Formal Lab # 2 – Titration Lab: Quality Control of Vinegar

Introduction:

Every consumer product is required to have the minimum quantity of the active ingredient listed on the label. The process of checking the product is called quality control. In this experiment, you will determine the concentration of acetic acid in vinegar using acid-base titration. You will then compare the actual concentration of acetic acid in the vinegar to the required amount of 5%.

The balanced chemical equation for the titration of vinegar with sodium hydroxide is shown below:

CH3COOH (aq) + NaOH à NaCH3COO (aq) + H2O (l)

Question: What is the concentration (in mol/L) of acetic acid in a sample of vinegar from the school cafeteria?

Hypothesis:

Purpose:

  1. To determine the molar concentration of acetic acid in a sample of vinegar.
  2. To determine whether the concentration of the sample of vinegar matches the label on the bottle.
  3. To list errors in this laboratory and solutions to these errors in order to obtain better results.

SAFETY PRECAUTIONS:

Sodium hydroxide is corrosive. Any spills on skin or clothing, or in the eyes, should be washed immediately with lots of cold water and reported to your teacher. Eye protection must be worn at all times. Extra caution must be undertaken when handling fragile glassware, such as the burets that are being used in this laboratory.

PROCEDURE:

1.  Fill the burette with 5 mL of the 0.10 mol/L NaOH solution. Drain it into a waste beaker (labeled “NaOH waste”).

2.  Fill the buret to the top with the 0.10 mol/L NaOH solution. Drain it into the NaOH waste beaker till the liquid falls between 0.00 and 0.10 mL. Record the starting volume in the burette to the nearest 0.01 mL.

3.  Obtain 25 mL of the vinegar solution (which has been diluted for you by a factor of 10 since it is too concentrated to be titrated directly) using a graduated cylinder. Pour this vinegar solution into an Erlenmeyer flask. This is the volume of the acid that should be used in the calculations.

4.  Add two to three drops of phenolphthalein indicator to the diluted vinegar solution. DO NOT ADD MORE THAN THIS. It should remain clear.

5.  Titrate the diluted vinegar sample with the standard solution of NaOH from the burette until a single drop produces a permanent colour change in the solution from colourless to pink.

6.  Record the final volume of the burette to the nearest 0.01 mL. Dispose the contents of the Erlenmeyer flask into the beaker labeled “Titration Waste”.

7.  Wash your Erlenmeyer flask and repeat steps 3-6 two more times. Record all observations and data.

8.  Clean and rinse all glassware. Make sure the final rinse of the buret is with distilled water.

Observations:

Trial / Initial Buret Reading (mL) / Final Buret Reading (mL) / Volume of NaOH
(mL)
1
2
3

Average Volume of NaOH = ______mL

Visual Observations (write some notes here):

Write Up:

o  For your write up (title page + report) you will be using the guide that was given in class earlier this term. If you do not have a copy of the guide, download it from the course website (http://robertschem.pbworks.com).

o  Your observation section should include the table from the previous page, as well as all other general observations made during the lab.

o  You should have a Calculations section – showing ALL calculations (formulas, work and units). Use the examples from the class notes to complete this section. More importantly, show all of your work!

o  IMPORTANT – Because the vinegar was diluted by a factor of 10, your answer (the concentration of acetic acid) MUST be multiplied by 10 to get the concentration in the vinegar sample.

o  The Discussion section should include:

a.  General overview of lab (what was the purpose and what was accomplished)

b.  Summary of findings

c.  Possible errors in the lab affecting results

d.  Possible solutions to fixing the errors

e.  Whether your hypothesis was correct or incorrect

f.  Answering the question whether the sample of vinegar passes the quality control test. In other words, in order to pass, it must match the label on vinegar concentration (5%, which is equal to 0.874 mol/L).

GROUP MEMBERS SHOULD NOT COPY FROM EACH OTHER. THE ONLY “WORK” YOU WILL BE DOING IS THE ACTUAL LAB AND RECORDING OF RESULTS! PLAGARIASM IS AN ACADEMIC FRAUD OFFENSE, WHICH MAY RESULT IN FURTHER CONSEQUENCES. HOWEVER, YOU MAY DISCUSS THE LAB, SUCH AS STEPS FOR CALCULATIONS, ERRORS, ETC.

Any questions, email me at

LAB IS DUE: ______, 2012