2 standards for measurement

8/6/2019 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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When dropping digits from a number, thelast digit retained is determined by theprocess known as rounding off numbers.


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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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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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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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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.

2 standards for measurement

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