Question

Procedure

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

Part 1: Prepare the NaOH Solution

1. Take a 250 mL volumetric flask from the Containers shelf and a balance from the Instruments shelf and place them on the workbench. Zero the mass of the volumetric flask on the balance.

2. Take sodium hydroxide from the Materials shelf and add 1 g to the flask. Record the mass from the balance display.

3. Place the volumetric flask on the workbench. Add 100 mL water from the Materials shelf to the volumetric flask to dissolve the solid NaOH. Add water again up to the maximum volume allowed. This produces a total volume in the flask of 250.00 mL and the meniscus of the liquid is level with the volume mark on the neck of the flask. Note: It is important to dissolve the solid before filling a volumetric flask to the line because the density of the overall solution is different from the densities of the separate solid and liquid. The volumetric flask is designed to contain a specific volume of liquid. In a typical lab, if the liquid level becomes greater than the mark on the flask, the concentration is no longer accurately known and the solution needs to be remade. In the virtual lab there is a limit on the volume of the flask that prevents you from adding liquid above the line.

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

5. 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 to reference later.

Part 2: Perform a Coarse Titration

1. Take a clean 250 mL Erlenmeyer flask from the Containers shelf and place it on the balance. Zero the mass of the flask on the balance.

2. Add the required mass of benzoic acid calculated in step 5 of Part 1 to the Erlenmeyer flask. Record the mass value from the balance and move the flask to the workbench.

3. Take water from the Materials shelf and add 100 mL to the Erlenmeyer flask. The 100 mL of water should be enough to dissolve the solid benzoic acid.

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

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

6. Take a burette from the Containers shelf and place it on the workbench. Place a 50 mL beaker from the Containers shelf underneath the tip of the burette as a waste container.

7. Take the prepared sodium hydroxide solution from the workbench and add 52 mL to the burette. Click and hold the stopcock of the buret for about 4 seconds to drain liquid from the buret into the waste container until the titrant level is at or below the 0 mL mark. Double-click on the buret to read and record the liquid volume at the meniscus to 2 decimal places. This initial volume reading does not need to be exactly 0.00 mL.

8. Remove the waste container and move the Erlenmeyer flask onto the base of the burette.

9. Perform a coarse titration, adding large increments of titrant (~2 mL) by pressing and holding the stopcock at the bottom of the burette for about 4 seconds. Pause after each dispensation. Record the volume dispensed.

10. 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.

11. Stop once you reach the end point. 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.

12. Discard the Erlenmeyer flask by emptying it into the waste then dragging it to the sink.

Part 3: Perform Fine Titrations

1. Prepare an Erlenmeyer flask as described in steps 1 - 5 in Part 2 of Experiment 1. The use of the pH meter is optional during the fine titrations.

2. Take the volumetric flask of sodium hydroxide solution and refill the burette to a total volume of about 3 mL less than the maximum volume. Drain the first few milliliters into the waste container until the titrant level is at or below the 0 mL mark. Double-click the burette to read and record the initial volume.

3. Remove the waste container and move the Erlenmeyer flask onto the base of the burette.

4. 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.

5. Add titrant one drop at a time using single clicks on the stopcock. This can be tedious, but if you click and hold you might miss the exact end point of the titration. Be sure to pause between each addition.

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

7. Clear your station by emptying the Erlenmeyer flask into the waste, then placing the flask in the sink.

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

9. 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.

Answer 1

Part 1

Weight of NaOH = 1 g

Molar mass = 39.997 g/mol

Moles of NaOH= Given wt / Molar mass

= 1 g / 39.997 g/ mol

= 0.025 mol

Volume of solution = 250 mL = 0.25 L

Molarity of NaOH solution = Moles / Volume (L)

= 0.025 mol / 0.25 L

= 0.1 M

Now,

C6H5COOH + NaOH C6H5COONa + H2O

Thus, 1 mol NaOH requires 1 mol benzoic acid for complete neutralization.

Mol of NaOH = 0.1 M * 0.02 L = 0.002 mol

Mol of benzoic acid = 0.002 mol

NOw,

0.002 mol = Wt of benzoic acid / Molar mass

0.002 mol = Wt of benzoic acid / 122 g/mol

WT of benzoic acid = 122* 0.002 = 0.244 g