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.)
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
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.
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 .
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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