1. Unit 1: Reliability of Measurements

2. Warm-up
With your lab partner, use the equipment available (1 pair has grad cylinder, 1 pair has beaker) and determine the density of water.
Write your answer (to 2 decimal places) in the table on board.

3. Density of Water Results Pd 2
Table #
Beaker
(g/mL)
Cylinder 1
.70
1.01 2
.77 3 4 5
.99
1.17 6
.82
1.00

4. Density of Water Results Pd 3
Table #
Beaker
(g/mL)
Cylinder 1 2 3 4 5 6

5. Density of Water Results Pd 4
Table #
Beaker
(g/mL)
Cylinder 1 2 3 4 5 6

6. Density of Water Lab: Confidence
How Confident are you in your answer for the density of water? Why?

7. Evaluating the Reliability of Results
Are my instruments precise?
Choose the most precise instrument possible
Are my results repeatable?
Always perform multiple Trials (at least 3)
Do 3x if data seems consistent, more if not
Vary sample size in each trial if possible
Experimental Answer is the Average of your trials

8. Warm-Up: Add the following ions to your ion page in syllabus
Positive Ions
+1 charge +2
charge +3
Cuprous
Ferrous
Cobaltous
Nickelous
Chromous
Ferric
Cobaltic
Nickelic
Chromic
Negative Ions -1 -2
hypochlorite
ClO-1
chromate
CrO4-2
perchlorate
ClO4-1
dichromate
Cr2O7-2
permanganate
MnO4-

9. Evaluating the Reliability of Results
Do I have good Precision?
Precision is how close together the results of each trial are or ---
Your “consistency” or “repeatability”
Good Precision does not mean experiment is accurate, but it is a good sign.

10. Evaluating Reliability: Precision
Which has better Precision?
Experiment A
Experiment B
Trial 1
7.36
7.12
Trial 2
7.58
7.35
Trial 3
7.45
7.75

11. Evaluating Reliability: Deviation
Average Deviation – measures the Precision
Find the Experimental Answer by averaging trials
Find how far off each trial is from that answer
Average those numbers

12. Finding Average Deviation: Example
Results for Density of Li
(g/mL)
Deviation
Trial 1
0.602
0.602 – = 0.029
Trial 2
0.504
0.504 – = 0.069
Trial 3
0.613
0.613 – = 0.040
Average
0.573
Avg Deviation = 0.046

13. Density of Water Lab: Precision
Do you think the “cylinder groups” or “beaker groups” will have the higher precision?
Compare the precision of the “cylinder groups” to the “beaker groups”. – Determine the Average Deviation for each group.
Which group do you feel has the best answer? Why?

14. Learning Check
What does the Average Deviation tell us about the data collected in an experiment?

15. Good Experimental Practices
Rejection of Data
Never “throw out” data because it looks wrong
If a known error occurs (spilled some sample, realized a measurement error, used a wrong chemical), then trial data may be discarded
Otherwise, the Q Test can be used on data points that look errant. The Q Test will statistically verify if the data is most likely bad and should be thrown out. Only one data point may be discarded using this test.

16. Evaluating Reliability: Accuracy
Do I have good Accuracy
Accuracy is how close your results are to the Accepted Answer

18. Evaluating Reliability: Accuracy
Do I have good Accuracy
Accuracy is how close your results are to the Accepted Answer
Is not applicable in situations where no accepted answer is known
Percent Error is a measure of Accuracy
|Experimental – Accepted|
% Error = x 100
Accepted

19. Example: Percent Error
Three trials in an experiment to determine the formula mass of water has yielded the following results:
Trial
Formula Mass 1
17.5g 2
17.9g 3
17.8g
The periodic table shows this mass to be g.
Calculate the % Error in the experiment.

20. Density of Water Lab: Accuracy
Calculate your percent error for the density of water. The accepted value at 25°C is g/ml.
Now calculate the percent error of all cylinder (or beaker) groups together.
Which had better accuracy, your single trial or the average Experimental Answer?

21. Learning Check
What does the percent error tell you about your data?

22. This is the data that looks wrong
The Q Test
Q = |outlier - value closest to the outlier|
|highest value - lowest value|
Look up Qc on Table of Q critical values for the number of trials run
If Q is larger than Qc, the outlier can be discarded with 90% confidence.
Range

23. Melting Point of Gallium (ºC)
Q Test Example
Melting Point of Gallium (ºC)
Trial
Experimental Values 1 2 3 4
28.7
23.5
29.1
28.2
Results from Trial #2 are significantly different.
| 23.5 – 28.2 |
Q = === > Qc(.76)
| 29.1 – 23.5 |
Yes, we can throw out Trial 2 with 90% Confidence

24. Table of Critical Q Values
(90% Confidence)
Number of Trials
(Data Points) Qc 3 4 5 6 7 8 9 10
0.94
0.76
0.64
0.56
0.51
0.47
0.44
0.41

25. Density of Water Lab:Bad Data?
Look at the beaker group data set and the cylinder group’s data.
Should we throw out any data in either group?

26. Learning Check
In your own words, write down when we would use the Q Test and the purpose for using it.