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Chapter R

Department

ChemistryCourse Code

CHEM 130Professor

Charles Mc MoryChapter

RThis

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Chem ch R

R-1 unit of measurement

• Metric vs English system

• Causes trouble, parts as simple as bolts are not interchangeable between machines

built according to the two systems

• Us starting to adopt metric system

• 1960 set up International system (SI system)- based on metric system and units derived

rom the metric system

• Volume is not a fundamental SI unit but derived from length.

• Cube 1m on each edge, volume of 1m^3

• Because there are 10 decimeters in a meter the volume is (1dm)^3 = (10dm)^3

=1000dm^3

• Cubic decimeter- (1dm)^3 is commonly called a liter L which is slightly larger than a

quart

• 1000 liters are contained in a cube with a volume of 1 cubic meter. Since 1 decimeter

squeals 10 cm, the liter can be divided into 1000 cubes each with a volume of 1 cubic

cm

• 1cm^3= 1 mL

• 1 liter= 100cm^3= 100 mL

•Mass : a measure of the resistance of an object to a change in its state of motion

• Measured by the force necessary to give an object a certain acceleration

•Weight: the force exerted on on an object by gravity

• Weight is the response of mass to gravity, it varies with the strength of the gravitational

ﬁeld

• Mass is same everywhere

R-2 Uncertainty in Measurement

Physical

Quantity

Name of

Unit

Abbreviatio

n

Mass

kilogram

kg

Length

meter

m

Time

second

s

Temperature

kelvin

K

Electric

current

ampere

A

Amount of

substance

mole

mol

Luminous

intensity

candela

cd

1

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Tuesday, January 8, 2019

• Measurement off volume of a liquid using buret: meniscus of the liquid

• Estimate the last number of the volume reading by interpolating between the 0.mL marks

• Since it is estimated its value may be different if another person makes same

measurements

• The numbers that remain the same between different interpretations of the volume are

called certain digits

• The numbers estimated are called uncertain digits

• Report a measurement by recording all the certain digits plus the ﬁrst uncertain digit

• Measurement always has some degree of uncertainty: characteristic that any measurement

involves estimates and cannot be exactly reproduced

• Uncertainty depends on the precision off the measuring devise

• Important to indicate the uncertainty in any measurement: done by always recording the

certain digits and the ﬁrst uncertain digits—> these are called signiﬁcant ﬁgures

• signiﬁcant ﬁgures indicated something about the uncertainty in a measurement: the

estimated number is usually assumed to be =/- 1 unless otherwise indicated

• 1.86 kg can be taken as 1.86 =/- 0.01 kg

• 25 mL means volume is between 24 mL and 26 mL vs 25.00 mL means volume

between 24.99mL and 25.01 mL

• Precision and Accuracy

•Accuracy: the agreement off a particular value with the true value

•Precision: the degree for agreement among several measurement of the same quantity;

the reproducibility of a measurement

• Errors

•Random error: an error that gas ab equal probability of being high or low: also called

indeterminate error

•Systematic error: an error that always occurs in the same direction: also called

determinate error

• Either always high to always low

• In quantitative work, precision is often used as an indication off accuracy

• We assume the average of a series is accrue or close to the true value- but this

assumption is only valid if systematic errors are absent

• ex: if using a defected balances the results would be constant in the same

direction and therefor the average would be inaccurate

• High precision among several measurements is an indication of accuracy only if systemic

errors are absent

R-3 signiﬁcant ﬁgures and calculations

• Rules for counting signiﬁcant ﬁgures

• Nonzero intriguers: nonzero integers always count as signiﬁcant ﬁgures

• Zeros: three classes of zeros

•Leading zeros: zeros that precede all nonzero digits. Do not count as signiﬁcant

ﬁgures

• 0.0025—> 3 zeros indicate position off the decimal point: only 2 signiﬁcant

ﬁgures

•Captive zeros: zeros between nonzero digits: always count as signiﬁcant ﬁgures

• 1.008 —> 4 signiﬁcant ﬁgures

•Trailing zeros: zeros at the right end o the number: signiﬁcant only if number

contains a decimal point

• 1.00x10^2 —>3 signiﬁcant ﬁgures 100 —> 3 signiﬁcant ﬁgures

2

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