012 Puzzle Titration

Expt 012 -- Puzzle Titration

Five solutions are provided. Of these solutions, some contain HCl and others NaOH. The HCl solutions also contain dissolved phenolphthalein. Devise and perform experiments that permit separating the chemicals into two groups based upon composition. Devise and perform experiments that permit ranking the chemicals within each group according to concentration.

Safety

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

Procedure

5 chemical solutions stored in pipet storage devices are provided. Determine as much as possible about these chemicals by mixing them two-at-a-time. If two or more chemicals have similar chemical make-up, attempt to rank them according to their concentration.
Devise and perform experiments that permit separating the chemicals into two groups based upon composition.

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Identify which group is acidic.

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Devise and perform experiments that permit ranking the chemicals within each group according to concentration.

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Summarize your observations. Identify each solution as acid or base, and rank the solutions by relative concentrations from most concentrated to least.

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Discard the used solutions into a disposal jar provided by the instructor.
Wash hands.

Questions

Does order-of-addition matter? Explain.
Does it matter whether a white background, a gray background, or a black background is used? Explain
Touching these chemicals is safe if fingers are washed immediately with lots of tap water after touching them. Can anything be learned by touching the chemicals? Explain
Suppose 1 M HCl were replaced in this experiment with 0.5 M H₂SO₄. Describe the net effect of this change.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 012 Puzzle Titration

Watch the movies and make careful observations.

Classify tubes 4, 5, and 6 as acid or base.
- 4
- 5
- 6
Two of the above are the different concentrations of the same chemical. Which of these two is the most concentrated?
Devise a plan for identifying the contents of tubes 1, 2, 3 and determining their concentration relative to all six solutions. Use the minimum steps required assuming the knowledge gathered with tubes 4, 5, 6. (Hint: it is much easier to see end point which is the appearance of a color than one which is a dissappearance of color.)

Curriculum-

This experiment works best after students have done a conventional acid base titration and after they have become familiar with the properties of phenolphthalein.

Safety-

Time-

Teacher Preparation: 15 minutes

Class Time: 40 minutes

Materials-

(per group)

2 mL of 1.0 M HCl containing phenolphthalein -- (dissolve 0.5 g phenolphthalein in 20 mL 95% ethanol. Add to 20 mL distilled water, with stirring. Add 17 mL of 3 M HCl. Add enough distilled water to bring the final total volume to 100 mL.)
2 mL of 0.5 M HCl containing phenolphthalein -- (Add 33 mL of the solution described above to 33 mL of distilled water.)
2 mL of 1.0 M NaOH -- (Add 33 mL of 3 M NaOH to enough water to make 100 mL of solution.)
2 mL of 0.5 M NaOH -- (Add 17 mL of 3 M NaOH to enough water to make 100 mL of solution.)
2 mL of 0.2 M NaOH -- (Add 6.7 mL of 3 M NaOH to enough water to make 100 mL of solution.)
96-well plate
5 toothpicks
2 cotton swabs

Disposal-

Neutralize the solution in to disposal jar (with vinegar or NaHCO₃). discard the neutralized solution at the sink with running water.

Answers-

Q1. Does order-of-addition matter? Explain. A1. Order of reaction doesn't affect the chemistry or the outcome.
Q2. Does it matter whether a white background, a gray background, or a black background is used? Explain A2. Phenolphthalein changes are readily seen against a white background but are very hard to see against a dark background.
Q3. Touching these chemicals is safe if fingers are washed immediately with lots of tap water after touching them. Can anything be learned by touching the chemicals? Explain A3. Strong bases react with fats to form soap, which feels slippery. By reacting with the fats in the skin of fingertips, strong bases feel slippery to the touch. If not quickly washed from the fingers, reaction with natural fats can cause damage.
Q4. Suppose 1 M HCl were replaced in this experiment with 0.5 M H₂SO₄. Describe the net effect of this change. A4. Substituting sulfuric acid for hydrochloric acid in this concentrations has no perceptible effect in this experiment.

Makeup ans.-

HQ1. Classify tubes 4, 5, and 6 as acid or base. HA1. 4 base 5 base 6 acid
HQ2. Two of the above are the different concentrations of the same chemical. Which of these two is the most concentrated? HA2. Tube 5 is more concentrated than 4.
HQ3. Devise a plan for identifying the contents of tubes 1, 2, 3 and determining their concentration relative to all six solutions. Use the minimum steps required assuming the knowledge gathered with tubes 4, 5, 6. HA3. Add 1 drop of tube 6, the acid with phenolphthalein, to 4 drops of solution from each tube. If a pink color is observed, classify the tube as a base. Place 4 drops of tube six in a well titrate to a pink end point with one of the bases. Repeat for each base. Place 4 drops of each acid in a separate well. Titrate with the more dilute base, tube 4 in the movies.

Reference-

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

Key Words 1-

solution stoichiometry, solutions, titration, acids, acid base, end point, pH, pH indicators, applied chemistry, assay, strong acid, strong base

Elements-

H O Cl Na