Post on 11-Jul-2020
A.0 SF’s-Uncertainty-Accuracy-Precision
Objectives:• Convert standard notation to scientific notation
• Convert scientific notation to standard notation
• Perform calculations with numbers in Sci-Not
• Determine the #SF’s in a measurement
• Round a calculated answer to the correct #SF’s
• Round a calculated answer to the correct decimal place
• Calculate % error
• Honors-Calculate relative precision
• Describe the difference between precision and accuracy
• Know how accuracy is quantified
• Know how precision is quantified
Scientific Notation
Scientific notation is a format for writing very large and very small numbers.
Numbers written in scientific notation have two parts:• The decimal part
• The exponential part
Numbers not in scientific notation are in standard notation. Examples:
43, 000,000,000
0.00567
6.78 x 10 -8
Scientific NotationA number is expressed in scientific notation when it is in the form:
a x 10 n
where a is between 1 and 10 and n is an integer. Both a and n can
be negative or positive
Sci-Not: 6.3 x 10 6
7.930 x 10 -4
- 5 x 10 24
Not in Sci-Not: - 63 x 10 9
0.62 x 10 - 8
When changing scientific notation to
standard notation, the exponent tells
how to move the decimal:
With a positive exponent (big number), move the decimal to
the right (you’re multiplying by 1000):
4.08 x 103 = 4080
When changing scientific notation to
standard notation, the exponent tells
how to move the decimal:
With a negative exponent (small number), move the decimal to
the left (you’re multiplying by 0.0001):
8.93 x 10-3 = 0.00893
When changing standard notation to scientific notation follow these steps:Dry erase board example:68,940,0000.000569
1) Move the decimal point in the number to obtain a number between 1 and 10 (the decimal part).
2) Write the decimal part multiplied by 10 raised to the number of places you moved the decimal.
3) The exponent is positive if you moved the decimal point to the left (it is a big number) and negative if you moved the decimal point to the right (it is a small number)
Standard notation to Sci-Not
835000 = ?
8.35 x 105 Positive exponent means big number
move decimal to the right.
0.000893 = ?
8.93 x 10-4 Negative exponent means small number
move decimal to the left.
Scientific Notation Practice
Change to Sci-Not:
1) 68700
2) 0.0043
Change to Standard Notation:
3) 5.78 x 10-5
4) 1.6 x 10 6
Answers
1)6.87 x 104
2)4.3 x 10-3
3)0.0000578
4)1 600 000
Uncertainty and Significant FiguresSF’s-Why Are They Important?
•All measuring devices have limitations and therefore measurements always involve some uncertainty.
•Significant figures are used to report all precisely known numbers + one estimated digit
•The # of SF’s indicates the precision of the instrument.
Measurement
The number of significant figures in a measurement.
Uncertainty•What is the diameter of a quarter ?
•The answer depends on the precision of the ruler used.
Uncertainty•The top ruler is more precise (and more expensive).
•In the measurement 2.33 cm, the last digit is estimated as a “3” by using the calibration marks. The other two digits are certain.
•In the measurement 2.3 cm, the last digit is estimated as a “3” by using the calibration marks. The other digit is certain.
Measurement and Uncertainty•When measuring an object, the last digit should always be estimated (between two calibration marks).
•Consider the line below. What is the length in cm ?
•When the measurement falls on a calibration mark, the last digit is always zero.
•Length = 1.70 cm
Measurement and Uncertainty
•Consider the line below. What is the length in cm ?
Length = 4.00 cm
Measurement and Uncertainty
What is the difference in the following lengths ?They represent the length of the same line.
4.23 cm4.2 cm4 cm
Measurement and Uncertainty
•What is the difference in the following lengths ?
•They represent the length of the same line.
•4.23 cm--scientist used a fairly precise ruler
•4.2 cm--scientist used a less precise ruler-he must have run out of $$
•4 cm--they cheaped-out ! This ruler would have no calibration marks !
0 cm 10 cm
Why do we care about Significant Figures (SF’s) ?
There are 2 kinds of numbers:
Exact: the amount of money in your account,
Metric unit equalities. These are known with
certainty.
Approximate: weight, height—anything
MEASURED, has a limit to the certainty,
depending on the instrument used.
Why do we care about Significant Figures (SF’s) ?
3.50 inch to a scientist means the
measurement is precise to within one
hundredth of an inch.
Every measurement is a reflection of
the precision of the measuring
instrument.
Why do we care about Significant Figures (SF’s) ?
To a mathematician 3.5 inches, or
3.50 inches is the same.
But, to a scientist 3.5 inches and
3.50 inches is NOT the same
Pacific-Atlantic Rule
P A
If a decimal point is present, start on the Pacific (P) side
and begin counting at the first non-zero digit all the way to
the end.
If a decimal is absent, start on the Atlantic (A) side and
begin counting at the first non-zero digit all the way to the
end.
P A
Decimal
Is present Is absent
0.0050074
5 sig figs
If a
6003895400
8 sig figs
then then
Pacific Rule Examples:
123.003 = decimal present, start on “P” side,
begin counting # SF’s = 6Examples (Do with a study buddy):
0.00024 How many SF’s ?
0.453 How many SF’s ?
40.9 How many SF’s ?
43.00 How many SF’s ?
1.010 How many SF’s ?
1.50 How many SF’s ?
2
3
3
4
4
3
Atlantic Rule
Examples:
204,000 = decimal is absent, start on “A” side, begin counting at first non-zero digit until you hit the end of the number. # SF’s = 3
Examples (CO):
7003 How many SF’s ?
300 How many SF’s ?
27,300 How many SF’s ?
56 How many SF’s?
4
1
3
2
Drill and Practice
Underline the significant digits in each of the following numbers.
0.00506970
500.6790
250005
4970350000
5678493
6 sig figs
7 sig figs
6 sig figs
6 sig figs
7 sig figs
Scientific Notation
All digits in the decimal part of in a
number written in scientific notation
are significant.
Examples:
5.6 x 10 4 Has 2 SF’s
8.90 x 10 3 Has 3 SF’s
Use the Pacific-Atlantic Rules to determine the # of SF’s
a) 18.3 b) 1.83 x 102 c)0.00183
d) 183.0 e) 50 f) 505
g) 0.0050
h) 5000 i) 200.00 j) 0.00220
k) 22020 l) 22.20
How many Sig-Figs ?
a) 3 b) 3 c) 3
d) 4 e) 1 f) 3
g) 2
h) 1 i) 5 j) 3
k) 4 l) 4
Significant Numbers in Calculations
A calculated answer cannot be more precise than the measuring tool.
A calculated answer must match the least precise measurement.
There are two different rules for rounding and reporting a calculated answer
--Multiplying or dividing
--Adding or subtracting
Sig. Fig. Math Rules
• Multiplication / Division:
Your answer can’t have more sig. figs. than the number in the problem with the least amt. of sig. figs.
Example = 60.56227892 m x 35.25 m
Calculator says – 2134.890832 m2 (too many SF’s !)
Don’t forget units!
How many SF’s should be in final answer?
4 SF’s
Answer - 2135 m2
Multiplication & Division
The final answer has the same number of sig-figs as the number with the least number of sig-figs.
123m x 5.35m = 658.05m2 = 658m2 (3 SF’s)
16cm2 x 2cm = 32cm3 = 30cm3 (1 SF)
16 x 2.0g = 32g = 32g (2 SF’s)
Multiplication & Division
The final answer has the same number of sig-figs as the number with the least number of sig-figs.
823dm3 ÷ 4.0dm = 205.75 dm2 = 210 dm2
(2 SF’s)
84.7 g ÷ 20 mL = 4.235 g/mL = 4 g/mL (1 SF)
Sometimes the answer must be written in scientific notation to express the correct number of SF’s
25 m x 4.0 m = 100 m2
The answer should have 2 SF’s. If left as 100 m2 , it would only have 1 SF.
So change it to Sci-Not: 1.0 x 10 2 m2 now it has 2 SF’s
Be Careful with “Domino Rounding”
A. Round the following to 4 SF’s:
37.4959
B. Round the following to 3 SF’s:
68.4499
Answer: 37.50
Answer: 68.4
Express Result in Correct # of SF’sPay attention to units!
5.35 x 20.2 x 5.0 = _______
2.00 x 500 = _______
2.50 cm x 55.5 cm = ________
5.0 m x 4.000 x 102 m = _________
Multiplication Examples
5.35 x 20.2 x 5.0 = 540.35 = 540
2.00 x 500 = 1000
2.50 cm x 55.5 cm = 138.75cm 2
= 139cm 2
5.0 m x 4.000 x 102m = 2000m 2 = 2.0 x 10 3 m 2
( 2 SF’s)
Express Result in Correct # of SF’s
0.030 ÷ 2.00 = __________
9101 ÷ 8.8 = __________
(3.94 x 10 7 m2 ) ÷ (8.4 x 10 -25 m) = _____________
Express Result in Correct # of SF’s0.030 ÷ 2.00 = 0.015
9101 ÷ 8.8 = 1034 = 1.0 x 10 3 ** must be is Sci-Not !
(3.94 x 10 7 m2 ) ÷ (8.4 x 10 -25 m) =
4.7 x 10 31 m
Addition and SubtractionWhen adding or subtracting, look at the decimal places. Find the measurement that is least precise (# places past the decimal). Round to the least precise place.
Ex. a) 13.64 + 0.075 + 67 b) 267.8 – 9.36
13.64
0.075
67.
80.71581
267.8
9.36
258.44258.4
–
+
+
Round to the
ones place.
Round to the
tenths place.
Examples
4.021 =
4.0283.1425 =
83.14
i)
83.25
0.1075–
4.02
0.001+
ii)
1.8183 =
1.82
0.2983
1.52+
iii)
Round to the
Hundredths place.
Round to the
Hundredths place.
Round to the
Hundredths place.
Examples
42, 000 53.9
+ 698 + 60
42, 698 = 113.9 =
43, 000 110
Round to the
Thousands place.
Round to the
Tens place.
Learning Check-Study Buddy
In each calculation, round the answer to the correct number of significant figures.
A. 235.05 + 19.6 + 2.1 =
1) 256.75 2) 256.8 3) 257
B. 58.925 - 18.2 =
1) 40.725 2) 40.73 3) 40.7
Solution
A. 235.05 + 19.6 + 2.1 =
2) 256.8
B. 58.925 - 18.2 =
3) 40.7
Sig. Figs.-Mixed Operations
• Significant Figures in Mixed operations (Use PEMDAS)
(1.7 x 106 ÷ 2.63 x 105) + 7.33 = ???
Step 1: Divide the numbers in
the parenthesis. How many sig
figs?
Step 2: Add the numbers. How
many decimal places to keep?
(6.463878327…) + 7.33
6.463878327…
+ 7.33
13.7938
Step 3: Round answer to the
appropriate decimal place. 13.8 or 1.38 x 101
Precision and Accuracy
Precision
--Refers to how close the measurements in a series are to
each other.
--Can indicate how well your tools are working, not what
the tools are measuring.
--Can indicatethe technique of the scientist-poor
technique leads to poor precision and vice versa.
Accuracy
--Refers to how close each measurement is to the
accepted, true, or literature value.
--Checks how “right” or “correct” your answer is.
--Only need one measurement and the “true” value.
Precision, Accuracy, and Error
Precision refers to how close the measurements in a series
are to each other.
Accuracy refers to how close each measurement is to the
actual value.
Systematic error produces values that are either all higher
or all lower than the actual value.
This error is part of the experimental system.
Random error produces values that are both higher and
lower than the actual value.
Figure 1.9
precise and accurate
precise but not accurate
Precision and accuracy in a laboratory calibration.
systematic error
random error
Precision and accuracy in the laboratory.Figure 1.9continued
Quantifying Accuracy
Percent error, sometimes referred to as percentage error, is an
expression of the difference between a measured value and the
known or accepted value. It is often used in science to report the
difference between experimental values and accepted (true)
values. The smaller the value, the better the accuracy. Units are
%, usually rounded to the hundredths place.
The formula for calculating percent error is:
Absolute value bars
Calculating Percent Error
The literature value for the atomic mass of an isotope of nickel is 57.9
g/mol. If a laboratory experimenter determined the mass to be 56.5
g/mol, what is the percent error (rounded to the hundredths place)?
See board.
Answer: 2.42 %
Quantifying Precision
Take multiple measurements of the same thing under
the same conditions. Describe the “spread” of the
data relative to the average. The smaller the value,
the better the relative precision. Units are %. Usually
rounded to the hundredths place.
Calculate the Relative Precision:
relative precision = |(largest value – smallest value)| 100%
average value
Honors
Only
Quantifying Precision
relative precision = |(largest value – smallest value)| 100%
average value
Honors
Only
Calculate the relative precision for the data given
below. Round your answer to the hundredths place
and include the appropriate units. (Answer: 1.78 %)
Trial Volume (mL)
1 28.22 27.93 28.44 28.1
Average = 28.15
Relative Precision =
(28.4 - 27.9) / 28.15 * 100 =
1.77620 = 1.78 %
Do #1 on p. 20 now (Do #2 for homework)
Why Perform Multiple Trials?
--Can show reliability and reproducibility, this can add
validity to the conclusions you draw
--Multiple trials can give you information about only
precision.
Poor technique can contribute to poor precision.
Performing many trials will NOT ensure good accuracy.
In fact, nothing can be determined about accuracy with
just data from trials. You need a true (literature) value in
order to determine accuracy,