honors chemistry making measurements and reading scales 1
TRANSCRIPT
Honors Chemistry
Making Measurements and Reading Scales
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How to measure – What is important?
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Instructions: Mass approximately 5.00 g of sucrose.
Recognize the meaning: Getting exactly 5.00 g of sucrose is NOT
important. You just need to be near 5.00 g of sucrose (i.e. +/-
0.10 g – between 4.90 g and 5.10 g). Key
Once you get near to the desired measurement you are ready to measure your EXACT mass of sucrose (i.e. 4.93 g)
IT IS A WASTE OF TIME TO GET 5.00 g!!!!
Reading Scales – What is important?
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The main idea: You are communicating scientific information to
fellow scientists. When you give a measurement you communicate
the following: The accuracy of your equipment. The sensitivity of your equipment. The uncertainty of your measurement.
Reading Scales – What is important?
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Example: Take your 4.93 g measurement we discussed
earlier. The person who reads your report and sees this
data knows: Your equipment can measure to 0.01 g. The uncertainty of your equipment is likely +/- 0.01 g
This information will help the scientific reader to evaluate the significance of your data and your findings/conclusions.
Reading Scales - Zeros
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Is reporting a zero at the end of a measurement important? Is 1.2 g the same as 1.20 g?
NO!!! These readings are from different equipment with
different sensitivities and different uncertainties.
What does this mean to you? When you are reading scales, use the following
rules. When reading a measurement that ends in zero,
report the zero.
Liquid Volumes When measuring
the volume of a liquid, always read the scale from the bottom of the meniscus. The meniscus is the curved surface of a liquid in a narrow cylindrical container.
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When reading a scale, always strive to avoid parallax errors. Parallax errors arise when a meniscus or needle is viewed from an angle rather than from straight-on at eye level.
Incorrect Correct7
Taking other measurements on an instrument with a needle that has a mirrored plate behind it, view the scale so that the needle's reflection is hidden behind the needle itself.
Incorrect Correct8
Reading a Scale A scale is made up of a series of
graduations. Usually, some of the graduations are labeled at regular intervals, with smaller, unlabeled graduations between them.
The rule for reading a scale: Read the scale using the markings and then
estimate one more place, past the last digit that is marked.
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Large Scale Increment
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To read the scale, you must first determine the large scale increment. The large scale increment is the quantity between any two adjacent marked graduations. To find the scale increment, subtract the values of any two adjacent labeled lines.
Small Scale Increment
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Next, to determine the small scale increment, count that there are ten lines between the labeled lines. Therefore, the scale increment is 10 mL/10 lines = 1 mL/line.
Remember – read this scale to 1 more place than marked – so it would be read to the tenths place.
Practice – Large Scale Increment What is the large
scale increment on this scale?
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Practice – Small Scale Increment What is the small
scale increment on this scale?
How many decimal places will be in your final reading?
Is the last place in your answer certain or estimated?
Are the other numbers in your answer certain or estimated?
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Measuring - Finding the Certain Digits Use the large scale
increment to find the first digit of your measurement.
The first digit of the reading is 5, since the last labeled graduation below the meniscus is 50 (i.e. the reading is between 50 mL and 60 mL).
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Finding the Certain Digits
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Next, use the small scale increments to determine the second digit of your reading.
The second digit of the reading is 2, since the last labeled graduation below the meniscus is 52 (i.e. the reading is between 52 mL and 53 mL).
We have now read as far as we can using the markings on the scale. The certain digits of the reading are 52 mL.
Estimate the uncertain digit and take a reading Finally, estimate the
distance that the meniscus lies between the two lines as a decimal.
On this graduated cylinder, the meniscus is about eight tenths (0.8) of the way to the next line.
So, the final digit in the reading is 0.8 mL. (The uncertain digit in the reading is always the last digit)
The correct volume in this graduated cylinder is 52.8 mL.
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What does this reading mean?
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Your reading is 52.8 mL.
Since the last digit (the 8) is estimated, readings of 52.7 mL and 52.9 mL would be equally correct.
On this scale, your uncertainty would be: +/- 0.1mL
25 mL Graduated Cylinder
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What is the large scale increment for this cylinder?
25 mL Graduated Cylinder
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What is the small scale increment between the lines that are unmarked?
Note: This is a tricky scale - although ten lines are between the labeled lines the short lines are used as guides in estimating the uncertain digit)
You could not really read this to the hundredths place accurately.
25 mL Graduated Cylinder What is the
volume of liquid in this graduated cylinder?
Answer
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Practice
AnswerAnswer
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Practice
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Measure this volume. (Be careful this is a little tricky)
Answer
50 mL Buret
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Before beginning to record a reading, examine the buret closely.
In the previous examples with the graduated cylinders, zero was at the bottom of the scale , with values increasing going up the cylinder.
However, a buret has zero at the top with values increasing going down the scale.
This is because the buret empties from the bottom.
50 mL Buret
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In examples where values on the scale increased going up, the digits in the reading were recorded by looking at the last graduation below the meniscus.
In examples where the scale values increase going down, record digits by looking at the last graduation above the meniscus
50 mL Buret
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What is the large scale increment for this scale?
What is the small scale increment for this scale?
How many decimal places will your reading have?
Which place in your reading is estimated?
50 mL Buret What is the reading
of the volume in this buret?
Answer
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Buret Reading Card
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Answer – 25 mL Graduated Cylinder
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Correct Reading 11.5 mL
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Answer – 100 mL Graduated Cylinder
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Correct Reading 76.0 mL
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Answer – 10 mL Graduated Cylinder
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Correct Reading 6.78 mL
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Answer – 25 mL Graduated Cylinder
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Correct Reading 21.6 mL
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Answer – 50 mL Buret
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Correct Reading 24.20 mL
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