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Units of Measurement And their Uses DR. C.’S PRE-AP CHEMISTRY FALL 2015

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Lesson Objectives: Distinguish between a quantity, a unit, and a measurement standard Name the SI units for length, mass, time, volume and density Transform a statement of equality into a conversion factor Distinguish between precision and accuracy Determine the number of significant figures in a number Perform mathematical computation using significant figures

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Accuracy vs. Precision Accuracy refers to the closeness of a measurement to the accepted value. Precision refers to the closeness of a set of measurements to each other.

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Percent Error

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Significant Figures

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Significant Figures Rules All non-zero numbers are significant. Leading zeros are not significant Zeros between two non-zero numbers are signficant Trailing zeros are where things get confusing… Trailing zeros are not significant unless: 1.They are after a decimal 2.There is a decimal at the end of the number

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How many significant figures? 28.6 3440 3440. 0.0004604 500000000000000000 this one makes my son upset! =)

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Significant Figures

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“Doing Math” with sig figs Always do the calculation and then round to the correct number of sig figs. Addition or Subtraction: fewest number of decimal places 18.4 + 3.45 = ? Multiplication or Division: fewest total # of sig figs 2.4 x 15.82

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Quantity Measurements represent quantities. A quantity is something that has magnitude, size or amount. Measurement ≠ quantity The teaspoon is a unit of measurement Volume is a quantity The choice of unit depends on the quantity being measured

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7 Fundamental Base Units

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Mass vs. Weight Reminder!! (You should already have this in your notes) Mass is the measure of the amount of matter present. Weight is a measure of the gravitational pull on matter. The SI base unit for mass is the kilogram

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Length Length is a measure of distance. The SI unit for length is the meter. The kilometer is used for longer distances, while centimeter is often used to express shorter distances.

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Derived SI Units

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Volume Volume is the amount of 3D space an object occupies. The derived SI unit is m 3 The cubic centimeter, cm 3, is often used Most often in chemistry you will see the liter (L) or milliliter (mL) even though it is a non-SI unit. 1 mL = 1 cm 3 1 L = 1000 cm 3

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Density

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Density is a characteristic physical property We can use density to help us identify a substance.

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Conversion Factors

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Dimensional Analysis Dimensional Analysis is a mathematical technique that allows you to use units to solve problems involving measurement. Steps to solving dimensional analysis problems: 1.Write the problem in question mark form. 2.Set up a “Picket Fence” 3.Start with the known (given) quantity 4.Cancel the unwanted unit by sliding it diagonally to the next part of the fence 5.Use a conversion factor to convert from one unit to another 6.Continue to cancel unwanted units until you end with the desired unit. 7.Multiply and divide

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Example (Done on white board) How many seconds are there in one year?

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Another Example Would you be breaking the speed limit in a 40 mi/h zone if you were traveling at 1 km/min? 1 km = 0.62 miles

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Lesson Objectives: Distinguish between a quantity, a unit, and a measurement standard Name the SI units for length, mass, time, volume and density Transform a statement of equality into a conversion factor Distinguish between precision and accuracy Determine the number of significant figures in a number Perform mathematical computation using significant figures

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