Question

Experiment 2: Using the Secondary Standard to Determine the Concentration of an Acid

1. In an Erlenmeyer flask, add 25 mL of Unknown #1 concentration of acetic acid (CH₃COOH) and 2 drops of phenolphthalein indicator.

2. Coarse Titration:

(a) Take a burette from the Containers shelf and place it on the workbench. Fill the burette with 50 mL of the standardized sodium hydroxide solution. Record the initial burette reading. Place the Erlenmeyer flask on the lower half of the burette.

(b) Perform a coarse titration, adding large increments of the sodium hydroxide solution from the burette by pressing and holding the black knob at the bottom of the burette. Each time you add the sodium hydroxide solution, check the volume remaining in the burette. As the sodium hydroxide is added to the acetic acid solution the pH increases. Watch for a change of the phenolphthalein color in the Erlenmeyer flask. The pink color will appear in Erlenmeyer flask when the endpoint is either reached or crossed. Record the burette volume at which this occurs.

(c) Place the Erlenmeyer flask and the burette in the recycling bin.

3. Fine Titration:

(a) Set up the tiration as before: (i) An Erlenmeyer flask filled with 25 mL of unknown #1 concentration of acetic acid and 2 drops of phenolphthalein indicator. (ii) A burette filled with 50 mL of the standardized sodium hydroxide solution. Record the initial burette reading. (iii) Place the Erlenmeyer flask on the lower half of the burette.

(b) Click and hold the black knob of the burette to quickly add enough standard sodium hydroxide solution to just get into the range of the coarse titration: 1 mL BEFORE the pink endpoint. This is near, but not yet at, the titration's endpoint.

(c) Add sodium hydroxide solution in small increments, down to one drop at a time, record the volume of the pink endpoint.

(d) Place the Erlenmeyer flask and the burette in the recycling bin.

4. Repeat the titration two more times for a total of three trials. Use 30mL of acetic acid for trial 2 and 35mL for trial 3. Calculate the acid concentration of each trial.
[If you run out of NaOH, simply prepare a second solution as described in step 2 of Exp. 1. You don’t need to standardize it a second time. Assume the standardized concentration is the same as before.]

Need help with the following!

Trial 1              Trial 2              Trial 3

a) Volume of acid (mL)                                     25.1mL             30.1mL            35.1mL

b) Volume of NaOH titrated (mL)                     _____              _____              _____

c) Moles of NaOH titrated                                 _____              _____              _____

d) Moles of acid                                               _____              _____              _____

e) Concentration of acid (M)                             _____              _____              _____

f) Average acid concentration (M)                                             _____

Using your Trial 1 data, show you work for calculating each of the following:

Moles of NaOH titrated

Moles of acid

Concentration of acid

Questions

1. Similar to other acids you've encountered in this class (HCl, HNO₃, etc.), benzoic acid is monoprotic (HA), meaning that only one of its hydrogens dissociates in water. With this in mind, write the net ionic equation for the reaction performed during this titration (as discussed in lecture).  

2. After dissolving the acid with water, a student noticed some undissolved solid, but continued with the titration as instructed by the procedure. He titrated until the pink color persisted for 30 seconds and recorded the volume of titrant required. Preparing for the second determination, he set the reaction solution aside and added some more NaOH to the buret. He was about to pour the reaction solution down the drain, so he could rinse and reuse the flask, when he saw that the pink color had disappeared!

a) What happened?

b) What can the student do to correct this error and salvage this trial, so he doesn't have to redo it? What corrections or modifications will have to be made to his recorded data?

3. Another student began adding titrant to the acid solution, but stopped when she suddenly realized that she forgot to add the 3 drops of phenolphthalein. When she did, the solution immediately turned dark red!

a) What happened?

b) What can the student do to correct this error and salvage this trial, so she doesn't have to redo it? What corrections or modifications will have to be made to her recorded data?

Answer 1

Answer 1) Benzoic acid reacts with NaoH in following way

C₆H₅COOH + NaOH---------------> C₆H₅COONa + H₂O

Answer 2)

a) Student has not dissolved benzoic completely, due to which only dissolved benzoic acid has reacted with NaOH. But at equivalence point the remaining benzoic acid has dissolved.Now reaction mixture becomes acidic, pH decreases and pink color disppears.

b) Student should not throw reaction mixture, instead add the few more drops of NaOH carefully till equivalence point is reached( apperance of pink color). While noting the burette reading, student should discard earlier reading and must note down reading after confirming the persistance of pink color.

Answer 3)

a) The reaction mixture has crossed the equivalence point.

b) To correct the error, add known volume of aliquot again in erlenmayer flask, now the color in solution will diappear. Continue the titration till the appearnce of pink color,but dont forget this reading corresponds to new volume. To find the concentration of unknown aliqout you have to take new volume while calculation.

I will explain with example

For Suppose you have taken 10ml of aliquot initially.You can add another 10ml . now the new volume would be 20ml and burette reading after equivalence corresponds to 20ml