometers, etc.) Mass (milligrams, kilograms, etc.) Time (seconds, hours, etc.)
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Precision and Accuracy The term precision refers to the closeness of the set of values obtained from identical measurements of a quantity. Precision: Accuracy is a related term; it refers to the closeness of a single measurement to its true value. Accuracy:
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Precision and Accuracy
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Precision of Measurement With a Centimeter Ruler What is the length of this steel rod? From the ruler markings, we know it is between 9.1 and 9.2 centimeters (cm). Reasonable guesses are 9.11, 9.12, 9,13
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A student measured the concentration of a solution three times, obtaining values of 0.010 M , 0.060 M , and 0.030 M . The average concentration was thus 0.033 M . The accepted “true” value was 0.034 M . The student’s data has: Precision and Accuracy Example a. good accuracy and good precision. b. poor accuracy but good precision. c. poor accuracy and poor precision. d. good accuracy but poor precision.
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A student measured the concentration of a solution three times, obtaining values of 0.010 M , 0.060 M , and 0.030 M . The average concentration was thus 0.033 M . The accepted “true” value was 0.034 M . The student’s data has: Precision and Accuracy Example a. good accuracy and good precision. b. poor accuracy but good precision. c. poor accuracy and poor precision. d. good accuracy but poor precision.
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Measurement and Significant Figures To indicate the precision of a measured number (or result of calculations on measured numbers), we often use the concept of significant figures . Significant figures are those digits in a measured number (or result of the calculation with a measured number) that include all certain digits plus a final one having some uncertainty .
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Precision and Uncertainty of Measurement With a Centimeter Ruler What is the length of this steel rod? From the ruler markings, we know “9” and “0.1” in the measurement exactly (certain). The guesses of 9.11 , 9.12 , 9,13 have uncertainty in the last digit.
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An exact number is a number that arises when you count items or when you define a unit. For example, when you say you have nine coins in a bottle, you mean exactly nine. Measurement and Significant Figures When you say there are twelve inches in a foot, you mean exactly twelve.
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