Transcript Scientific Measurement - Central Valley School District

Scientific Measurement
• What is density?
• From your
experimental data,
were the densities of
the similar objects the
same or different?
Why?
• What does this tell
you about density?
• Can you look up the
density of a particular
substance?
• Does the size of the
substance play a role
in changing its
density?
• For the irregular
shaped objects, did
you get similar
densities for each?
Why or why not? If
you didn’t, can you
give a reason as to
why? (accuracy and
measuring tools)
Scientific Measurement
• Qualitative and Quantitative
• What is the difference between qualitative
and quantitative measurements?
• Qualitative- results that are descriptive and
nonnumeric
• Quantitative- results are given in a definite
form, usually as numbers and units
• Most of the things that we will be doing in
chemistry will be quantitative, but there will
qualitative elements as well
Scientific Notation (Review)
• What is scientific
notation?
• Example:
11000000000m
• s.n. : 1.1 *1010m
• Example: 8.1 *10-3m
= 0.0081m
• Example: diameter
of a hair: 0.000008m
= 8.0*10-6m
• Multiplication
• 3.0 *106 x 2.0*103 = (3.0
*2.0) x 10(6+3) = 6.0*109
• 2.0 *10-3 x 4.0 *105 = 8.0
*10(-3+5) = 8.0*102
• (Add exponents)
• Division
• 3.0*104/2.0*102 =
3.0/2.0 x10(4-2) = 1.5
*102
• 6.0*10-2/2.0*104 =
3.0*10(-2-4) = 3.0*10-6
• (Subtract denominator
from the numerator)
Accuracy and Precision, Percent Error
• Accuracy- measure
of how close a
measurement
comes to the actual
or true value of
whatever is being
measured
• Precision- measure
of how close a
series of
measurements is to
one another
• Percent error
compares the
experimental value
to the correct value
• Accepted valuecorrect value based
on reliable
references, what
types of references,
your neighbor?
• Experimental valuevalue measured in
the lab
Accuracy and Precision
Percent Error
• Difference between • Density of water=
accepted and
1.0 g/mL
experimental values
(accepted)
is called error
• 0.98 g/mL
• Error= accepted
(experimental)
value-experimental • Percent Error =
value
[1.0 g/mL• % Error=
0.98g/mL]/1.0 g/mL
[error]/accepted
* 100% = 2%
value * 100%
Significant Figures in Measurements
• The calibration of your measuring tool
determines how many sig. Figs. you
can have.
Significant Figures in Measurements
•
•
•
•
•
Example #1:
This ruler measures to the .1 (in this case
centimeters)
However, I can see that the measurement lies
between the 2.8 and 2.9 measurement, so I
can make the estimate that it is
approximately 2.83 cm.
You see!!! All of those numbers are significant,
because they all tell me about the
measurement!
If I went out any further, it would not be
accurate, because my measuring device is
not that accurate!
Significant Figures in Measurements
•
•
•
•
•
This works for other measuring
devices as well. Just
remember to always go one
digit further than the device
does
Example #1:
What temp does the
thermometer on the left
indicate?
The thermometer has whole
number digits , so for sig figs I
can go to the tenths.
The temp is 28.5oC
Significant Figures in Measurements
•
•
This also works for Graduated Cylinders
Example #1
•
The drawing above indicates you are
looking at a graduated cylinder from the
side (note the dip or meniscus, which you
always read from the bottom)
This graduated cylinder measures to the
whole number so we will read it to the
tenth
This graduated cylinder has a reading of
30.0 ml
•
•
Rules for Significant Figures
• Every nonzero digit reported in
measurement is assumed to be significant
• How many sig. Figs.?
•
-24.7m
•
-0.743m
•
-714m
• three
• Zeros appearing between nonzero digits
are significant
• How many sig. Figs.?
•
-7003m
•
-40.79m
•
-1.503m
•
Four
Rules for Significant Figures
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•
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Leftmost zeros appearing in front of nonzero
digits are not significant (Act as
placeholders)
0.0071m
0.42m
0.000099m
two
Zeros at the end of a number and to the right
of a decimal point are always significant.
43.00m
1.010m
9.000m
Four
Rules for Significant Figures
•
•
•
•
Zeros at the rightmost end of a
measurement that lie to the left of an
understood decimal point are not
significant if they serve as placeholders to
show the magnitude of the number.
•
•
•
300m (1)
7000m (1)
27210m (4)
If 300 was found from careful measurement
and not a rough guess, then the zeros
would be significant. To avoid this, write in
scientific notation.
3x102m - not significant
3.00x102m – significant
Significant Figures in Calculations
• Calculated values cannot be more
precise than the measured values
used to obtain it.
• Addition and Subtraction
• round to the same number after the
decimal place as the measurement
with the least number after the
decimal place.
• 12.54m + 349.0m + 8.24m = 369.76m =
369.8m
• 74.626m – 28.34m = 46.286m = 46.29m
Significant Figures in Calculations
• Multiplication and Division
• round answer to the same # of
significant figures as the measurement
with the least # of significant figures.
• 7.55m * 0.34m = 2.6 (2)
• 0.365m * 0.0200m = 0.00730 (3)
• 2.4526m / 8.4m = 0.29 (2)
SI Units
• Factor Name Symbol
• 10-1
deci
d
• 10-2
centi
c
• 10-3
milli
m
• 10-6
micro
µ
• 10-9
nano
n
• 10-12 pico
p
• 10-15 femto f
• 10-18 atto
a
SI Units
• Factor
• 106
• 103
• 102
• 101
Name
mega
kilo
hecto
deka
Symbol
M
k
h
da
Glassware
• Which are used to measure
approximate volumes?
• Which are used to measure more
precise volumes?
• Which one would you use to
measure a large volume, such as
100 mL, accurately?