Acid-Base Titration of Vinegar

Procedure:

1)    Check the set-up of your lab station.  Make sure you have two burets labeled acid and NaOH.  Make sure the burets have a minimum of 40 mL of either acid or base in them to start with.  You should have a beaker for waste, a wash bottle of distilled water and an Erlenmeyer flask in which to perform the titration.  You will also need phenolphthalein indicator solution in a dropper bottle.

2)    Record exact initial volume of acid buret.  Dispense about 10 mL HC₂H₃O₂ into Erlenmeyer flask (the exact amount is not critical – just make sure you record the exact amount you dispense).  Record the exact final volume of the buret.

3)    Add approximately 10 mL of distilled water to the Erlenmeyer flask (measure out 10 mL in a graduated cylinder, then pour into the flask) and 2-3 drops of phenolphthalein. 

4)    Record the exact initial volume of the NaOH buret.  Carefully add NaOH with constant swirling of the flask until a very pale, persistent pink color is reached.  This is your equivalence point (endpoint).  Record the exact final volume of the buret.

4b.  If you go past the equivalence point (very dark pink) add HC₂H₃O₂ into the flask until the

color clears (no more pink), then carefully add NaOH drop by drop until you reach the endpoint (pale, persistent pink).  Record your new final volumes of HC₂H₃O₂ and NaOH.

5)    Solutions may be emptied down the drain.  Rinse flask several times with distilled water.  It is not necessary to dry the flask.

6)    Repeat steps 2-5 two more times.  Add more vinegar and NaOH as necessary, using the correct funnel.

Data Collection

 Molarity of standard NaOH solution: ___________

Data Analysis

1.    Write and balance the neutralization equation for acetic acid and sodium hydroxide.

2.    Calculate the molarity of acetic acid for each trial.

3.    Using the concentration from each trial, calculate the average molarity of HC₂H₃O₂.

4.    The label for vinegar says that the acid has been diluted to 5% by mass, which translates into roughly 0.830 M.  Were your results accurate?  Were they precise?  Explain your answers to both.  (Remember the difference?  If you don’t remember, you better look it up!)

5.    Quantitatively express accuracy by calculating % difference for EACH trial and the average concentration of vinegar you found.