units of measurement and their uses dr. c.’s pre-ap chemistry fall 2015
TRANSCRIPT
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Units of MeasurementAnd their UsesDR. C.’S PRE-AP CHEMISTRYFALL 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• Percent error is a mathematical way to determine how
accurate a measurement is.
Percentage error = x 100
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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
•Combinations of SI base units form derived units.• A Joule (Unit for energy) is
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Volume• Volume is the amount of 3D space an object occupies.
• The derived SI unit is m3
• The cubic centimeter, cm3, 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 cm3
• 1 L = 1000 cm3
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Density
•Density is the ratio of mass to volume.
density =
• The SI unit for density is kilogram per cubic meter, kg/m3
• In chemistry most often we will use grams per milliliter, g/mL
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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
•A conversion factor is a ratio derived from the equality between two different units that can be used to convert from one unit to another.
Example: There are four quarters in a dollar.
is the same mathematically as
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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