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Chapter R

CHEM 130 Chapter R: Reading notes

6 pages96 viewsWinter 2019

Department
Chemistry
Course Code
CHEM 130
Professor
Charles Mc Mory
Chapter
R

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Tuesday, January 8, 2019
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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