PROCEDURES

Preparation of stock base solution:

As before, you will need to make a stock 0.5 M NaOH solution for titration.  This is performed by combining approximately 35-36 mL of 7M NaOH stock solution with approximately 500 mL of distilled water.  Mix well to ensure uniformity of the solution.  This solution will be used for both standardization and molar mass determination.

Standardization of the ~0.5 M NaOH solution:

In this section, you will determine the precise concentration of the NaOH solution you just made.  If properly diluted, you should have an approximate 0.5 M solution.  Prepare the burette by first rinsing with a 10 mL portion of water followed by three 5 mL portions of the prepared 0.5 M base solution.

Dissolve approximately 1.5 grams of potassium hydrogen phthalate (KHP, 204.23 g/mol) in approximately 50 mL of distilled water in a 250-mL Erlenmeyer flask.  Record the exact mass of the KHP in your data table and calculate the number of moles used.  Add a few drops of phenolphthalein solution (indicator).

Titrate the KHP solution with your prepared NaOH solution until a light pink color is produced that persists for 30 seconds.  Record the amount of NaOH solution that was required to neutralize the KHP solution to the closest 0.01 mL.  Using this information, calculate the number of moles of NaOH used and the molarity of the NaOH solution.  Repeat this process three more times for a total of four titrations.  Average the closest three titrations.

Molar mass determination of unknown carboxylic acid:

In this section, you will determine the molar mass of an unknown carboxylic acid using the standardized NaOH solution.  This process should occur concurrently with the standardization or you will not finish the experiment on time.

In a 250-mL Erlenmeyer flask, add between 1.0 - 1.3 grams of your unknown carboxylic acid and dissolve in approximately 50 mL of distilled water.  A few drops of ethanol may be needed to fully dissolve the carboxylic acid.  Add a few drops of phenolphthalein solution (indicator).

Titrate the unknown carboxylic acid solution with your prepared NaOH solution until a light pink color is produced that persists for 30 seconds.  Record the amount of NaOH solution that was required to neutralize the unknown carboxylic acid to the closest 0.01 mL.  Using this information, calculate the moles of NaOH used, moles of unknown carboxylic acid used, and finally the molar mass of the unknown carboxylic acid.  Repeat the process three more times for a total of four titrations.  Average the closest three titrations.  From a list provided by your instructor, identify the carboxylic acid you have.

Your unknown carboxylic acid choices are glutaric acid, maleic acid, or malonic acid (structures below).