2 standards for measurement
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Standards forMeasurement
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Chemistry:The Experimental Science
Doing an experiment in
chemistry is like doingour daily lives.
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Measurement
The most commonly measured quantitiesin elementary experiments are mass,
length, volume, pressure, temperatureand time.
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Mass Vs. Weight
Often used interchangeably
Have quite different meanings
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Mass Vs. Weight
Mass of a body the amount of matterin that body
It is fixed and unvarying quantity that isindependent of the objects location.
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Mass Vs. Weight
Weight of an object the measure ofthe Earths gravitational attraction for that
object. Measured using a scale, which measures
force against a spring.
It varies in relation to the position of anobject on Earth or its distance from Earth.
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Measurement andSignificant Figures
Numbers obtained from a measurementare never exact values
They always have some degree ofuncertainty due
to limitations of the measuring instrument
the skill of the individual making themeasurement
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Measurement andSignificant Figures
Measurement must have some sort ofreliability (precision).
must contain all digits that are known plusone digit that is estimated
the last digit introduces uncertainty thus,measurement must have limited number of
digits.
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Significant Figures
Digits used to express a measuredquantity with certainty.
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Evaluation: SignificantFigures
All non-zero digits are significant.
Leading zeros are never significant.
Confined or captive zeros are alwayssignificant.
Trailing zeros are significantif itis after a
non-zero digit and a decimal point.
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Practice Exercise
4.5 inches
25.0 grams
3.025 feet 12.20 liters
125.0 meters
100,000 people
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How many significant figures?
1. 48,923
2. 3.967
3. 900.06
4. 0.0004
5. 8.1000
6. 501.040
7. 3,000,000
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Rounding Off Numbers
In calculations, it is necessary to dropexcess digits to express answer with theproper number of significantfigures.
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Rounding Off Numbers
When dropping digits from a number, thelast digit retained is determined by theprocess known as rounding off numbers.
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Rounding Off Numbers
If the next digit after the last significantfigure is 5 or greater, round up.
Increase the last significantfigure by 1.
Example: 2.136 becomes 2.14
rounded to 3 SF
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Rounding Off Numbers
If the next digit after the last significantfigure is less than 5, round down.
Do not change the last significantfigure.
Example: 2.132 becomes 2.13
rounded to 3 SF
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Scientific Notation ofNumbers
Very large and very small numbers areoften used in chemistry and can besimplified and conveniently written using apower of10.
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Scientific Notation ofNumbers
To express a number greater than 1 inscientific notation, we count the numberof places the decimal point has to bemoved to the left to put just after the firstdigit of the number.
The number of movement of decimal point
equals the positive exponent.
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Scientific Notation ofNumbers
To express a number smaller than 1 inscientific notation, we count the numberof places the decimal point should beplaced after the first non-zero digit.
The number of movement of decimal pointto the right equals the negative exponent.
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Significant Figures inCalculation
For addition and subtraction:
Round off the sum or difference based on
the least number of digits after thedecimal point.
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Addition:
5.67 J (twodecimal places)
1.1 J (onedecimal place)
0.9378 J (fourdecimal place)7.7 J (onedecimal place)
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NOTE:
Whendoingmulti-step calculations, keep
at least one more significant digit in
intermediate results thanneededin your
finalanswer.
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The Two Greatest Sins Regarding
Significant Digits
Writingmoredigitsinananswer
(intermediateor final)than justified by the
numberofdigitsinthedata.
Rounding-off, say, totwodigitsinan
intermediateanswer, andthenwriting
threedigitsinthe finalanswer.
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Significant Figures inCalculation
For multiplication and division:
Round off the product or quotient based
on the number that contains the leastnumber of SF.
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The Metric System
International System (SIfrom SystemInternational)
Decimal system o
funits
for measurementsof mass, length, time and other physical
quantities
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The Metric System
Uses factors of10 to express larger orsmaller numbers
Pre
fixes are added to name the units(represents multiples of10)
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Calculation
Measurement ofLength
Measurementfor Mass
Measurementfor Volume Measurement ofTemperature
Density and Specific Gravity
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Calculation
In all the calculations, dimensionalanalysis or factor label method is used.
Provides a systematic, straightforward way to set
up problems. Gives a clear understanding of the principles
involved.
Trains to organize and evaluate data.
Helps to identify errors, since unwanted units arenot eliminated if the setup of the problem isincorrect.
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