measurement in science scientific observation… empirical knowledge is gained by conducting...
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Measurement in ScienceMeasurement in Science
Scientific Observation…
Empirical knowledge is gained by conducting experiments and making observations. There are 2 types of observations that can be gathered from experiments.
Qualitative Observations: Describe the features of an object or substance using the senses. Ex: Colour, gas bubbles, odour, precipitate.
Quantitative Observation: requires some sort of measuring equipment, usually numerical with a unit. Ex: Temperature, Volume, pH.
Measurement – A Quantitative Observation
• Measured results are required for quantitative observations.• Various factors will affect your confidence in your measured
results. Such as…
– Type of measuring equipment used– Amount to be measured (too large or too small)– Condition of equipment
All these factors must be minimized and controlled in order to increase confidence and decrease “uncertainty” in your measurements
The International System of Units
Length meter m
Mass kilogram kg
Time second s
Amount of substance mole mol
Thermodynamic temperatureKelvin K
Electric current amperes amps
Luminous intensity candela cd
Quantity Name Symbol
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 16
Prefixes in the SI System
Power of 10 for Prefix Symbol Meaning Scientific Notation_______________________________________________________________________
mega- M 1,000,000 106
kilo- k 1,000 103
deci- d 0.1 10-1
centi- c 0.01 10-2
milli- m 0.001 10-3
micro- 0.000001 10-6
nano- n 0.000000001 10-9
The Commonly Used Prefixes in the SI System
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 118
Laboratory Equipmentused for accurate measurements
Burette GraduatedCylinder
PipetteVolumetricFlask
Laboratory Equipmentused for approximate measurements
Beaker ErlenmeyerFlask
Units of Measuring Volume
1 L = 1000 mL
Timberlake, Chemistry 7th Edition, page 3
Reading a Meniscus
The graduated cylinder on the left has scale marks 0.1 mL apart, so it can be read to the nearest 0.01 mL.
Reading across the bottom of the meniscus, a reading of 5.72 mL is reasonable (5.73 mL or 5.71 mL are acceptable, too).
Accuracy vs. PrecisionCertainty of Measurements
•Accuracy refers to the ability of the measurement to match the “true” value. How close are you to the real number?
•Precision refers to the ability of a measurement to be consistently reproduced
Accuracy vs. Precision
Random errors: reduce precision
Good accuracyGood precision
Poor accuracyGood precision
Poor accuracyPoor precision
Systematic errors: reduce accuracy
• Example: At STP, 5mL of pure water should have a mass of exactly 5 grams. The following students weighed a cylinder containing 5mL of pure water three times. Comment on their accuracy and precision:
Student A Student B Student C
5.0g 6.2g 6.4g
5.1g 6.1g 5.9g
4.9g 6.2g 4.2g
Estimating the last digit in measurements:
• The maximum possible precision of a measurement is 1/10 (0.1) times the smallest division on the measuring instrument
• Eg. If your ruler’s smallest division is the tenth’s place, your measurement should be to the hundredths place
• If your ruler’s divisions are to the one’s, you estimate to the tenth’s.
Rules for Rounding
1. If the last digit to be removed is…a. less than 5, the preceding digit stays the same.
For example, 1.33 round to 1.3.b. equal to or greater than 5, the preceding digit is
increased by 1.For example, 1.36 rounds to 1.4, and 3.15 rounds to 3.2.
If you have more than one step in a calculation, do not round until you arrive at the final answer!!!
Significant Digits
• Significant figures are used to show the accuracy of a measurement.
• All measurements consist of a number of digits about which we are certain, and a final digit that has been estimated.
• The expression of this measurement must show this certainty
RULES FOR SIGNIFICANT DIGITS
1. All non-zero digits are significant. (Ex. 367 has 3 sigfigs)
2. All zeros between non-zero digits are significant. (Ex 307 has 3 sfs)
3. Zeros to the right of the last number smaller than one are significant. (Ex. 0.300 has 3 sfs)
4. All zeros to the right of the last whole number are not considered significant unless measured directly by the measuring device. (Ex. 6400 km has 2 sfs; 70. g has 2 sfs; 32.00 has 4 sfs)
5. All zeros to the left of a number less than one, are not significant. (Ex. 0.012 g has 2sfs)
6. Exact numbers (numbers derived from counting) are not considered measurements. When multiplying or dividing an uncertain value by an exact number, the answer has the same place setting as the measured value. (Ex. 3 x 14.7 mL will be expressed to the tenth)
SIGNIFICANT DIGITS IN CALCULATIONS
7. When adding or subtracting, the answer is expressed to the same place setting as the quantity with the highest place setting, which means round off your answer to the least number of decimals in the problem.
8. When multiplying or dividing, the answer should be rounded off to the same number of significant digits as the number having the fewest significant digits.
State the number of significant digits in the following:
a) 0.00123 g
b) 205 000 g
c) 370.0 g
d) 560. g
e) 1.23x104 g