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 *10
10 m
• Example: 8.1 *10
-3 m = 0.0081m
• Example: diameter
of a hair: 0.000008m = 8.0*10 -6 m
• Multiplication • 3.0 *10
6 x 2.0*10 *2.0) x 10 (6+3) 3 = (3.0 = 6.0*10 9
• 2.0 *10
*10 (-3+5) -3 x 4.0 *10 = 8.0*10 2 5 = 8.0
• (Add exponents) • Division • 3.0*10
4 /2.0*10 2 3.0/2.0 x10 (4-2) *10 2 = = 1.5
• 6.0*10
-2 /2.0*10 3.0*10 (-2-4) 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 value-
correct value based on reliable references, what types of references, your neighbor?
• Experimental value-
value measured in the lab
Accuracy and Precision
Percent Error
• Difference between
accepted and experimental values is called error
• Error= accepted
value-experimental value
• % Error=
[error]/accepted value * 100%
• Density of water=
1.0 g/mL (accepted)
• 0.98 g/mL
(experimental)
• Percent Error =
[1.0 g/mL 0.98g/mL]/1.0 g/mL * 100% = 2%
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 28.5
o C
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
• 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.
• 3x10
2 m - not significant
• 3.00x10
2 m – 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
• 10
-3
• 10
-6
• 10
-9
• 10
-12
• 10
-15
• 10
-18 centi c milli m micro µ nano n pico p femto f atto a
SI Units
• Factor Name • 10
6 Symbol mega M
• 10
3
• 10
2
• 10
1 kilo k hecto h deka 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?