Molecular Weight and pKa of an Unknown Acid

Molecular Weight and pKa of an Unknown Acid

General Information

  • Purpose: To determine the molecular weight and pKa of a given unknown monoprotic acid through titration.

  • Materials:

    • Standard NaOH solution of known concentration available for titration.

    • A solid, water-soluble monoprotic acid.

Titration Procedure

  1. Sample Preparation:

    • Accurately weigh between 0.5 to 0.7 grams of the unknown acid.

    • Dissolve the sample in approximately 20 mL of deionized water (dH2O). Note that this volume does not need to be precise.

  2. Titration Steps:

    • Begin titrating by adding NaOH incrementally to achieve an increase in pH of 0.5 units at a time until the pH reaches about 6.5.

    • After each 0.5 unit increase in pH, record the following in your lab notebook:

      • Volume of NaOH added

      • Corresponding pH value

    • Once the pH is 6.5:

      • Proceed to add NaOH in increments of 5 drops until the pH reaches about 10.5.

      • Continue to record the volume of NaOH and pH at each addition.

    • Continue titrating until the pH reaches 12.0, recording all volume and pH data throughout the process.

  3. Graphing the Titration Curve:

    • Plot the graph of the titration curve using the recorded pH values and volume of NaOH added.

    • Determine the equivalence point from the plotted titration curve.

Reporting Requirements

  • In addition to the completed worksheet for the titration, you must:

    • Submit an Excel graph of your titration curve.

    • Clearly label:

    • Volume at the equivalence point

    • Volume at the half-equivalence point

    • Demonstrate on the curve how the pKa was determined.

Safety Precautions

  • Always wear safety glasses whenever handling chemicals in the laboratory.

Calibrating the Vernier pH Probe

  1. Obtain two different standard buffer solutions, such as pH 4.00, 7.00, or 10.00.

  2. Connect the pH probe to the Vernier device.

  3. Navigate to the sensors tab and select the calibrate function for the pH probe.

  4. Choose calibrate now.

  5. Remove the pH probe from its storage solution and rinse it with dH2O.

  6. Place the probe into one of the standard buffer solutions and enter the pH value in the Vernier tool. Once the voltage reading stabilizes (displayed at the top right of the screen), select KEEP (do not click OK here).

  7. Rinse the probe with dH2O again, then place it in the second standard buffer solution. Enter the pH in the Vernier and select KEEP after stabilization.

  8. Press OK to finalize calibration. Your pH probe should now be ready for titration use.

  9. Make sure to rinse the probe with dH2O between each pH measurement.

  10. Upon completion of the experiment, return the pH probe to its storage solution. If the storage solution requires replacement, contact your instructor or TA.