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Unformatted text preview: Answer Key Chemistry 223 Quantitative Analysis Laboratory Post‐lab Quiz 9/24/10 1. Drying ensured that you could accurately weigh your KHP and your acid unknown. Why couldn't the same procedure be used to make up a standard NaOH solution by drying solid NaOH, accurately weighing it, and dissolving it in water to avoid needing to do a KHP standardization? First, NaOH absorbs CO2, so the temperature would have to be high enough to decompose bicarbonate and carbonate into Na2O. That would also dehydrate NaOH into Na2O. This compound is even more hygroscopic than NaOH (Na2O + H2O gives 2 NaOH). Second, Na2O could also soak up CO2 from the air and directly form Na2CO3. Thus, drying is insufficient to generate a stable, weighable compound. One would have to store Na2O in vacuum and weigh it in vacuum to use it as a primary standard 2. It's time to do a titration with NaOH. You obtain a buret, and find there is water clinging to the inside. You don't want that water to dilute your solution, and you realize that clinging drops are an indication that there is dirt in the buret. What is the quickest, easiest way to deal with the clinging water, while still ensuring you get accurate results? Pour some NaOH into the buret, roll it around to coat the walls, then drain and repeat. The contamination in the buret will then be the NaOH solution, not water drops, and any titrable matter will have been neutralized. "Phenolphthalein indicator is so old‐fashioned, and besides some students are color‐blind." Your instructor has heard this statement and agrees with it. Questions 3 and 4 are based on it. 3. Why use an indicator to find the KHP/NaOH endpoint instead of using some other approach? It's cheap, it's as precise as any automated approach, and any number of students can do titrations simultaneously since a drop of phenolphthalein is a lot cheaper than a pH meter plus electrode. 4. If you were to use another approach, what would you recommend and why? (2) Use a pH meter and plot the shape of the titration curve. By fitting the shape of the curve or finding where the second derivative of the curve crosses zero, one can interpolate a precise endpoint. No ability to sense color is required. ...
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This note was uploaded on 01/06/2011 for the course CHEM 223 taught by Professor Scheeline during the Fall '08 term at University of Illinois at Urbana–Champaign.
- Fall '08