Puzzle Titration

Procedure

You will receive 5 chemical solutions stored in pipet storage devices. By mixing these chemicals two-at-a-time, determine as much as you can about their composition. If two or more chemicals have a similar chemical make-up, attempt to rank them according to their concentration.

You may wish to use the two-piece device consisting of a plastic bulb and a pipet tip described earlier.

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The chemicals used in this experiment are corrosive to eyes and can cause blindness. Wear eye protection at all times. Do not ingest the chemicals. Wash any spilled chemicals with water as quickly as possible after the spill. Have an eye wash available.

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• This experiment works best after students have done a conventional titration and after they have become familiar with the properties of phenolphthalein.
• Here are a few "question" clues you might give your students:
  • Does order-of-addition matter?
  • Does it matter whether you use a white background, a grey background, or a black background?
  • Touching these chemicals is safe if you wash your fingers immediately with lots of tap water after touching them. Can you learn anything by touching the chemicals?
• If you must, tell the students that these chemicals are acids and bases, and ask them to group them and order them according to concentration.
• An excellent question is to ask students for a description of the chemical role of thymolphthalein. (Thymolphthalein is a pH indicator very much like phenolphthalein except that it turns blue instead of pink in a basic solution.)

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• 0.5 M HCl containing phenolphthalein (dissolve 0.5 g phenolphthalein in 20 mL 95% ethanol. Add to 20 mL distilled water, with stirring. Add 4.2 mL of concentrated HCl. Add enough distilled water to bring the final total volume to 100 mL.)
• 1.0 M HCl containing thymolphthalein (dissolve 0.5 g thymolphthalein in 20 mL 95% ethanol. Add to 20 mL distilled water, with stirring. Add 8.4 mL of concentrated HCl. Add enough distilled water to bring the final total volume to 100 mL.)
• 1.0 M NaOH (dissolve 4.0 g NaOH in enough distilled water to make 100 mL of solution)
• 0.5 M NaOH (dissolve 2.0 g NaOH in enough distilled water to make 100 mL of solution)
• 0.2 M NaOH (dissolve 0.8 g NaOH in enough distilled water to make 100 mL of solution)
• 96-well plate
• 5 toothpicks
• 2 cotton swabs

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This experiment was brought to our attention by faculty from the Chemistry Department at the State University of New York, Cortland.

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