Summary

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 K_sp of calcium hydroxide. K_sp will be calculated using the molar concentration of ions in the solution and the equilibrium expression for the dissociation of calcium hydroxide.

Grade Level

High School

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 K_c or K_p based on experimental observations of concentrations or pressures at equilibrium.

NGSS Alignment

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

Objectives

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 K_sp 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.

Chemistry Topics

This lab supports students’ understanding of

• Solutions
• Solubility
• Conductivity
• Concentration
• Equilibrium
• Equilibrium Constant
• Stoichiometry

Time

Teacher Preparation: 45 minutes
Lesson: 90 minutes (see details in Teacher Notes)

Materials

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

Safety

• 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 Notes

• 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 K_sp expression for the dissociation, and calculate K_sp.
• 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.