Determining the Concentration of Vinegar
Name:_______________
Period:____
PURPOSE: To use what we have learned about molarity and acids to determine the
concentration of commercial vinegar.
THEORY: Vinegar is a common substance that was first made when wine was left to
ferment too long, thus producing vin aigre (or sour wine). It is produced when certain
bacteria metabolize a sugar all the way to acetic acid, HC2H3O2. As we have learned in
class, acetic acid belongs to the class of compounds known as weak acids, meaning they
do not completely donate their proton to water. However, they will completely donate
their proton to a base such as sodium hydroxide. Therefore, we can use the NaOH in a
titration to determine the number of moles of acetic acid in a given volume, and hence
determine its molarity. Since we will be mixing two clear solutions, we need a method
by which to know when we have used up all the acetic acid. Since the amount of
hydronium ion will drop as the acetic acid is reacted, we will use an indicator to tell us
when the reaction is completed. A simple change of color will indicate that the reaction
is completed.
MATERIALS:
Three labeled microtip pipets
24 well plate
phenolphthalein solution
standardized NaOH solution – Caution, corrosive
white vinegar
universal indicator
red and blue litmus paper
10-ml graduated cylinder
Procedure and Data:
What we are going to do in this experiment is to measure the volume of a known
concentration of sodium hydroxide solution needed to react completely with a known
volume of vinegar. Then, by doing a simple stoichiometric calculation involving
molarity, we can determine the concentration of the vinegar. In this experiment we are
going to use drops as our standard unit of measure. Thus, we need a conversion factor
between ml and drops. Unfortunately, the size of each drop coming out of the pipet
depends on the substance being dropped (for reasons we will mention later). In order to
get this we need to count drops. Thus, you and your lab partner will find in front of you
two filled pipets, one with NaOH, the other with HC2H3O2. Holding your pipet
vertically, add 50 drops of NaOH into the 10-ml graduated cylinder in front of you.
Record the volume after each 50 drops, up to 200 drops. Clean the cylinder with some
distilled water and then dry it by rolling a paper towel up in a cylinder shape and rubbing
it in the graduated cylinder. Then repeat the dropping experiment with vinegar, recording
the volume every 50 drops up to 200.
Data Table I – Drop Counting for Vinegar and NaOH
Unknown Letter:
Volume after addition
of 50 drops
Volume after addition
of 100 drops
Volume after addition
of 150 drops
Volume after addition
of 200 drops
NaOH
Vinegar
Before we go any further in the lab, we need to make sure you have “dropped” well.
Specifically, that you can consistently release drops that are the same size. To do this, let
us calculate the number of drops per ml in each of your trials. Simply divide the volume
of the solution added to the cylinder by the number of drops added at that time. Show a
sample calculation now:
Sample Calculation of drop volume for Vinegar:
Sample Calculation of drop volume for NaOH:
Results Table I – Average ml per drop for Vinegar and NaOH
Substance:
ml/drop after 50 drops
ml/drop after 100 drops
ml/drop after 150 drops
ml/drop after 200 drops
Average ml/drop
NaOH
Vinegar
Many of you have probably seen or even used litmus paper, which is perhaps the most
famous indicator of acids and bases. In front of you are two pieces of litmus paper, one
red and one blue. Rip each piece in two and then place a drop of the NaOH on both a red
piece and a blue piece. Record what happens below:
Observations after adding NaOH to blue litmus paper:
Observations after adding NaOH to red litmus paper:
Now do the same for the vinegar, adding one drop to each of the remaining red and blue
pieces. Record what happens below:
Observations after adding vinegar to blue litmus paper:
Observations after adding vinegar to red litmus paper:
The problem with litmus paper is that is will just tell you if a substance is an acid or a
base, but yields little other information. Better indicators exist to give you an estimate of
just how basic or acidic a solution is. One of these is universal indicator, which is a
solution that produces several possible different colors depending on the amount of H3O+
in the solution. To test universal indicator, take the well plate in front of you and add one
drop of universal indicator to two different wells of your choice. Then add several drops
of vinegar to one of the wells with UI, several drops of NaOH to the other well. Observe
the color change, and using the indicator color chart, record your observations and
estimate of the pH of the solutions below:
Observations after adding vinegar and NaOH to universal indicator:
Estimate of pH’s for vinegar and NaOH:
We can now begin to determine the concentration of the vinegar solution. Although
vinegar is a weak acid, it will react completely with sodium hydroxide according to the
following balanced molecular equation:
NaOH(aq) + HC2H3O2(aq)  H2O(l) + NaC2H3O2(aq)
If the vinegar becomes the limiting reactant in this reaction, the concentration of
H3O+(aq) willl drop, and the pH will increase. When OH-(aq) is in excess, the pH will be
so high (or equivalently the concentration of H3O+(aq) will be so low) that
phenolphthalein will turn pink, as we have seen in previous labs. Thus your task is to add
NaOH drop by drop, stopping after the solution stays pink for at least 30 seconds after
stirring. However, you will not be able to do any of the calculations of you do not know
the concentration of the NaOH, so write this number in Data Table II now!
At this time add 25 drops of vinegar to an empty, clean well. Make sure that you hold
the pipet vertically at all times. Then add one drop of phenolphthalein solution to the
well. When you are ready, begin to add the NaOH to the well, drop by drop, using your
plastic stir stick to stir the solution. Continue adding NaOH until the solution stays pink
after stirring. It will only take one drop, so do not get anxious and add too many drops at
a time!. Record the number of drops of NaOH it takes to make the solution pink in Table
II below. In addition, record any observations about the reaction in the space below the
table.
Repeat the experiment by adding 25 drops of vinegar to another clean well. Add a drop
of phenolphthalein and add NaOH dropwise until the reaction is completed. Record the
number of drops necessary in the table. If your two trials agree to within in drop, you are
finished. If there is discrepancy among the number of drops between trials, repeat the
experiment for a third time.
Data Table II – Titration Data for the Reaction between Vinegar and
NaOH
Molarity of NaOH solution:
Trial Number
I
II
III (if necessary)
Initial Drops of
25
25
25
Vinegar
Phenolpthalein
added?
Drops of NaOH
needed to
complete reaction
Observations:
Now that the reaction if completed, drain the reactants into the sink, and rinse the well
plate with soap solution, tap water, and finally distilled water. Leave the plate on the
table to dry. Dispose of your stir stick and litmus paper, and return to your desk and
begin calculations.
CALCULATIONS:
For each of the steps that follow, simply show the calculation in the space provided. You
only need to show the calculation for trial one, and just report the rest in the results
table below.
1) First determine how many liters of NaOH solution you added to each well. To do
this, you need to know how many drops you added and the average volume per
drop. Show your calculation with conversion factors:
2) Now determine the number of moles of NaOH that reacted in each trial.
Remember, you wrote down the molarity of your NaOH solution in Data Table II.
3) From the balanced chemical equation, determine the number of moles of vinegar
that reacted in each trial.
4) Now determine the volume of vinegar used in each trial. This will be done in the
same way as calculation 1) above:
5) Finally, find the molarity of your vinegar solution in each trial. Average these
results
Results Table II: Vinegar Molarity
Trial:
I
II
III (if
necessary)
Volume of
NaOH (L)
Moles of
NaOH
Moles of
Vinegar
Volume of
Vinegar (L)
Molarity of
Vinegar (mol/L)
Average
Molarity(mol/L)
When you buy vinegar, it as sold as a solution of 5% acidity. What this confusing
term really means is that there should be 5.0 g of vinegar in 100 g of solution. Thus,
to make a comparison between your calculation and the published concentration, we
need to convert your molarity in these percent units. These final calculations outline
this conversion. In all the work that follows, we will use your average molarity.
6) We are going to assume that the vinegar solution has a density of 1.00 g/ml
(which is a good assumption because the solution is not very concentrated). If
this is the case, then 100. ml of your solution should have a mass of 100. g. So, to
convert your molarity into a percentage, we simply need to know how many
grams of vinegar are in 100 ml (and hence 100. g) of solution. From your
calculated molarity, determine the number of moles and number of grams of
vinegar in 100. g of the solution.
7) The number of grams you have calculated if the acid % of the solution. Do a
percent error calculation to the true value, assuming that this value is the 5%
stated on the bottle. (Don’t worry about sig. figs. here, just report the value to the
nearest 0.1%)
DISCUSSION:
1) Using the pH value you estimated for the vinegar solution from universal
indicator (in your observations above), calculate the concentration of hydronium
and hydroxide in your vinegar.
2) Based on your results, what would the pH of vinegar be if it were a strong acid?
Explain.
3) You know that NaOH is a strong base. Based on that information, and the
concentration of the NaOH that your wrote down in your data, what should the
pH of the NaOH solution be? Did your observations from the universal indicator
back this up? Why or why not?
4) Kelley gets a job testing the pH’s of various solutions, and she needs to know if
the pH of a solution is below 5. She has at her disposal red litmus paper, blue
litmus paper, phenolphthalein, and universal indicator. Which of those solutions
would be the best choice to determine if the pH is below 5? As part of your
answer, explain why the other solutions are poorer choices by describing what
you would see when you added a drop of the solution to each of the indicators.
4) What are the hydronium and hydroxide concentrations of a solution with a pH of
5?
CONCLUSION: