Acid Strength

Purpose

To illustrate the difference in strength between a weak acid (acetic acid) and a strong acid (hydrochloric acid).

Procedure

Conductivity Meter

Setup:

• Use a light bulb conductivity meter with samples of acetic acid (HOAc) and hydrochloric acid (HCl) in solution.

Observation:

• Show the difference in intensity of light when both acids are measured.
• Discuss why the difference exists (HCl dissociates into more ions).

Serial Dilution:

• Perform a serial dilution of both acids until the light bulbs light up to an equal degree.
• Optionally, calculate relative strength and dissociation constant.
• If needed, “juice” the dilute HOAc with NaCl crystals (from Shakhashiri).

pH Rainbow Tube

Setup:

• Stopper one end of a diffusion tube tightly.
• Place the stoppered end into a large beaker to catch any leaks.
• Clamp the tube in the middle using a ring stand.

Procedure:

• Add a pellet of NaOH into the tube.
• Slowly pour in deionized water (dI water) mixed with universal indicator.
• Carefully add 2 drops of concentrated HCl to the top of the tube without agitating.
• Clamp vertically to the ring stand and observe the rainbow formation.

Dry Ice and NaOH

Setup:

• Fill a 500 mL or 1 L graduated cylinder with 1.0 M NaOH.
• Add universal indicator.
• Introduce dry ice into the cylinder.

Acidity Reactions

Setup:

• Use 250 mL graduated cylinders with the following solutions:
  • 1.0 M boric acid
  • Ethanol
  • 1.0 M acetic acid (HOAc)
  • 1.0 M hydrochloric acid (HCl)

Observation:

• Add magnesium (Mg) turnings to each solution and watch the reactions.

Titration Curves

Setup:

• Use a Vernier drop counter:
  • Calibrate the drop counter.
  • Plug it into DIG 1.
  • Mount it on a ring stand with a burette filled with 0.5 M NaOH.
  • Place a 10 mL graduated cylinder under the drop counter.
  • Record the initial volume of the titrant.

Procedure:

• Press sensors to calibrate the drop counter and start the process.
• Start adding titrant at about 1 drop/second until the 10 mL level is reached.
• Turn off the burette and record the burette reading.
• Enter the exact volume into the box on screen.

pH Measurement:

• Attach a pH electrode to CH 1 and mount it on the drop counter.
• Calibrate and wash the electrode with deionized water.
• Place 10.0 mL of analyte (0.5 M HCl, 0.5 M HOAc, or 0.17 M H3PO4) into a 150 mL beaker.
• Add about 30 mL deionized water and a stir bar.
• Place the beaker on a magnetic stirrer under the drop counter.
• Immerse the pH electrode so it is protected from the stir bar but fully immersed in the analyte.
• Hit start and begin adding titrant.
• Let it run until a clear titration curve is obtained.

Data Analysis:

• Obtain the second derivative by adding an additional column to the data table.