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Quiz3 - Chemistry 223 Quantitative Analysis Laboratory...

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Unformatted text preview: Chemistry 223 Quantitative Analysis Laboratory Post‐lab Quiz Answer Key 9/17/10 1. A dried powder is stored in a beaker in a desiccator. An aliquot of the powder is to be used for standardization. The total mass of beaker and contents is 78.3472 g. After an aliquot is placed in an Erlenmeyer flask, the remaining mass of beaker and contents is 76.1822 g. What is the mass of the aliquot in the Erlenmeyer flask (assuming no spillage)? (1) 78.3472 g – 76.1822 g = 2.1650 g Number of significant figures must be correct. 2. In the first half of the twentieth century, it was common to calibrate burets by measuring the mass of water delivered from the buret and comparing that to the buret's readings. Since about 1970, it has been reasonably common to simply trust the calibration on the buret. Why? (1) a) Glass tube diameter is more precise now than it was a century ago, so variation in diameter along the length of the buret is small. b) The precision of the glass is better than the precision of measuring delivered volume and mass (i.e. the errors in calibration exceed the actual error in making the buret) c) Since the same buret is typically used for standardization and titration of unknowns, any absolute volume errors influence both measurements in ways that cancel. d) All of the above 3. You are doing an experiment in which you need to deliver 10.00 mL, so of course you use a 10 mL TD pipette to deliver the solution. You later decide that you would like to redo the experiment using 7.50 mL. Which strategy for delivering this volume would you choose (1) and why (2)? a) Use a buret, fill it sufficiently so that 7.50 mL can be drained from it, and then use the buret to deliver 7.50 mL. b) Use a 10 mL graduated pipette. Fill it to the 0.00 mL line, then carefully drain it until the reading is 7.50 mL. Let the remaining solution in the graduated pipette drain into a waste container. c) Use a 5.00 mL pipette, a 2.00 mL pipette, and a 0.50 mL pipette to add up to 7.50 mL. Using the buret, the delivered amount will be 7.500.03 mL. The graduated pipette is harder to control, but is approximately as precise. Using 3 different pipettes means you have 3 pieces of glassware to clean instead of one, and the total volume error is (5.002 + 2.002 + 0.502)1/2. Thus, the error for each pipette would have to be at least a factor of 31/2 less than that of the burette or graduated pipette, or about 0.01 mL. This may be the case for the 0.50 mL pipette, but it's not the case for the others. Thus, either a) or b) are acceptable. ...
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