CHEMISTRY P
MRS. BENIPAL, MRS. EDWARDS, MR. NELSON, DR. SELGRATH
NAME:
SPRING 2012
EXPERIMENT #12: ACID/BASE TITRATION
PURPOSE:
To determine the molecular mass of a solid acid by titration against a standard base solution, and
identify the unknown acid based on its experimental molecular mass.
INTRODUCTION:
Amounts of reactants and products in a reaction are commonly investigated in two ways in the laboratory:
gravimetrically (by mass) and volumetrically (by volume and concentration), as in this experiment. Titration is
the name given to the process used to determine the volume of a solution needed to react with a given mass, or
volume, of a sample. We will use this process to study quantitatively the reaction between an acid and a base.
The hydrogen ion from the acid reacts with the hydroxide ion from the base to produce water.
Phenolphthalein will be used as the indicator in this experiment, since its color change occurs when the same
number of moles of hydrogen ion and of hydroxide ion are present. This point in the reaction is called the
equivalence point.
PRE-LAB QUESTIONS:
1. Determine the number of moles in 34.52 milliliters of a 0.2608 Molar NaOH solution.
PROCEDURE: (Include with your laboratory report, or refer to this page)
1. Fill buret with the standardized base at your lab station, and record the concentration of the NaOH(aq) as
stated on the stock bottle in your data section. Do not use titrant from another lab station! Do not fill burets
above the top division.
2. Select a solid acid unknown, and record the number of the unknown acid in the data section.
3. Determine the mass of the solid acid, cap and container to the nearest 0.001 gram and record this mass in
your data section.
4. Place the solid acid in a clean 125 milliliter Erlenmeyer flask. Measure the mass of the empty container
and cap and record.
5. Add approximately 15 milliliters of distilled water to the flask containing the acid. Add three drops of the
indicator phenolphthalein to the flask.
6. Before titrating, be certain that the titrant has filled the tip of the dropper and that there are no air bubbles
present. If air bubbles are present, use techniques discussed in class to remedy problem. When the above
conditions have been met, record the initial fluid level in the buret in the data section.
7. Hold the neck of the flask in one hand and manipulate the buret with the other. Add the sodium hydroxide
and gently swirl the flask so that the solutions mix thoroughly.
8. Continue adding NaOH until the first faint fuchsia/pink color develops. If the color disappears upon mixing
the solution, add more NaOH, drop by drop, until a pink color persists for at least ten seconds. You have
arrived at the equivalence point. Record the final fluid level in the buret.
9. Repeat steps #1-8 above, twice more, using the same unknown acid as in the first trial.
The Unknown Acids:
Maleic acid
Sodium bisulfite
Oxalic acid
Potassium bisulfate
©BHS Chemistry, 2012
(H2C4H2O4, 116.0 g/mole)
(NaHSO3H2O, 122.1 g/mole)
(H2C2O4, 126.0 g/mole)
(KHSO4, 136.2 g/mole)
Ascorbic acid
(H2C6H6O6, 176.0 g/mole)
Potassium bitartrate (KHC4H4O6, 188.1 g/mole)
Citric acid
(H3C6H5O7, 192.0 g/mole)
Potassium biphthalate (KHC8H4O4, 204.1 g/mole)
CHEMISTRY P
MRS. BENIPAL, MRS. EDWARDS, MR. NELSON, DR. SELGRATH
NAME:
SPRING 2012
DATA AND OBSERVATIONS: (Record on a separate page)
Unknown Acid # __________
Trial #1
Trial #2
Trail #3
Trial #1
Trial #2
Trail #3
a) Concentration of NaOH (in moles/liter)
b) Mass of acid, cap and vial (in grams)
c) Mass of empty cap and vial (in grams)
d) Initial volume of NaOH (in milliliters)
e) Final volume of NaOH (in milliliters)
CALCULATIONS: (Show all calculations on a separate page)
1. Mass of unknown acid (in grams)
2. Volume of NaOH reacted (in milliliters)
3. Moles of NaOH (in moles)
4. Moles of acid (in moles)
5. Molar mass of acid (in grams/mole)
6. Average molar mass of acid for three trials:
__________ g/mole
7. Name of Acid:
_______________________________________
8. Formula of Acid:
_______________________________________
9. Percent error:
(Use the given molar mass of the acid that you have identified)
__________%
QUESTIONS: (Answer on a separate page)
1. Describe the role of the phenolphthalein indicator, and the principle assumption it permits you to make.
2. How does the value for the molar mass of your solid acid have been affected (would it be larger or smaller)
if not all of had been dissolved at the time that you ended the titration? Explain in words, or show by way of
sample calculation how this error gives the results you claim.
3. How does the value for the molar mass of your solid acid have been affected (would it be larger or smaller)
if you over-shot the equivalence point? Explain in words, or show by way of sample calculation
how this error gives the results you claim.
4. Determine the molar mass of a solid unknown acid for which the following data was collected:
a) Concentration of standard base (NaOH)
b) Mass of acid and vial
c) Mass of empty vial
d) Initial level in buret
e) Final level in buret
©BHS Chemistry, 2012
0.2341
17.689
16.574
3.32
32.79
moles/L
g
g
mL
mL