1. Measurement and Identification
2nd Class
Measurement and Identification
Today’s Agenda:
Develop the day’s problem.
Perform first challenge: measure density of knowns.
Discuss reporting measurements and significant figures.
Develop a consensus on the correct values.
Perform second challenge: identify unknown plastics.
Learn how to write a scientific report.

2. Make-up Labs New for this Spring
To perform an experiment in a make-up lab you are required to complete and bring with you a make-up lab contract.
For details, see the make-up lab policy.

3. Safety Agreement
Have you done it? We can not let you work in our labs if you don’t agree to be safe.

4. One of the central goals of Chemistry is to analyze chemical substances.
Detection: Does the sample contain substance X?
Identification: What is the identity of the substance in this sample?
Central Goals and Questions
Separation: How can the substances present in the sample be separated?
Quantitation: How much of substance X is in the sample?
Point out that the ones on the left are QUAL and the ones on the right QUANT. We will revisit this theme slide from time to time during the course.
Here in the lab you will learn how do we do it

5. How do you propose to do it?
Imagine that you are employed by a recycling firm.
You have been assigned to devise a quick and cheap method for identifying different types of plastics.
How do you propose to do it?
The separation needs to be based on the identification of a property that differentiates one type of plastic from another (differentiating characteristic)

6. Analyze the plastic samples.
Let′s think!
In your groups:
Analyze the plastic samples.
Make a list of properties that differentiate one type of plastic from another.
Identify those properties that depend on the size of the sample (extensive) from those that are not expected to vary with sample size (intensive).
Select “the property” that you think is the best to differentiate and separate one plastic from another.

7. Density is an intensive property that is commonly used to differentiate and separate plastics during recycling.
What do you need to measure to determine the density of each of the plastic pieces?
Density is a measure of the mass of material per unit volume:
r = m/V (mass and volume are extensive properties)

8. Intro to Measurement Computer-Graded Pre-lab assignment

9. Notebooks
As a group, make a list of the things you expect to be putting in your notebook today.
Title, date, time, names of partners.
observations, measurements, descriptions of what was done.
signature at the end of the session.
instructor initials and submission of copies

10. Revisiting Mass Measurement
Let′s think!
Revisiting
Mass Measurement
DON’T FORGET
Balances must not be moved
Only glass, metal or plastic objects can be put on the pan
They must be left clean.
In your groups revisit what you will need to do to make reliable mass measurements.

11. Let′s think!
Volume Measurement
Do you know how to properly use a graduated cylinder? What will you have to do to ensure an accurate, reliable reading?

12. Volume Measurement: making linear measurements.
Let′s think!
Volume Measurement: making linear measurements.
In your groups review your responses to the following pre-lab question and come up with a consensus answer.
Lengths are measured using rulers. Describe why a vernier caliper is preferred over a common ruler for this experiment.

13. Volume Measurement: linear measurements vs displacement.
Let′s think!
Volume Measurement: linear measurements vs displacement.
In your groups review your responses to the following pre-lab question and come up with a consensus answer.
Consider the objects illustrated in figure 1-3 of the Measurement guide. Describe two different methods you could use to determine the volume of the objects. Describe how you would go about using those methods..
How could you prove one to be more accurate?

14. Volume Measurement: displacement.
Let′s think!
Volume Measurement: displacement.
In your groups discuss how you intend to determine the volume by displacement.
Write down what you expect to do. Include all the equipment and materials you will need to use.
What if the plastic does not fit in the cylinder?
What if it floats?
What data do you need?

15. Identifying Unknown Samples Density values for candidate plastics
Let’s apply
Identifying Unknown Samples
Once you have determined the density of an unknown piece of plastic, what else will you need to know to identify it?
Density values for candidate plastics

16. Determining Known Values
You will need to know the densities of the six possible plastics to identify your unknown. The following samples are provided for your use.
Your group will be responsible for determining the density of one plastic
High Density Polyethylene ( HDPE)
Low Density Polyethylene (LDPE)
Polyvinyl chloride ( PVC)
Polyethylene terephthalate( PETE)
Polypropylene ( PP)
Polystyrene ( PS)

17. Design a feasible procedure and implement it!
Let′s explore!
First Challenge
Determine the density of your assigned plastic. Measure the volume both ways and compute the density separately for each.
Design a feasible procedure and implement it!
Available resources:
4 pieces of PVC.
Rulers, vernier calipers.
Balances (TC 4).
Glassware.
You have 30 minutes

18. Let′s think!
Within your group, review your results and decide on a value for the density of your plastic. Prepare a report to the class.
Choose a reporter. That person is to report to the class the one value to use for that plastic, your justification for selecting that value and your confidence in that value.

19. While preparing your report consider the following
Let′s think!
While preparing your report consider the following
Did everyone have the same value? If not, how did you decide on one number to report?
Did both volume determination methods produce the same value? If not, which one did you chose to use and why?
What is your confidence in your answer? Does your reported value reflect your confidence? How so?

20. Is there another way to analyze these data?
Let′s think!
Is there another way to analyze these data?

21. Graphing Data in Excel®
Volume (mL)
Mass (grams)
Density (g/mL)
x-axis
y-axis
calculation
7.0
6.565
0.94
1.6
1.398
0.87
6.80
5.237
0.77
Average
0.86

22. REPORTING MEASUREMENTS
To be considered correct a measurement MUST be reported with the correct number of figures. Improperly reported measurements have less credibility and may be considered wrong, even if the digits are correct.
Your grade on assignments and tests will depend on your correct use of significant figures and units!

23. Danger Will Robinson !!!
Calculators and EXCEL know NOTHING about measured digits and significant figures! They will put as many digits on the screen as it will hold.
YOU need to round to the correct
number of measured digits from your
computations.
(Significant Figures)

24. Known values for plastic densities.
Reports to the class
Each group is to report.
The known value to use for that plastic.
The justification for selecting that value.
The confidence in that value.
Plastic
Density (g/mL)
HDPE
LDPE
PET PP PS
PVC

25. Identify your unknown plastic pieces.
Let′s explore!
Final Challenge
Identify your unknown plastic pieces.
Available resources:
Densities of known plastics
Pieces of unknown plastics.
Balances;
Rulers, vernier calipers;
Glassware;
Just Do It!
You have 20 minutes

26. The Report
To be submitted BEFORE leaving class:
Raw data. A copy of all measurements and observations you collected in lab.
Calculations. Examples of the various calculations performed.
You will now write your report for this experiment. It is a limited report. Only the indicated sections are required.
To be submitted next week:
Results. An organized presentation of the results obtained.
Discussion. A discussion of the results obtained. For this report (only) this is to be the answers to the Wrap-up Discussion questions.

27. Calculations From p. 65 of your Technique Book:
Calculations. Examples of all calculations performed to manipulate the numerical data. Include one example for each type.
From p. 70 of your Technique Book:
One example for each type of calculation. Each example should consist of three parts:
The formula that describes the calculation
The formula in use
The answer

28. Calculations: Outline
Let′s think!
Calculations: Outline
In your groups:
What were the major goals of today’s lab?
Find densities of known plastics
Identify unknown plastic

29. Calculations: Outline
Let′s think!
Calculations: Outline
In your groups:
Identify the important calculations that should be included in your calculations section
Known Plastics
Volume of plastic
Density (can be taken from the graph rather than actually calculating)
Unknown Plastics
Density of plastic

30. Calcuations Calculations should always use a three step format:
Let’s apply
Calculations should always use a three step format:
Write out the equation
Plug in your numbers
Give the answer
This method ensures that your instructor can follow your thought process during the calculation.
Do not forget to add units and watch significant figures during your calculations!

31. Calculations: Goal 1 The first calculation we listed is volume:
Let’s apply
Calculations: Goal 1
The first calculation we listed is volume:
Volume of a rectangular prism:
length x width x height = volume
1.05 cm x 1.26 cm x 1.82 cm =volume
volume=2.41 cm3

32. Let’s apply
Calculations: Goal 1
The second calculation we have listed is density. We obtained this value from the graph, but it would be helpful to make a note in the calculations so that the reader does not think you forgot one: Density of known plastic: Determined from the slope of the graph

33. Let’s apply
Calculations: Goal 2
The first calculation of goal 2 is the same as that of goal 1: volume. Since we have already shown this calculation, we do not need to show it again. If we calculated the volume a different way, then we would need to show a second calculation.

34. Calculations: Goal 2 Density:
Let’s apply
Calculations: Goal 2
The last calculation we have is density of the unknown plastic. Since we only had one piece of plastic, we do not have a graph to use to calculate density, so we have to use a formula.
Density:

35. Calculations: Things to Remember
Include a calculation for all types of calculations that you do, but only include one example for the type of calculation
Watch significant figures and units

36. Results From p. 65 of your Technique Book:
Results. An organized presentation of the observations and calculated results. This section is frequently dominated by figures, tables and graphs. It will contain little to no text.
From pp of your Technique Book:
Results. An objective and clear description of experimental findings following an appropriate order and including relevant:
Tables
Graphs
Figures

37. Results:Outline In your groups:
Let′s think!
Results:Outline
In your groups:
What were the major goals of today’s lab?
Find densities of known plastics
Identify unknown plastic

38. Results: Outline In your groups:
Let′s think!
In your groups:
Identify the important results that should be included in your results section
Known Plastics
Dimensions of plastic
Linear measurement method
Displacement method
Mass of plastic
Density of known plastic
Unknown Plastics
Density of unknown plastic
Proposed identity of unknown plastic

39. Let’s apply
Results: Goal 1
In a labeled table, compile all of the items on your list in a table that follows a logical order.
Table 1. Density determination of known plastic PETE.

40. Let’s apply
Results: Goal 1
It also might be helpful to display this information in a graph as well. If we plot the volume on the x-axis and the mass on the y-axis, the slope will be the density of the plastic. Graph 1. Known PETE mass vs. volume measurements.

41. Let’s apply
Results: Goal 1
Sometimes, more than one table should be used to portray all of the results from a single goal.
Goal 1 can be split in to two parts:
A detailed table for your plastic and graph
A summary table for all unknowns

42. Table 1. Densities of known plastics
Let’s apply
Results: Goal 1
Table 1. Densities of known plastics

43. Let’s apply
Results: Goal 2
Sometimes, it makes more sense to list numbers and answers in a clear order rather than putting them in a table. Density of unknown plastic: 1.38 g/cm3 Identity of unknown plastic: PET (Polyethylene terephthalate)

44. Results: Things to Remember
Only include results that are important to the conclusions that you make
Be careful about the amount of significant figures you have, especially when copying tables that you’ve made from Excel

45. Data Presentation - Results Data Manipulation - Calculations
Let’s apply
Data Presentation - Results
Organize all your measurements into tables.
Include all calculated results along with measured quantities.
Organize your table to “flow”. Your presentation should lead the reader to your conclusion.
You will be graded on credibility. Your presentation must be credible. Proper use of significant figures and labeling (units) strongly impacts credibility.
Data Manipulation - Calculations
Decide which calculations to present and generate examples as described in technique chapter 2.

46. Make sure to address the following points
Let’s apply
Wrap-up Discussion
Make sure to address the following points
Explain why significant figures are important to measurement and what they signify.
Of the measurements you made, which were more precise; the mass measurements or the volume measurements? Defend your position.
Is it practical to use density to separate plastics? Explain.
Propose another way to separate plastics making use of a different property than density.

47. Looking Ahead to Next Week
We will investigate how chemists use their knowledge and understanding of the properties of substances to separate the different components of materials.
Thinking ahead
Make sure you remember to take the pre-lab quiz and do the pre-lab assignment, both of which are found in D2L.