Precipitation Stoichiometry

Introduction

The purpose of this experiment is to determine the approximate combining ratios for reactions of calcium chloride.

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Calcium Oxalate

1. Select a 12-well strip. Place 4 drops of distilled water into each of the first 9 wells of the strip.
2. Add 1 drop of 0.1 M calcium chloride to well 1, 2 drops to well 2, etc., until you add 9 drops to well 9.
3. Add 9 drops of 0.1 M sodium oxalate to well 1, 8 drops to well 2, etc., until you add 1 drop to well 9.
4. Mix each of the wells with a toothpick. Allow to settle for 5 minutes.
5. Hold the strip up and observe the level of the precipitate in each of the wells by looking through the side of the wells. Identify the well with the largest amount (volume) of precipitate. Determine the drop ratio for this well.
6. On the basis of the ratio of drops, write a balanced equation for the formation of calcium oxalate.
7. Wash the 12-well strip at the sink. Use a cotton swab to scrub each well.

Calcium Phosphate

1. Select a 12-well strip. Place 4 drops of distilled water into each of the first 9 wells of the strip.
2. Add 1 drop of 0.1 M calcium chloride to well 1, 2 drops to well 2, etc., until you add 9 drops to well 9.
3. Add 9 drops of 0.1 M sodium phosphate to well 1, 8 drops to well 2, etc., until you add 1 drop to well 9.
4. Mix each of the wells with a toothpick. Allow to settle for 20 minutes. (Heating the strip in the steam from a hot water bath sometimes speeds the settling process.)
5. Hold the strip up and observe the level of the precipitate in each of the wells by looking through the side of the wells. Identify the well with the largest amount (volume) of precipitate. Determine the ratio of drops for this well.
6. On the basis of the ratio of drops, write a balanced equation for the formation of calcium phosphate.
7. Wash the 12-well strip at the sink. Use a cotton swab to scrub each well.

Calcium Oleate

1. Select a 12-well strip. Place 4 drops of distilled water into each of the first 8 wells of the strip.
2. Add 1 drop of 0.1 M calcium chloride to well 1, 2 drops to well 2, etc., until you add 8 drops to well 8.
3. Add 8 drops of 0.1 M sodium oleate to well 1, 7 drops to well 2, etc., until you add 1 drop to well 8.
4. Mix each of the wells with a toothpick. Allow to settle for 20 minutes. (Heating the strip in the steam from a hot water bath sometimes speeds the settling process.)
5. Hold the strip up and observe the level of the precipitate in each of the wells by looking through the side of the wells. Identify the well with the largest amount (volume) of precipitate. Determine the ratio of drops for this well.
6. On the basis of the ratio of drops, write a balanced equation for the formation of calcium oleate.
7. Wash the 12-well strip at the sink. Use a cotton swab to scrub each well.
   

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• Wear safety goggles and an apron in the lab at all times. Do not ingest the chemicals. Wash your hands after working with the chemicals.
• Sodium oxalate is toxic.

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• One way to teach this is to do the calcium phosphate first and then pose the "determination of the charge of the oxalate ion" as a question for the class.
• Drop sizes must be uniform in this experiment. See the special technique in which a thin stem pipet and a plastic pipet tip are combined to make a device that delivers uniform drops.

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• 0.10 M calcium chloride (dissolve 1.47 g CaCl₂•2H₂O in enough water to make 100 mL of solution)
• 0.10M sodium oxalate (dissolve 1.34 g Na₂C₂O₄ in enough water to make 100 mL of solution)
• 0.10 M sodium phosphate (dissolve 3.8 g Na₃PO₄•12H₂O in enough water to make 100 mL of solution)
• 0.10 M sodium oleate (dissolve 3.04 g NaOOCC₁₇H₃₃ in enough water to make 100 mL of solution)

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