Transcript Slide 1

Topic 11: Measurement and Data
Processing
IB Core Objective
11.1.2 Distinguish between
precision and accuracy.
Distinguish: Give the differences
between two or more different items.
(Obj. 2)
11.1.2 Distinguish between precision and
accuracy.
Accuracy: How close you are to the true
value
Precision: How reproducible your
measurements are.
11.1.2 Distinguish between precision and
accuracy.
Random
Systematic
(Not precise not accurate)
(precise but not accurate)
Good reading
(precise and accurate)
IB Core Objective
11.1.1 Describe and give examples of
random uncertainties and systematic
errors.
Describe: Give a detailed account. (Obj. 2)
11.1.1 Describe and give examples of random
uncertainties and systematic errors.
Types of error:
 Random error: Is caused by measurement

estimation when reading equipment. If the
measurements are inconsistent then the lab
technique is poor.
Systematic error: Is caused by
instrumentation error. Technique is good but
equipment is faulty or un-calibrated. This will
result in consistent but wrong readings.
11.1.1 Describe and give examples of random
uncertainties and systematic errors.
A random uncertainty can arise from inadequacies
or limitations in the instrument, such as
pinpointing the reading of a burette or
graduated cylinder.
Examples of a systematic
error can be from reading
a burette from the wrong
direction, reading the top
of the meniscus instead of
the bottom, or using
equipment that is not well
calibrated.
IB Core Objective
11.1.3 Describe how the effects of
random uncertainties may be
reduced.
Describe: Give a detailed account.
(Obj. 2)
11.1.3 Describe how the effects of random
uncertainties may be reduced.
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We will be learning more about this when
we do labs.
This is also why we ask you to collect data
several times (3-5 times) for an
experiment.
Repeating should increase the precision of
the final result since random variations
can be statistically cancelled out (or
dropped if it is way off).
IB Core Objective
11.1.4 State random uncertainty as an
uncertainty range (±)
State: Give a specific name, value, or
other brief answer without
explanation or calculation.
(Obj. 1)
11.1.4 State random uncertainty as an uncertainty
range (±)
Absolute uncertainty: Is the
measurement you are guessing
Ex: 25.0 cm3 pipette has an absolute
uncertainty of ±0.1cm3
100cm3 beaker has an absolute
uncertainty of ±1cm3
11.1.4 State random uncertainty as an
uncertainty range (±)
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Instruments may have the tolerance (i.e.
uncertainty) clearly labeled.
If the tolerance is not labeled on the instrument,
you will have to determine the uncertainty
yourself.
A digital scale may bounce around on the last
digit (i.e. between 3.759 and 3.760). The
uncertainty would be ±.001. If it bounces
around by five on the last digit, then it would be
± .005.
We will practice this in labs.
IB Core Objective
11.1.5 State the results of calculations
to the appropriate number of
significant figures
State: Give a specific name, value, or
other brief answer without
explanation or calculation.
(Obj. 1)
11.1.5 State the results of calculations to the
appropriate number of significant figures
Estimating the number
Bathroom scale
Grape fruit 1
11.5kg
Grape fruit 2
11.5kg
Balance
1.476kg
1.518kg
Certain digits: The numbers we know
Uncertain digits: The estimated number. The bolded
numbers represent the guessed digit.
Significant Figures


The number of figures known + one guessed figure.
The bathroom scale has: 2 sig. Figs.
The balance has: 4 sig. Figs
11.1.5 State the results of calculations to the
appropriate number of significant figures
Leading Zeros (Zeros to the Left of the decimal place)
Don’t count! They are just place holders.
Value
0.0056
000.334g
0.01
0.0000105
0.0056
# of sig figs
Sci. notation
11.1.5 State the results of calculations to the
appropriate number of significant figures
Trailing Zeros (Zeros to the Right End of the number) Only
count when the number contains a decimal place.
Value
1.00
300.
300.0
1000
6.02 x 1023
Sig figs
11.1.5 State the results of calculations to the
appropriate number of significant figures
Addition/Subtraction:
When adding and subtracting data, use the
measurement with the least number of decimal
places.
Value
0.0056 + 1.0010
5.5 – 0.13
5.12 x 103 + 0.10
1.5 – 0.0055
# of d.p.
Answer
11.1.5 State the results of calculations to the
appropriate number of significant figures.
Multiplication/ Division
When multiplying and dividing, your answer should have
the number of sig. figs as the one with the least number
of sig figs.
Value
4.56 x 1.4
.50 x 100
25.0 ÷ 5.00
1.0 x 102 ÷ 5
# of sig figs
Answer
IB Core Objective
11.2.1 State uncertainties as absolute
and percentage uncertainties.
State: Give a specific name, value, or
other brief answer without
explanation or calculation.
(Obj. 1)
11.2.1 State uncertainties as absolute and
percentage uncertainties.
Percent uncertainty: Absolute uncertainty x 100
Amount used
If we take a 30cm3 sample in the 100cm3 beaker (with a
± 1 uncertainty) what is the % uncertainty?
% uncertainty = 1/30 x 100  3.33%
If we take a 90cm3 sample in the 100cm3 beaker what is
the % uncertainty?
% uncertainty = 1/90 x 100  1.11%
This is why taking small samples with a large
beaker is not a good idea! Use the proper tool!!
IB Core Objective
11.2.2 Determine the uncertainties in
results.
Determine: Find the only possible
answer. (Obj. 3)
11.2.2 Determine the uncertainties in
results.
Adding/ Subtracting uncertainties
Just add the uncertainties of each piece of
equipment
Add the two volumes from the previous
example
30 (±1)
+90(±1)
120 (±2)  (So range is 118-122)cm3.
% uncertainty = 2 ÷ 120 x 100  0.83%
11.2.2 Determine the uncertainties in
results.
Multiply/Dividing uncertainties
 Each measurement must have the %
uncertainty calculated.
 The % uncertainties are then added
 The final % uncertainty is then used to recalculate the final absolute uncertainty.
Scale = 5.000g (±0.001)
Pipette = 50.00cm3 (±0.01)
Graduated cylinder = 25.0cm3 (±0.05)
11.2.2 Determine the uncertainties in
results.
Answer
0.001 ÷ 5.000 x 100 = 0.02%
0.01 ÷ 50.00 x 100 = 0.02%
0.05 ÷ 25.0 x 100 = 0.2%
Total percentage = 0.24%
If the molar mass in the end was determined to be
64.0 g/mol, then
0.0024 x 64.0 = 0.1536,
So final answer is 64.0 g/mol ± 0.2g