Experimental Procedure

Calibration of the pH Probe (This must be done before moving on to the procedure).

• Do NOT allow the pH probe to sit outside of solution at any time.  The probe must be submerged to keep it in working order.  The bulb of the glass should NOT touch the bottom of the solution vessel.  The probe should always be stored in a buffer solution when not actively seeking a measurement.
• Connect the pH probe to its console and plug the instrument in.
• Select “Standby” to turn on the pH probe.
• Carefully uncap the probe and hold it over a small beaker. Rinse the residual buffer solution from the pH probe with water by spraying from a deionized water bottle.
• Place the probe into the pH 4.00 buffer solution and wait for the pH reading to stabilize.  Once the numbers are stable, select STD (standardize) to begin calibrating.  *If after the pH reading reappears it is not between 3.95 and 4.05, select STD again.
• Once the probe has been calibrated at a low pH range, you will need to calibrate it with a high pH (pH = 10.0 buffer).
• Carefully rinse the residual pH = 4.00 buffer from the probe by holding the prove over the waste beaker and spraying it with deionized water.
• Place the probe into the pH 10.00 buffer solution and wait for the pH reading to stabilize.  Once the numbers are stable, select STD (standardize) to begin calibrating.  *If after the pH reading reappears it is not between 9.95 and 10.05, select STD again.
• If at any time you get an ELECTRODE ERROR message, select Standby.  Wait 5 seconds and hit Standby again.  If the error message is still there, consult with your instructor.
• Remove residual solution from the probe by spraying with deionized water over a beaker ANY time you switch solutions.

Part A:  Calculating the Molar Solubility and Solubility Constant for Ca(OH)₂

• *Be careful that when pouring the calcium solutions you do not agitate the precipitate at the bottom of the bottles.  The lab works best if solid calcium hydroxide does not get into the solution you will test.
• Carefully decant ~ 20-30 mL of the saturated Ca(OH)₂ into a clean, dry 50mL beaker.  The exact amount does not matter as long as it is deep enough to cover the bulb of the probe.
• Remove the pH probe from the buffer solution and rinse by spraying with deionized water (over your “rinse” beaker).
• Hold the probe in the solution until the pH reading stabilizes.  Record this number in the data section.
• Rinse the probe and place it back into the buffer solution.  If necessary, recalibrate the solution.
• Repeat the steps above to collect a second pH reading.  Record this in your data section.
• Be sure to secure the probe in the buffer solution before moving on.
• Repeat steps 1-7 with the Ca(OH)₂ solution made in the presence of Ca²⁺ ions.
• Use the pH of your solution to calculate pOH, [OH^–], molar solubility and K_sp for your solution.  Show all calculations for full credit.

Part B:  Calculating the Molar Solubility and Solubility Constant for Ba(OH)₂ in the Presence of Ba²⁺

• *Be careful that when pouring the calcium solutions you do not agitate the precipitate at the bottom of the bottles.  The lab works best if solid calcium hydroxide does not get into the solution you will test.
• Carefully decant ~ 20-30 mL of the saturated “Ba(OH)₂ with Ba²⁺” solution into a clean, dry 50mL beaker.  The exact amount does not matter as long as it is deep enough to cover the bulb of the probe.
• Remove the pH probe from the buffer solution and rinse by spraying with deionized water (over your “rinse” beaker).
• Hold the probe in the solution until the pH reading stabilizes.  Record this number in the data section.
• Rinse the probe and place it back into the buffer solution.
• Use the pH of your solution to calculate pOH, [OH^–], molar solubility and K_sp for your solution.  Show all calculations for full credit.

Prelab Questions

*This prelab needs to be completed prior to lab.  Obtain a signature prior to leaving class for full credit.

• Students test a saturated solution of Cr(OH)₂ and find its pH to be 8.94.  Fill in the table below.

• Would the molar solubility of Cr(OH)₂ increase, decrease, or remain unchanged in the presence of NaOH?  Justify your answer.
• The Ksp for lead II hydroxide (Pb(OH)₂) is recorded as 1.3 x 10^-5.   What should the pH of a saturated solution be?

Experimental Data and Results

Part A:  Calculating the Molar Solubility and Solubility Constant for Saturated Ca(OH)₂ Solutions

*Show all calculations for full credit.

• If the K_sp value for Ca(OH)2 is 5.5 x 10^-6, what was the percent error in your measurements?

• How did the molar solubility of Ca(OH)2 change between the solutions?  Why did this happen?

Part B:  Calculating the Molar Solubility and Solubility Constant for Ba(OH)₂ in the Presence of Ba²⁺

*Show all calculations for full credit.

• If the Ksp value for Ba(OH)₂ is 7.11 x 10^-22, what was the percent error in your measurements?

• How did the molar solubility of Ba(OH)₂ change between the solutions?  Why did this happen?

Post-Lab Questions

• If tap water was used instead of deionized water, how would the molar solubility of calcium hydroxide be affected in the lab?  (Would it be too high, too low , or not affected?)  Explain your answer.

• If the solution of saturated Ca(OH)₂ was allowed to sit for several minutes between pouring it into the beaker for measurement and actually taking the pH measurement, how would the molar solubility be affected?  (Would it be calculated as too high, too low or not affected).  Explain your answer.

• Name at least two sources of error that could have occurred in your procedure.  How would each have affected your measurements?