Download Understanding Standard Deviation & Experimental Error in College Lab: A Weight Experiment and more Study notes Chemistry in PDF only on Docsity! 1 CH204 Experiment 1 Are the Densities of Coke and Diet Coke Different? d = m V Dr. Brian Anderson Spring 2007 Today • Random error in experimental data • Calculating standard deviation • Reporting significant digits • Quick look at Experiment 1 But first... Sections 53215 and 53315 Yushan Yeh is out. Ashley Garcia is in. Stay tuned for any changes in the TA office hour schedule. 2 How much does a quarter weigh? Weigh a few more... 5.7128 5.6947 5.7085 5.6907 5.6106 5.6339 5.6009 5.7205 5.6466 5.7195 Now what does a quarter weigh? Average = 5.67387 Every data point is an estimate! But how good of an estimate is it? If we don’t know the true value, how do we know how much variability (random error) there is in our measurement? 5 So what does a quarter weigh? 5.67 ± 0.05 g 68% of all quarters should weigh between 5.62 and 5.72 grams. 5.7128 5.6947 5.7085 5.6907 5.6106 5.6339 5.6009 5.7205 5.6466 5.7195 Variability limits significant digits There were five significant digits in the mass of each quarter, but only three significant digits in the final result. The last two digits are insignificant because they are less than the variability in the measurement. “Variability in the measurement” = experimental error. Ways of Determining Experimental Error For a single reading: Precision of the equipment Tolerance of the glassware For many readings: Statistics This is what we’re gonna do in lab today. 6 Std. Dev. = random error We’ll use this same procedure to determine random error and significant digits in Experiment 1. And speaking of Experiment 1... Graduated cylinder Volumetric pipette Burette 0.01 mL Analytical balance Equipment 0.0001 grams! TwoPart Lab Part One: • Calculate density. Total of six data points. • Enter your results into the spreadsheet on the computer nearest the printer, and use all the class data in your report. • Measure the mass of 5 mL of sample using the analytical balance and three different types of glassware (pipette, burette, and graduated cylinder). 7 TwoPart Lab Part Two: • Do NOT calculate density. • Enter your mass and volume measurements into the spreadsheet on the computer nearest the door, and use all the class data in your report. • Dispense your assigned volume using a burette, and measure the mass of the sample on the analytical balance. Important! You will need all three graphs: Part One: 1 Density chart and graph comparing different methods (includes average and standard deviation for each method). Part Two: 2 Mass vs volume graph for Coke 3 Mass vs volume graph for Diet Coke Handling bad data If you know it’s bad – because you know something went wrong, or because the number is physically impossible you can discard it. If you don’t like it because it’s widely scattered, you can’t just toss it, you have to apply the Qtest (see the appendix of the lab manual).
