Question

Experiment 1: Standardize an NaOH Solution Using Benzoic Acid as Primary Standard

Part 1: Prepare the NaOH Solution

Take a 250 mL beaker from the Containers shelf and place it on the workbench.

Take sodium hydroxide from the Materials shelf and add 0.800 g to the beaker.

Take water from the Materials shelf and add 200 mL to the beaker.

Calculate the expected concentration of the sodium hydroxide solution. The molar mass of NaOH is 39.997 g/mol.

Calculate and record the expected mass of benzoic acid required to react with 20.00 mL of a 0.100 M sodium hydroxide solution. Record the mass of benzoic acid using three significant digits

Part 2: Perform a Coarse Titration

Take a clean Erlenmeyer flask from the Containers shelf and place it on the workbench.

Take water from the Materials shelf and add 100 mL to the Erlenmeyer flask.

Add the required mass of benzoic acid calculated in step 5 of part 1 to the Erlenmeyer flask. The 100 mL of water should be enough to dissolve the solid benzoic acid.

Take phenolphthalein from the Materials shelf and add 2 drops to the Erlenmeyer flask.

Take a pH meter from the Instruments shelf and place it into the flask. Record the initial pH of the solution.

Take a burette from the Containers shelf and place it on the workbench.

Take the prepared sodium hydroxide solution from the workbench and add 50 mL to the burette. Pass the mouse cursor over the burette and a gray tool tip will briefly display the total volume in the burette as well as the volume dispensed. Record these initial volumes.

Move the Erlenmeyer flask onto the base of the burette.

Perform a coarse titration, adding large increments of titrant (~ 2 mL) by pressing and holding the stopcock at the bottom of the burette for 1-2 seconds. Pause after each dispensation. Record the volume dispensed.

Check if the end point has passed. When the reaction reaches the end point, the solution changes color. Also, as sodium hydroxide is added to the benzoic acid solution, the pH increases.

Stop once you reach the end point. Record the volume dispensed and pH at which this occurs. Record both the last dispensed volume where the solution was colorless (right before the end point) and the first dispensed volume where the solution changed color. You will use the volume dispensed before the end point in the fine titration.

Discard the Erlenmeyer flask by dragging it to the recycling bin beneath the workbench.

Part 3: Perform Fine Titrations

Prepare an Erlenmeyer flask as described in steps 1 - 5 in part 2 of experiment 1.

Take sodium hydroxide solution from the workbench and refill the burette to a total volume of 50 mL. Record this initial volume.

Move the Erlenmeyer flask onto the base of the burette.

Click and hold the stopcock of the burette to quickly add the before the end point volume of sodium hydroxide determined in the coarse titration.

Add titrant in small increments, down to one drop at a time. This can be tedious, but if you click and hold you might miss the exact end point of the titration.

When the solution changes color, stop adding titrant. Record the pH and volume dispensed.

Clear your workstation by dragging the Erlenmeyer flask to the recycling bin.

Repeat the fine titration two more times, and record the results.

Calculate the concentration of the sodium hydroxide solution. The rest of the sodium hydroxide solution can now be used in further lab work as a secondary standard with a reliably known concentration equal to the average of the three titrations.

Experiment 2: Use the Standardized NaOH Solution to Determine the Concentration of an Acid

Part 1: Perform a Coarse Titration

Take an Erlenmeyer flask from the Containers shelf and place it on the workbench.

Take acetic acid #1 (unknown concentration) from the Materials shelf and add 25 mL to the flask.

Take phenolphthalein from the Materials shelf and add two drops to the flask.

Take a pH meter from the Instruments shelf and place it into the Erlenmeyer flask. Record the pH.

Take the NaOH solution from the workbench and refill the burette to a volume of 50 mL. Pass the mouse cursor over the burette and a gray tool tip will briefly display the total volume in the burette as well as the volume dispensed. Record these volumes.

Move the Erlenmeyer flask onto the base of the burette.

Perform a coarse titration, adding large increments of titrant (~ 2 mL) from the burette by pressing and holding the stopcock at the bottom of the burette. Pause after each dispensation. Record the volume dispensed.

Check if the end point has passed. When the reaction reaches the end point, the solution changes color. Also, as more sodium hydroxide is added to the acetic acid solution, the pH increases.

Stop once you reach the end point. Record the dispensed volume and pH at which this occurs. Record both the last dispensed volume where the solution was colorless (right before the end point) and the first dispensed volume where the solution changed color. You will use the volume dispensed right before the end point in the fine titration.

Clear your station by dragging the Erlenmeyer flask to the recycling bin.

Part 2: Perform Fine Titrations

Set up the titration as described in steps 1 – 6 in part 1 of experiment 2.

Click and hold the stopcock of the burette to quickly add the before the end point volume of sodium hydroxide determined in the coarse titration.

Add sodium hydroxide in small increments, down to one drop at a time. This can be tedious, but if you click and hold you might miss the exact end point of the titration.

When the solution changes color, stop adding titrant. Record both the pH and volume dispensed.

Clear your station by dragging the Erlenmeyer flask to the recycling bin.

Repeat the fine titration twice more