Unit 1: Reliability of Measurements

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.

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

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

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

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

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

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-

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.

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

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

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

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?

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

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.

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

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

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 18.015g. Calculate the % Error in the experiment.

Density of Water Lab: Accuracy Calculate your percent error for the density of water. The accepted value at 25°C is 0.997 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?

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

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

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 = ----------------- === .839 > Qc(.76) | 29.1 – 23.5 | Yes, we can throw out Trial 2 with 90% Confidence

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

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?

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