Acid-Base Titration
Neutralization of a Weak Acid with a Strong Base
Purpose
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To standardize a solution of sodium hydroxide by titration with a primary
standard.
To determine the equivalence point of the titration of a sample of commercial
vinegar using a visual indicator.
To determine the equivalence point of the titration using a pH meter.
To determine the Ka of the acid from the plot of pH versus volume of base.
To determine the mass % of acetic acid in a commercial vinegar.
Introduction
A solution of sodium hydroxide of exact molarity cannot be made because of the
tendency of the solid to absorb water and carbon dioxide from the air. Therefore, the
sodium hydroxide solution must be standardized. In this experiment the standardization
of the solution is done by titrating it against a pure sample of potassium hydrogen
phthalate (KHC8H4O4) of known mass. This monoprotic acid is abbreviated KHP and has
a molar mass of 204.2 g/mol.
After the sodium hydroxide solution is standardized, it is used to titrate acetic
acid, the active ingredient in commercial vinegar. The vinegar is first titrated using
phenolphthalein as an indicator. The titration is then repeated using a pH meter to
monitor it.
From a graph of pH versus mL NaOH, the equivalence point and the halfequivalence point will be found. From these points, the amount of acid present in the
solution and the pKa of the acid can be determined.
Procedure
Part I Preparation of 0.100 M Sodium Hydroxide Solution
1. Heat about 1 liter of distilled water to boiling in a 1 L Erlenmeyer flask to remove
the dissolved carbon dioxide.
2. Cool the flask in tap water and transfer about 400 mL of the contents to a 500 mL
Florence flask.
3. Add about 4.00 mL of stock solution of carbon dioxide-free sodium hydroxide
(about 14.3 M) to the flask and fill the remainder of the flask with CO2 free water.
Mix thoroughly. Save the rest of the water for Part II.
Part II Standardization of Sodium Hydroxide Solution
1. Weigh out a sample of KHP between 0.25 - 0.40 g and put it into a clean
Erlenmeyer flask.
2. Add about 100 mL of CO2 free water and agitate until the KHP is dissolved.
3. Add 2-3 drops of phenolphthalein indicator solution and swirl gently.
Acid-Base Titration: Neutralization of a Weak Acid with a Strong Base
4. Rinse a clean buret with about two to three 5 mL portions of the sodium hydroxide
solution previously prepared. Discard washings down the drain.
5. Fill the buret with NaOH, running some out through the tip. Take an initial reading
to the nearest 0.01 mL.
6. Slowly add the NaOH to the flask until a pink color persists for 30 seconds, which
signals the equivalence point. Record the final buret reading to the nearest 0.01 mL.
7. Repeat two more times.
Part III Titration of Vinegar Using Phenolphthalein Indicator
1. Into a clean and dry 250 mL beaker, pour approximately 50 mL of the vinegar
sample.
2. Rinse a 10 mL volumetric pipet with the vinegar by drawing about 5 mL into it .
Repeat 2 more times. Discard washings into sink.
3. Carefully pipet 10.00 mL of the vinegar into a 250 mL Erlenmeyer flask.
4. Add about 40 mL of distilled water to the vinegar sample.
5. Add three drops of phenolphthalein and swirl to mix.
6. Titrate with your standardized NaOH.
7. Repeat two more times. You should have three trials that agree to within 0.05 mL.
Part IV Titration of Vinegar Monitored with pH Meter
1. Fill the buret with your standardized NaOH to the 0.00 line.
2. Carefully pipet 10.00 mL of the vinegar into a clean 250 mL beaker. Add 90 mL of
distilled water and a magnetic stirring bar.
3. Place the beaker on a magnetic stirrer. Clamp the pH meter into place so that the
magnet is not touching the electrode. Turn on the stirrer.
4. Set up CBL with pH probe. Use stored calibration. Under collect data, select
TRIGGER/PROMPT. Follow directions on the calculator screen to allow the
system to warm up (about 30 seconds), then press enter. Before adding NaOH,
monitor the pH value on the CBL screen. Once the pH has stabilized, press
TRIGGER on the CBL and enter “0” (the buret volume, in mL) in the calculator.
You have now saved the first data pair for this experiment. Select MORE DATA to
collect another data pair.
5. Begin by adding 5.00 mL increments of NaOH for the first 30 mL. Then add 1 mL
increments until the pH reaches pH 6. When the pH reaches 6.0, add 0.10 mL
increments of NaOH solution until the pH reaches 11.0. Once pH 11.0 has been
reached, take at least five additional pH readings after additional 1.00 mL
increments are added. After each increment, allow pH to stabilize, press TRIGGER
and enter volume on buret.
Calculations
Part II
1. Calculate the molarity of the sodium hydroxide solution for each trial and then
average the three trials. The molar mass of the standard acid, KHP, is 204.2 g/mol.
Acid-Base Titration: Neutralization of a Weak Acid with a Strong Base
Part III
1. Calculate the moles of acetic acid in 10.00 mL of vinegar using the average volume
of NaOH used in the titration.
2. Calculate the mass of acetic acid in 10.00 mL.
3. Assuming the density of vinegar is 1.008 g/mL, calculate the mass % acetic acid in
the vinegar.
Part IV
1. Make a graph of pH versus the volume of NaOH by transferring your data to
graphical analysis.
2. Locate the approximate equivalence point by one of the three following methods.
(a) Extend the vertical portion of the curve with a ruler. Mark the tangent points at
the top and bottom of this extended line and locate the midpoint.
(b) An alternative way to determine the equivalence point is to examine the data in
your calculator in the vertical portion of your graph. Find the largest increase in pH.
Find the volume of NaOH required to give this largest increase. This volume
represents the equivalence point.
(c) A second alternative for determining the equivalence point is to use graphical
analysis to take the first derivative of the points on the vertical portion of your
graph. This number represents the slope of the line at that point. The steepest point
on the line (greatest slope) is the equivalence point. To find the first derivative,
create a third column and under Data, choose New Column Calculations, Other
Functions, and Derivative. Type in pH for dependent column and volume for
independent column. Make another graph, plotting the first derivative on the y-axis
and the volume of NaOH on the x-axis. The graph should have a peak at the
equivalence point.
3. Calculate the mass % of acetic acid in vinegar based on this equivalence point.
4. Determine the half-equivalence point (one-half of the volume of NaOH needed for
the equivalence point). Find the pH of the solution at that point.
5. Determine the Ka of the acid.
Data
Part II
Trial 1
Mass of KHP
Initial reading on buret
Final reading on buret
Volume NaOH used
Trial 2
Trial 3
Acid-Base Titration: Neutralization of a Weak Acid with a Strong Base
Part III
Trial 1
Initial reading on buret
Final reading on buret
Volume NaOH used
Trial 2
Trial 3
Acid-Base Titration: Neutralization of a Weak Acid with a Strong Base
Part IV
Volume NaOH
(mL)
pH
Volume NaOH
(mL)
pH
Acid-Base Titration: Neutralization of a Weak Acid with a Strong Base
Results
Part II
Trial 1
Trial 2
Trial 3
Average
Molarity NaOH
Part III
Moles acetic acid
Mass acetic
Mass % acetic acid
Part IV
1
2
3
4
4
5
Approximate equivalence point
Precise equivalence point
Mass % acetic acid
Half-equivalence point
pH at half-equivalence point
Ka of acetic acid
mL NaOH
mL NaOH
mL NaOH
Questions
1. Explain how the Ka of an acid can be determined from the titration of the acid with a
strong base.
2. You followed the titration with a visual indicator and with a pH meter. Which
technique was most accurate? Explain why.