ch 2: scientific measurement renee y. becker chm 1025 valencia community college 1
TRANSCRIPT
Measurements
• Measurement
– A number with an attached unit
Examples: 15 inches 3 cups 36 cm
Every measurement must include units!!
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Example 1: Measurements
In the following, what are the measurements and what are the units?
a) 12 trees
b) 2.3 mm
c) 100 ¢
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Measurements
• Instrument– A device for recording a measurement• Examples: Ruler (length), electronic balance (mass),
Graduated cylinder (volume)
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Uncertainty
• An instrument may give a very sensitive reading, but EVERY measurement has UNCERTAINTY
• No measurement instrument is perfect and neither is the person using it
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Example 2: Length Measurements
• If we need to measure the length of this candycane, which ruler should we use? Why?
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Mass
• Mass– Measure of the amount of matter it possesses– Measured by a balance– Not affected by gravity– Typical units: kilogram (kg), gram (g), pound (lb),
ounce (oz)
• Weight– Force exerted by gravity on an object
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Example 3: Mass
• Would you have the same mass on the moon as on Earth? Why?
• Would you have the same weight on the moon as on Earth? Why?
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Volume
• Volume– The amount of space occupied by a solid, gas, or
liquid
– Graduated cylinder, pipet, buret, volumetric flask, syringe
– Typical units: milliliter (mL), Liter (L), centimeter cubed (cm3), quart (qt), gallon (gal),
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Accuracy, Precision, and Significant Figures in Measurement
• Accuracy is how close to the true value a given measurement is.
• Precision is how well a number of independent measurements agree with one another.
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Accuracy, Precision, and Significant Figures in Measurement
• Significant Figures are the total number of digits in the measurement.
• The results of calculations are only as reliable as the least precise measurement!!
• Rules exist to govern the use of significant figures after the measurements have been made.
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Accuracy, Precision, and Significant Figures in Measurement
• Rules for Significant Figures:
– Zeros in the middle of a number are significant
– Zeros at the beginning of a number are not
significant
– Zeros at the end of a number and following a
period are significant
– Zeros at the end of a number and before a period
may or may not be significant.
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Example 4: Significant Figures
How many Sig. Figs ?
a) 0.000459
b) 12.36
c) 36,450
d) 8.005
e) 28.050
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Accuracy, Precision, and Significant Figures in Measurement
• Rules for Calculating Numbers:
– During multiplication or division, the answer
can’t have more sig figs than any of the original
numbers.
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Example 5: Significant Figures
a) 238.5 x 79 =
b) 12 / 0.1272 =
c) 0.2895 x 0.29 =
d) 32.567 / 22.98 =
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-During addition or subtraction, the answer can’t have more digits to the right of the decimal point than any of the original numbers.
Accuracy, Precision, and Significant Figures in Measurement
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Example 6: Significant Figures
a) 238.5 + 79 =
b) 12.3 - 0.1272 =
c) 0.2895 + 0.29 =
d) 32.567 - 22.98 =
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Accuracy, Precision, and Significant Figures in Measurement
• Rules for Rounding Numbers:
– If the first digit removed is less than 5 - round down
– If the first digit removed is 5 or greater- round up
– Only final answers are rounded off, do not round intermediate calculations
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Example 7: Rounding
Round off each of the following measurements
(a)3.774499 L to four significant figures
(b) 255.0974 K to three significant figures
(c) 55.265 kg to four significant figures
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Scientific Notation
• Changing numbers into scientific notation– Large # to small #– Moving decimal place to left, positive exponent
123,987 = 1.23987 x 105
– Small # to large #– Moving decimal place to right, negative
exponent0.000239 = 2.39 x 10-4
• Correct scientific notation: #.#### x 10n
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Example 9: Scientific Notation
Put into or take out of scientific notation
a) 87542
b) 2.1956 x 10-3
c) 0.784
d) 2.78 x 106
e) 92000
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