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Units and Measurement

Units, Sci. Not., Sig Figs.
by

Nancy Cope

on 22 October 2013

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Transcript of Units and Measurement

Units and Measurement
In science, we use the international system of measurements - the SI units. Many of these units agree with the metric system.
The metric system is based on the number 10.
Scientific Notation
Dimensional Analysis
expressing a number as a decimal between one and ten, and a factor of 10.
Metric System
a base unit is the standard unit of measure for a particular measurement
Prefixes are added to the basic unit, or root name, to indicate magnitudes of 10 larger or smaller than the basic unit.
Base Units
Property

Unit
second
time
meter
length (distance)
liter
volume
gram
Mass
Temperature
the base unit or SI unit for temperature is Kelvin
There are three scales to measure temperature:
Farhenheit
Celcius
Kelvin
Density
density is a derived property
Density is the amount of mass an object contains per unit volume.
the unit of density is gram per cubic centimeter
154
2
0.008003
8.003x10
-3
6,789,038,000
6.789038x10
9
1.54 x 10
a method of converting units using conversion factors, while writing down all units.
24 eggs
1 dozen eggs
12 eggs
2 dozen eggs
=
x
This is a conversion factor. Here's some ones you should know.
Useful Conversion Factors

Sometimes we can do this in our heads. In Chemistry, you must show all of your work, fully, with units.
Units may be abbreviated.
meters (m)
kilometers (km)
centimeters (cm)
millimeter (mm)
liters (L)
kiloliters (kL)
centiliters (cL)
milliliters (mL)
grams (g)
kilograms (kg)
centigrams (cg)
milligrams (mg)
Accuracy and Precision
accuracy is the closeness of a measurement to an accepted value
precision is the closeness of measurements to one another.
your accuracy can be affected by the instruments you use.
Instrumentation
the accuracy of an instrument is determined by its graduations.
You can always estimate one more digit than the smallest graduation
52.8 mL
Significant figures
the digits which come from a direct measurement and show the degree of accuracy of the measurement
All digits 1-9 are always significant.
Ending zeros with a decimal are significant.
Zeros between numbers are significant.
Beginning zeros are never significant.
Ending zeros without a decimal are never significant.
These zeros do not show precision.
Let's Practice
5,200.

How many significant figures are in the following numbers?
8.35
60038
5.280
0.00528
5,200
Let's Practice
5,200.

How many significant figures are in the following numbers?
8.35
60038
5.280
0.00528
5,200
3
5
4

4
2
3
The zero is significant because it shows precision.
These zeros are estimated because they do not have a decimal point.
The decimal point shows the significance of the zeros.
These zeros are not significant because they can be eliminated in scientific notation, without changing the value of the measurement.
Representing Data
The title of the graph tells what information is on the graph. Dependent Vs. Independent.
The vertical axis is the dependent variable.
the horizontal axis is the independent variable.
Slope Equation
The slope of the line describe how the variables change.
If the two variables change the same way, they are directly proportional.
If one increases while the other decreases, they are inversely proportional.
-1
0.20400
2.0400x10
Percent Error
a measure of the accuracy of measurements
percent error must be a positive number
Adding and subtracting numbers with correct significant figures
the numbers are added or subtracted normally, then rounded to the correct number of significant figures.
answers should have as many digits after the decimal place as the number with the smallest number of digits after the decimal place.
8.52cm + 9.3cm + 1.002cm = 18.822cm
Answer: 18.8cm
Add normally
Identify the number with smallest number of digits after the decimal place
Round the answer to the same decimal place
Multiplying and dividing numbers with correct significant figures
the answer should have the same number of significant figures as the number with the smallest number of significant digits.
then round the answer to the correct number of significant digits.
multiply or divide the numbers correctly
5.0 x 5.0 = 25
5 x 5 = 25
5.0 x 5 = 25
Answer = 25
Answer = 30
Answer = 30
multiply normally
round answer to match smallest number of significant figures
identify the number with the least significant figures
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