In this lab, students will predict and measure the relationship between the conductivity of a solution of calcium hydroxide and the mass of substance added to it. From the relationship, students will determine solubility and Ksp of calcium hydroxide. Ksp will be calculated using the molar concentration of ions in the solution and the equilibrium expression for the dissociation of calcium hydroxide.
AP Chemistry Curriculum Framework
This lab supports the following units, topics, and learning objectives:
- Unit 7: Equilibrium
- Topic 7.4: Calculating the Equilibrium Constant
- TRA-7.B: Calculate Kc or Kp based on experimental observations of concentrations or pressures at equilibrium.
- Topic 7.4: Calculating the Equilibrium Constant
This lab will help prepare your students to meet the performance expectations in the following standards:
- HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
- HS-PS1-6: Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.
- Scientific and Engineering Practices:
- Using Mathematics and Computational Thinking
- Analyzing and Interpreting Data
By the end of this lab, students should be able to
- Determine the solubility of a sparingly soluble substance in grams for every liter.
- Calculate the molar solubility of a sparingly soluble substance in moles for every liter.
- Calculate the molar concentration on ions in a saturated solution of calcium hydroxide.
- Analyze data from a conductivity vs. mass of solute graph.
- Calculate the Ksp of the sparingly soluble substance using an equilibrium expression for the dissociation of the substance.
- Predict what factors may influence the dissociation of the substance.
This lab supports students’ understanding of
- Equilibrium Constant
Teacher Preparation: 45 minutes
Lesson: 90 minutes (see details in Teacher Notes)
The materials needed reflect the amount for one setup using a demonstration-type approach. Students can do this activity in groups, but more equipment will be needed.
- Conductivity probe (Vernier or similar), one
- Probe clamp (to support Probe away from stir bar), one
- Ring stand, one
- User interface for the probe (Vernier Labquest or similar), one
- Simple LED type conductivity device (available from Flinn, or make your own), one
- 1-liter beaker, one
- 0.500 L of distilled water
- Magnetic stirrer with stir bar, one
- 12 – 50 mg samples of Ca(OH)2 previously weighed out
- Always wear safety goggles when handling chemicals in the lab.
- Students should wash their hands thoroughly before leaving the lab.
- When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
- Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
- Teacher Preparation: The teacher will need time to weigh out ten to twelve 50-mg samples of calcium hydroxide. This may take up to 30 minutes to complete. 15 minutes or so will be needed to gather the other equipment and glassware necessary.
- Lab procedure: 20-25 minutes are necessary for all of the additions of the calcium hydroxide to the solution. After each addition of calcium hydroxide, it will be necessary to wait a minute for the conductivity reading to level off. Students will need 15 minutes to plot the data and determine the solubility of the solution. This leaves a 45-50 minutes for students to determine molar solubility, write the Ksp expression for the dissociation, and calculate Ksp.
- Before this activity, students should be able to: write a balanced chemical equation, calculate the number of moles from mass, calculate molarity, and be familiar with the rules for writing equilibrium expressions.
- Before the lab, I demonstrate the difference between a soluble compound and an insoluble one – compare the solubility of two compounds such as calcium hydroxide vs. table salt. A simple conductivity tester can be used to show that both solutions contain dissolved ions, but one solution has more solute that dissolves (soluble) than the other (insoluble). Students should also be able to observe that the soluble solution has no remaining solute visible, whereas the insoluble solution has solute remaining in the container.
- The data collection for this activity is done as a whole group - that way you only need one conductivity meter, and the data can be collected in a more controlled environment. Data is shared on a large whiteboard for students to record after every addition of calcium hydroxide.
- Often this experiment is done using titration – I like the conductivity method instead because students can actually see the point when the dissolving process stops (from observation and from the data), and the excess begins to remain in the undissolved phase.
- The tip of the conductivity probe should be submerged in 500 mL of distilled water. Add a stir bar and place the beaker on a magnetic stirrer set to a reasonable speed.
- Before starting the experiment, zero the meter so that the conductivity of the water is zero. Add 50 mg of calcium hydroxide, let the solution stir until the conductivity reading levels off, and then record the conductivity of the solution. Continue with additions of calcium hydroxide until the conductivity shows very little change for several data points in a row.
- Using a graphing utility, or by hand, students should plot a graph of conductivity vs. amount of calcium hydroxide added to 500 mL of water at room temperature. The graph will look similar to the one below:
- Students should create two best-fit lines, one on the sloped data points, and one on the horizontal data points, like this:
- The point of intersection between the two best-fit lines can be used to determine the total mass that the 500 mL of water will dissolve at a particular temperature. Note that this is a typical point for students to differ, particularly if graphing by hand. You can tell them to analyze the graph this way, but I prefer to let students come up with a method to determine the maximum mass of solute that will dissolve into the solution themselves.
- LoggerPro software from Vernier allows you to make two best fit lines, but this can be done with a ruler and a printed graph from Graphical Analysis or with a graph made by hand.
- A complete Answer Key document created using sample data is available for download for teacher reference. Note that the collected data in the answer key varies from accepted data values for calcium hydroxide.
For the Student
Kidney stones are solid masses formed in the kidney when there are high levels of certain substances present, such as calcium oxalate. Kidney stones can be caused by a variety of factors including certain medical conditions, obesity, consumption of too much protein or sodium, and dehydration from not drinking enough fluids.
The photograph on the right shows a kidney stone in this person’s left kidney. Kidney stones may sometimes pass through the urine stream on their own. But if they are large like this one, they get stuck in the kidney, and a medical procedure is necessary to remove them. Kidney stones are made up of insoluble ionic compounds, like calcium oxalate, which don’t readily dissolve in water.
Calcium hydroxide and calcium carbonate are sparingly soluble compounds – that is, they don’t readily dissolve in water either. Good thing too, because the ocean’s coral reefs are made of calcium carbonate. If these calcium compounds were soluble in water, then the ocean’s coral reefs would dissolve.
The truth is that a little bit of these calcium compounds will and do dissolve in water. The question becomes, how much of one of these calcium substances dissolve in water? The answer, of course, is: it depends.
To get an idea of how much substance will dissolve in water, you will use a conductivity meter. The conductivity meter is an electronic device that measures the electrical conductivity of a solution that has dissolved charged particles. You will first determine whether there is a relationship between the mass of substance dissolved in water and the electrical conductivity of the solution; then you will determine the solubility of a specific substance, calcium hydroxide, Ca(OH)2.
You will determine:
- If there is a relationship between solution conductivity and the mass of Ca(OH)2 added to water.
- If there is a maximum amount of Ca(OH)2 that can be dissolved in water, how much can be dissolved, and what is its significance.
- The numerical value of the molar solubility of Ca(OH)2.
- The solubility product constant, or Ksp, of Ca(OH)2.
The solubility product constant, or Ksp, of a substance is the equilibrium constant for the dissolving of a substance. It indicates the extent to which a substance dissolves, or is soluble, in water. Ksp is in important tool for determining the solubility of a substance, predicting the likelihood of a precipitation reaction, and in separation of ions in chemical qualitative analysis. By comparing Ksp values of substances, you can gain an idea of which is slightly more soluble than the other. The higher the value, the more ions are released from the seemingly insoluble salt. Solubility product constant has useful industrial applications.
- Write a balanced chemical equation for the dissociation of calcium fluoride in water.
- Determine the molar solubility of calcium fluoride, (in moles for every liter) if 1.6 mg of the substance can be dissolved in 100 mL of water at 20°C.
- Write the equilibrium expression for the dissociation of calcium fluoride in water.
- Using stoichiometry, calculate the molar concentration (in moles for every liter) of each of the ions in the solution of calcium fluoride based on the molar solubility.
- Using the equilibrium expression and the molar concentration of calcium and fluoride ions, calculate the equilibrium constant, or Ksp, for the dissociation of calcium fluoride in water.
What is the numerical value of the solubility product constant of calcium hydroxide?
As a class, we will observe and collect data for the dissolving of calcium hydroxide in water. We will measure the conductivity of the resulting solution as 50 mg samples of calcium hydroxide are added to 500 mL of distilled water. We will then graph the data and determine the Ksp of calcium hydroxide.
In an organized and efficient way, collect the data for conductivity vs. amount of calcium hydroxide added to the water.
In the space below, or by using a graphing utility, create a graph of conductivity vs. mass:
In your group, come up with a method to determine the solubility of calcium hydroxide in grams for every liter.
- Write a balanced chemical equation for the dissociation of calcium hydroxide in water.
- Calculate the molar mass of calcium hydroxide.
- Determine the molar solubility of calcium hydroxide, (in moles for every liter) from the solubility in grams for every liter that you determined from the graph of the data.
- Calculate the molar concentration of each of the ions in the saturated solution of calcium hydroxide.
- Write the Equilibrium Expression for the dissociation of Calcium hydroxide in water.
- Using the equilibrium expression and the molar concentration of each ion at equilibrium, calculate the equilibrium constant, or Ksp, for the dissociation of calcium hydroxide in water.
- The accepted value for the molar solubility of calcium hydroxide at 20 oC is 0.023 mol/L. Determine the percent error using your data.
- The accepted value for the dissociation of calcium hydroxide in water at 20 oC is 1.3x10-6. Determine the percent error compared to your calculation.
- What happened to the extra calcium hydroxide that did not dissolve in the water?
- What factor or factors may affect the solubility of calcium hydroxide, or any other substance?
- In what way does drawing the two best fit lines by hand affect the value of the calculated Ksp?