NAME PERIOD DATE

SOLUBILITY

Initial QuestionMost ionic compounds are considered to be soluble or slightly soluble in water. Even compounds considered insoluble will dissolve to some small extent. This small extent can become very important when the dissolved substance is poisonous like lead, thallium, or cadmium. Over time, tiny amounts of these heavy metals can build up in your body and cause severe health problems. By knowing the amount of a dissolved ion in a solution, we can determine any potential health risks. How can you determine the amount of dissolved ions in a solution?

Materials and Equipment

Model 1 Data collection system Unknown A, solid, 1.0 g Conductivity sensor Unknown B, solid, 1.0 g Beakers (3), 150-mL Unknown C, solid, 1.0 g3

Balance (1 per class) Distilled water, 300 mL Stirring rod Wash bottle with distilled deionized water Graduated cylinder, 100-mL

Model 2 Data collection system Balance (1 per class) Conductivity sensor Beaker, 150-mL Magnetic stir bar Clamp Stir plate Potassium bitartrate (KHC4H4O6), solid, Ring stand 4.2 g Graduated cylinder, 100-mL Distilled water, 100 mL

Model 3 Erlenmeyer flask, 125-mL Quantitative filter paper Mohr pipet, 25-mL Magnetic stir bar Pipet bulb Stir plate Beaker (2), 150-mL Phenolphthalein, 3 drops Buret 0.10 M Sodium hydroxide (NaOH), 80 mL Buret clamp Distilled water, 20 mL Funnel

SafetyAdd these important safety precautions to your normal laboratory procedures: In addition to goggles and an apron, wear gloves.

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Getting Your Brain in Gear

1. Why would the conductivity of a solution change when solute is added? ______________________________________________________________________________________________

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2. Based on conductivity, how can you distinguish between soluble ionic and molecular solutes when they are dissolved in water?

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3. Consider the three beakers below. Label the solutions either saturated or unsaturated. Explain your reasoning.

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4. Bitartrate is a polyatomic ion with the formula HC4H4O6–. In a saturated solution of potassium bitartrate, the following equilibrium reaction occurs:

KHC4H4O6(s) ⇌ K+(aq) + HC4H4O6–(aq)

Describe the chemical processes during the forward and reverse reactions.______________________________________________________________________________________________

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MODEL 1

Building Model 1 – Saturated or Unsaturated?

1. Obtain three 150-mL beakers and label them “A,” “B,” and “C.”

2. Pour 100 mL of distilled water into each of the beakers.

3. Connect the conductivity sensor to the data collection system.

4. Insert the conductivity sensor into the water in Beaker A. NOTE: Make sure the small hole on the side of the conductivity probe is submerged. Gently tap the conductivity probe to remove any trapped bubbles.

5. Start data collection. Record the conductivity in the Model 1 Data Table.

6. Measure and add 1.0 g of unknown solid A to the beaker.

7. Stir the solution with a glass stirring rod and then record the conductivity and appearance on the Model 1 Data Table.

8. Make a particulate drawing for the unknown in solution in Model 1.

9. Rinse the conductivity probe with distilled deionized water and then repeat the previous steps with unknowns B and C in their respective beakers.

Model 1 – Saturated or Unsaturated?

Table 1: Model 1 Data Table—Identifying a saturated solution

Unknown AppearanceConductivity (μS/cm)

Water Water Plus UnknownUnknown A

Unknown B

Unknown C

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Analyzing Model 1 – Saturated or Unsaturated?

10. Which of the solutions would you describe as unsaturated? Explain your reasoning.______________________________________________________________________________________________

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11. Which of the solutions would you describe as saturated? Explain your reasoning.______________________________________________________________________________________________

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12. Which of the unknowns would you describe as ionic? As covalent? Explain your reasoning for each.

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13. How could you experimentally verify your answers above?______________________________________________________________________________________________

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14. If you are going to use conductivity to help determine the saturation of a solution, what can you say about the substance you use for this study?

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MODEL 2

Building Model 2 – Reaching Saturation

1. Connect the conductivity sensor to the data collection system.

2. Pour 100.0 mL of distilled water into a 150-mL beaker.

3. Add a stir bar to the beaker and place the beaker on a magnetic stirrer, placed on the base of the ring stand.

4. Use the ring stand and clamp to hold the conductivity sensor immersed in the water in the beaker and then turn on the magnetic stirrer. NOTE: Make sure the small hole on the side of the conductivity sensor is submerged. Gently tap the conductivity probe to remove any trapped bubbles.

5. Start data collection. Record the conductivity and the appearance of the solution in the Model 1 Data Table.

6. While constantly stirring, add 0.2 g of potassium bitartrate to the beaker.

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7. After 4 minutes, record the conductivity and appearance in the Model 2 Data Table.

8. With constant stirring, add an additional 0.2 g of potassium bitartrate to the beaker. After 4 minutes, record the conductivity and appearance of the solution.

9. Continue this process of adding 0.2 g of potassium bitartrate to the beaker and stirring for 4 minutes until 1.2 g have been added.

10. Make particulate level drawings of the solutions before potassium bitartrate is added, after 0.6 g are added, and after 1.2 g are added.

11. Sketch a graph of conductivity on the y-axis and total mass of potassium bitartrate added on the x-axis.

12. Do not discard the contents in your beaker. Save the solution for Model 3.

Model 2 – Reaching SaturationTable 2: Model 2 Data Table—Detecting the saturation point

Trial Total Potassium Bitartrate Added

(g)

Conductivity Appearance Saturated or Unsaturated?

1

2

3

4

5

6

7

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SOLUBILITY / STUDENT HANDOUT

Analyzing Model 2 – Reaching Saturation

13. In the Model 2 Data Table, describe the solution after each trial as either saturated or unsaturated.

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14. What is the approximate mass of KHC4H4O6 required to saturate 100 mL of water? Explain your reasoning using at least two instances of data-based evidence.

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MODEL 3

Building Model 3 – Quantifying Solubility

1. Use the following steps to remove any solids from the solution you saved from Model 2. NOTE: Do not add any water to the filtrate.

a. Assemble a filtration setup as in the diagram shown at the right, using a 125-mL Erlenmeyer flask, filter paper, and a funnel.

b. Fold the filter paper and place it into the funnel.

c. Slowly pour the contents of the beaker into the funnel. Do not over fill the filter paper.

d. Continue until all of the filtrate has moved through the filter paper into the flask.

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2. Is the filtered solution saturated or unsaturated? ______________________________________________________________________________________________

3. Insert the magnetic stir bar into a clean, dry, 150-mL beaker. Then, using a Mohr pipet, deliver 25 mL of the filtered potassium bitartrate solution from the Erlenmeyer flask into the beaker. Save the rest of the solution for a second trial.

4. Add approximately three drops of phenolphthalein indicator to the potassium bitartrate solution in the beaker.

5. Use a ring stand to set up a buret, as shown.

6. Rinse the buret:

a. Open the stopcock and then place a waste beaker under the buret.

b. Rinse the buret three times with small amounts of distilled water. This will remove any residue.

c. Rinse the buret three times with small amounts of 0.10 M NaOH. This will push out any water or other impurities.

7. Close the stopcock. Replace the waste beaker with the 150-mL beaker that contains the 25 mL of potassium bitartrate and phenolphthalein. Dispose of the waste according to your teacher’s instructions.

8. Fill the buret with 0.10 M NaOH. Record the initial volume in the Model 3 Data Table.

9. Turn on the magnetic stirrer. Then open the stopcock to achieve a steady drip of approximately one drop per second. Add the 0.10 M NaOH until you get a permanent, faint pink color in the solution, and then close the stopcock.

10. Record the final volume of sodium hydroxide in the Model 3 Data Table.

11. Repeat the previous steps for a second trial using another 25-mL sample of the KHC4H4O6 solution. Calculate the volume of titrant added for each trial and determine the average volume.

Model 3 – Quantifying SolubilityTable 3: Model 3 Data Table—Using titration to determine solubility

Parameters Trial 1 Trial 2 AverageVolume of KHC4H4O6 solution (mL)

Concentration of NaOH used (M)

Initial titrant volume (mL)

Final titrant volume (mL)

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Volume of titrant added to reach the endpoint (mL)

Analyzing Model 3 – Quantifying Solubility

12. Bitartrate is a weak acid and sodium hydroxide reacts with it according to the following net ionic reaction:

OH–(aq) + HC4H4O6–(aq) ⇌ H2O(l) + C4H4O62–(aq)

a. Calculate the average number of moles of OH– used in the titrations.

b. Calculate the average number of moles of HC4H4O6– in the 25.0-mL sample of the

solution.

c. Calculate the average molarity of the HC4H4O6– in the solution.

d. Calculate the average mass of the KHC4H4O6 dissolved in 100 mL of water.

13. Compare this mass of KHC4H4O6 to the approximated mass in Model 2.______________________________________________________________________________________________

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SOLUBILITY / STUDENT HANDOUT

Connecting to TheoryThe ability of a compound to dissolve in a solvent is its solubility. The solubility of an ionic compound refers to the maximum amount of solute that can dissolve in a given amount of solvent at standard temperature and pressure. The solution of an ionic compound containing the maximum amount of solute is known as a saturated solution and is in a state of equilibrium between dissolved and undissolved solute. The solubility product constant, Ksp, is a temperature-dependent constant that refers to this state. If a salt, MxAy, dissociates into cations [Mm+] and anions [Aa–] the expression for the solubility product will be

where “[ ]” indicates the molar concentration of the ion in solution. For example, the equation below describes the equilibrium of a saturated solution of calcium carbonate in water:

CaCO3(s) ⇌ Ca2+(aq) + CO32–(aq)

Calcium carbonate dissolves in the solution in the forward reaction and at the same time calcium carbonate precipitates from the solution in the reverse reaction. When both reactions occur at the same rate, a state of equilibrium is established. The Ksp expression for calcium carbonate would be:

Applying Your Knowledge – Determining Solubility Equilibrium

1. Write the Ksp expression for potassium bitartrate using the following equilibrium reaction:KHC4H4O6 (s) ⇌ K+ (aq) + HC4H4O6

– (aq)

2. Based on the formula of potassium bitartrate and the average molarity at saturation of bitartrate from Model 3, what are the molarities of potassium ions and the bitartrate ions in a saturated solution?

a. Calculate the value of Ksp for potassium bitartrate.

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3. Copper(II) tartrate, CuC4H4O6, dissolves in water according to the following equation CuC4H4O6(s) ⇌ Cu2+ (aq) + C4H4O62–(aq)

The Ksp of copper(II) tartrate is 4.0 × 10–4.

a. What is the Ksp expression of copper(II) tartrate?

b. What is the molarity of a saturated solution of copper(II) tartrate?

c. If 0.200 grams of copper(II) tartrate are added to 100 g of water, is the solution saturated?

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