Chemistry
Date: _____________
Name:
Lab Table:
Lab Partner:
340.04.04L
______________________
Concentration of Acetic Acid in Household Vinegar
Task
Your task is to determine the concentration of acetic acid (HC2H3O2) in a sample of household
vinegar using a weak acid / strong base titration. The strong base is sodium hydroxide
(NaOH).
Safety
1. Look up the Material Safety Data Sheets (MSDS or MSD) for KHP, NaOH, and vinegar.
2. Sodium hydroxide (NaOH) is a caustic base. If you spill any, on yourself, the table, etc.,
immediately wash with copious amounts of water. You should check that it is completely
cleaned up using a drop or two of diluted phenolphthalein. Vinegar is a weak concentration
of acetic acid, but it is an acid with a strong smell. Again, handle with care and clean up any
spilled liquid. Phenolphthalein is a laxative so be careful.
3. Disposal: KHP *& NaOH (in the waste container); vinegar (down the drain). The instructor
will show you how to make sure that the stir bar does not go down the drain or waste
container.
In addition to SOPs (e.g., goggles, closed-toe shoes), students should wear aprons to minimize
damage should there be any chemical spillage.
Background
The acid in vinegar is acetic acid. An inorganic chemist would write acetic acid1 as HC2H3O2,
which is the newest naming system. An organic (or old) chemist would write it as CH3COOH.
Both are acceptable. The reason that the old naming system was used is because it provides
more information about the actual structure of acetic acid (i.e., there’s more that one way to put
together two carbon atoms, three hydrogen atoms, and two oxygen atoms). The hydrogen
attached to the oxygen is the one lost:
CH3COOH(aq)
H+(aq)
+
CH3COO–(aq)
The quantity of acetic acid in a sample of vinegar may be found by titrating the sample against a
standardized basic solution. Most commercial vinegar is labeled as 5% acetic acid, but can have
a mass percentage that ranges between 4.0% and 5.5% acetic acid. By determining the volume of
standardized sodium hydroxide solution needed to neutralize a measured quantity of acetic
acid, the molarity and mass percentage of the vinegar can be calculated.
However, the concentration of an NaOH solution cannot be know, with certainty, unless it is
compared with an acidic solution of known concentration. NaOH is hygroscopic and reacts
with carbon dioxide, even in the air inside the container. So, by the time you’ve weighed out
the required mass of NaOH, it is already impure.
1
Acetic acid is often abbreviated as HOAc (sodium acetate as NaOAc, etc.)
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LAB: Acetic Acid in Vinegar
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The most commonly used chemical to standardize NaOH is potassium
hydrogen phthalate (KHP). This acid salt is very stable and reacts with
NaOH in a simple 1:1 stoichiometric ratio.
Pre-Lab
After completely reading this handout, answer the following questions before preforming the
experiment.
Part A
1. What are the equivalence point and endpoint? If they are not the same thing, explain why
the terms are essentially interchangeable in a lab.
2. Why does the buret need to be rinsed with small amounts of the solution that will going
through it, and making sure it flows out the tip? ______________________________________
(Yes, the answer is obvious.)
3. The procedure suggests using between 0.20 g – 0.30 g KHP.
a. Why is there no exact amount of KHP given?
b. Is it important to know the exact mass that you do use?
i. Why or why not?
c. Is it important to know the exact volume of the water in the KHP solution?
i. Why or why not?
d. Is it important to know the exact volume of the water in the NaOH solution?
i. Why or why not?
4. KHP is a monoprotic acid. What is the number of moles of possible H+ ions in 0.25 g KHP?
Part B
You will be using 1-2 grams of vinegar. It is helpful to know the volume of base you’ll need to
reach the equivalence point. Assume the density of vinegar is ca. 1.0 g/mL.
5. Calculate the mass of acetic acid in 2.0 mL of a 5.0%(m/m) vinegar solution.
6. Calculate the number of moles of acetic acid.
7. Calculate the volume of a 0.15 M NaOH solution that would be required to neutralize this
number of moles of acetic acid.
Materials
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ring stand for buret
beaker, 100-mL (for filling NaOH buret)
weighing boats (for KHP & vinegar)
scoop (dispensing chemicals for weighing)
magnetic stirrer and stir bar
Erlenmeyer flask, 50-mL
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buret clamp (to hold buret to ring stand)
potassium hydrogen phthalate (KHP)
sodium hydroxide (NaOH)
vinegar (3-5% acetic acid)
phenolphthalein (1%)
di H2O
LAB: Acetic Acid in Vinegar
p. 3
Procedures
Part A: Standardization of the NaOH solution
The purpose of Part A is to standardize your NaOH solution with KHP (Part B). When you titrate
to the equivalence point, you can conclude that the number of moles of acid (KHP) is equal to the
number of moles of base (NaOH). Knowing the moles of base and its volume, allows you to
calculate the concentration of the NaOH solution. N.B., use only d.i. water.
1. Follow good laboratory procedures by preparing the buret by rinsing with (1) di H2O, and
(2) a small amount of the solution that will be going through it, making sure to allow it to
run through the tip of the buret.
2. Measure 0.200 g – 0.300 g of KHP into an Erlenmeyer flask. Dissolve the KHP in enough
water to just barely cover the stir bar. This will allow the stir bar to make mixing more
consistent and make the endpoint easier to see. Add a few (ca. 10) drops of phenolphthalein
and insert a small stir bar. Place the flask onto the magnetic stirrer.
3. Place a piece of small piece of white paper between the magnetic stirrer and the flask – this
will make it easier to see when there is a color change (equivalence point). Fill the buret with
ca. 0.15 M NaOH, but no more than the calibrated volume of the buret: you need only to
know the volume entering the flaks, which is a difference between beginning and ending.
4. Titrate the KHP solution with the NaOH solution (as shown by the teacher) until the
endpoint is reached. The endpoint is signaled by the solution turning from colorless to pale
pink – and staying the very light pale pink color. If the solution is dark pink (magenta), you
have added too much base (NaOH). Record the final volume of the NaOH used.
Part B: Analysis of Acetic Acid in Vinegar.
The purpose of Part A is to determine the concentration of acetic acid in vinegar using he NaOH
you standardized in Part A.
1. Record the number/brand of vinegar that you will use.
2. Clean the inside and outside of the buret. (Make sure the outside is dry – our tap water will
leave salt marks.)
3. Using a pipette, add 1-2 grams of vinegar to a tared flask. Record the exact mass and
volume of the vinegar. As in Part A, add just enough water to the cover the stir bar, and
phenolphthalein. (Adjust the amount of phenolphthalein based on your observation in Part
A.)
4. Add NaOH to the buret and titrate as you did in Part A until you have just reached the
endpoint. Record the volume of NaOH used.
LAB: Acetic Acid in Vinegar
p. 4
Data & Results
Data Table
Part A. Mass KHP (g):
Part B.
Mass Vinegar (g)
Volume NaOH (mL):
Volume Vinegar (mL)
Volume NaOH (mL)
Part A.
1. Calculate the number of moles of KHP used in the experiment.
2. Calculate the molarity of the NaOH solution.
(KHP is monoprotic; NaOH is ‘mono-basic’. Thus the molar ratio is 1:1.)
Part B.
3. Calculate the molarity of the vinegar solution.
a. Acetic acid is a monoprotic acid and NaOH is ‘mono-basic’, therefore you can use
MA * VA = MB * VB
b. Because vinegar is mostly water, you can assume its density is 1 g/mL. Thus the mass
of vinegar you originally measured is approximately equal to the volume (mL) of the
solution.
4. Calculate the molarity of the vinegar solution.
a. Calculate the number of moles of acetic acid in the vinegar sample.
i. Using the average standardized molarity that you calculated in Part A, calculate the
number of moles of NaOH that were necessary to neutralize the vinegar.
ii. Again, assume a stoichiometric ratio of 1:1 for NaOH:HC2H3O2.
b. Determine the mass of acetic acid in the vinegar.
i. Using the number of moles of acetic acid from above with the molar mass of acetic
acid.
c. Calculate the % (m/m) of acetic acid in the vinegar solution.
i. Because % is a 1 part out of a total of 100 parts, divide the mass of the acetic acid in
vinegar by the mass of the vinegar solution and multiply by 100%.
Questions
1. It was necessary for you to standardize the NaOH solution prepared by your instructor. This
is necessary it is not possible to be sure of the exact amount of NaOH. This is because
NaOH is so hygroscopic that it will deliquesce.
a. What does it mean to standardize a solution?
b. What does hygroscopic mean? And why is the word hydroscopic not used?
c. What does deliquesce mean?
2. Why is potassium hydrogen phthalate used to standardize a solution?
3. Write the ionization equations for (in water) (a) KHP, (b) NaOH, and (c) acetic acid. Show
the predominate species in each.
4. Vinegar is a weak acid; NaOH is a strong base. Write the balanced net ionic equation for
this neutralization reaction.
5. Suppose you want to make 250.0 mL of a 0.15 M NaOH solution.
a. What mass of NaOH would you need?
b. If you prepared the solution on a hot and humid day, would the molarity be greater or
smaller than 0.15 M.
6. Why is there a temperature of a volumetric flask, labeled TC? Why are flasks and
graduated cylinders labeled TD instead?