Lab 05
Introduction to Acid-Base Titrations
Introduction
Among the many types of quantitative chemistry techniques, volumetric analysis is a timehonored classical method. The characteristic feature of volumetric analysis is measuring the
volume of a solution that is required to carry a chemical reaction from inception to completion.
One way to observe when a reaction reaches completion is to include in the reaction mixture an
indicator, a substance whose color changes when the reaction is completed. The occurrence of
this completion is called the end point, and the procedure by which the volume is measured is
called a titration. If the solution whose volume is being measured is of precisely-known
concentration, it is referred to as a standard solution. The ultimate basis for standardization is
always a primary standard, which is a substance of known and stable composition and purity.
The chemist may dissolve a precise mass of a primary standard in a solvent to make a definite
volume of solution, thereby formulating a standard solution, or they may standardize a solution
by using it to titrate a definite mass of a primary standard. The data of the titration are used to
calculate the precise molarity of the solution, which is then considered standardized.
In any case, a standard solution is eventually used to titrate an unknown, and the observed
volume of the standard solution required to reach the end point is used to calculate the
quantitative composition of the unknown. As you follow the procedures of this experiment,
these perhaps abstract concepts will become clear realities to you. (I hope!)
Reactions
In this experiment, you will learn to perform titrations. You will standardize a solution of a base
(NaOH) against a primary standard acid (potassium hydrogen phthalate, KHP, or KHC8H4O4).
This standard has a very high molecular weight, and allows you to precisely weigh out a very
small number of moles.
KHC8H4O4 + NaOH  NaKC8H4O4 + H2O
You will then titrate household vinegar with the standard NaOH solution to determine the
percent by mass of acetic acid.
HC2H3O2 + NaOH  Na C2H3O2 + H2O
In addition, you will titrate samples of an unknown HCl solution to determine its molarity.
Pre Lab Problems (answer on separate paper)
1.
2.
3.
4.
Find the molecular weight of potassium hydrogen phthalate (KHP).
A chemist weighed out 0.450 g KHP, dissolved it in 20.00 mL of water, and titrated with
a NaOH solution of unknown molarity. 38.64 mL of the NaOH solution were required to
reach the end point. Calculate
(a) the moles of KHP titrated;
(b) the moles of NaOH used in the titration;
(c) the molarity of the NaOH solution.
If 0.450 g of a primary standard acid required 38.64 mL of NaOH titrant to reach the end
point, what volume of titrant would be required to reach the end point in a second
titration using 0.485 g of the primary standard acid? (This problem can be solved by a
simple proportion.)
A 30.00 mL sample of dilute HCl is titrated to the end point with 41.35 mL of 0.1025M
standard KOH solution:
Introduction to Acid-Base Titrations
HCl + KOH  KCl + H2O
What is the molarity of the acid solution?
Materials &Equipment
NaOH solution, approximately 1.25M
Vinegar
Phenolphthalein indicator
Distilled water
Buret, stand, and clamp
Weighing paper or tin
Potassium hydrogen phthalate
HCl solution of unknown molarity
250 mL Volumetric flask
50 mL Graduated cylinder
Metal spatula or scoop
250 mL Erlenmeyer flasks
Procedure
In preparing for this experiment, be sure to read Appendices A and B, which contain important
information about the use of a buret and the procedure for titrating. Bring a calculator with you
to the lab.
Contamination of your buret and flasks by an acidic or basic material from previous experiments
can seriously affect your results. It is therefore advisable that you rinse your glassware with
distilled water prior to use. To conserve distilled water, you should first rinse copiously with tap
water, and then use several small portions of distilled water from a wash bottle for the final
rinsing.
1.
Preparation of NaOH solution
(a)
2.
Rinse a 250 mL volumetric flask with distilled water, but do not dry it. With a
graduated cylinder, measure approximately 50 mL of 1.25M NaOH and empty it
into the flask. Immediately rinse the cylinder with tap water so that it will not be
etched by the fairly concentrated NaOH solution.
(b)
Fill the flask to the mark with distilled water. It is not necessary to be precise
about filling to the mark, because you will determine the molarity of the solution
later. Stopper the flask, hold the stopper in with the thumb of one hand, and
thoroughly mix the contents by inverting the flask completely several times and
moving the bulbous body in a rotary motion. The NaOH solution will be the titrant
(the solution measured with a buret) in the titration.
(c)
Clean and rinse a buret, support the buret with a stand and clamp (be careful that
the buret is clamped securely, but not tightly enough to snap the glass), and rinse
and fill the buret using about 60 mL of NaOH solution.
Standardization of NaOH solution
(a)
Use a spatula to place 0.4 to 0.5 g of KHP crystals on a piece of clean, dry
paper, and record the weight of paper plus acid. Precision is important in this
step. Weigh the empty paper after transferring the sample to a clean (not
necessarily dry) 250 mL Erlenmeyer flask. The difference in the two weights is
the sample weight. Repeat with a second sample, making sure to somehow
differentiate between the two. Avoid losses by spills.
(b)
With distilled water rinse down any crystals on the inside walls of the flasks.
Bring the total volume in each flask up to about 20 mL. Add 2 to 3 drops of
2
Introduction to Acid-Base Titrations
3.
4.
5.
phenolphthalein indicator to each flask. DO THIS PART FIRST SINCE KHP
CAN TAKE A LONG TIME TO DISSOLVE.
(c)
PERFORM THIS TITRATION LAST TO GIVE THE KHP LONGER TO
DISSOLVE. Titrate both samples by the procedure described in Appendix B.
For each sample, record the initial and final buret readings to 0.01 mL and
calculate the titrant volume (the difference between the initial and final readings).
Once you have titrated the first sample, you can estimate the titrant volume
needed for the second sample by a calculation similar to Pre Lab Problem 3.
(d)
For each sample, divide the titrant volume by the sample weight to obtain the
milliliters of titrant per gram of sample. For accurate results, the two volumes
values of milliliters of titrant per gram of sample should agree to within 1%; that
is, the ratio of these two values should be between 0.99 and 1.01. If this is not
true, prepare and titrate a third sample (or as many as are necessary to obtain
reproducible results). Careful work will prevent you from repeating this part.
Titration of vinegar
(a)
At the front of the lab, you will find a buret containing household vinegar. Into
clean 250 mL Erlenmeyer flasks, dispense duplicate samples of 7 to 8 mL
vinegar. Record the volume of each sample to the nearest 0.01 mL.
(b)
Dilute each sample to about 20 mL with distilled water. Add 2 to 3 drops of
phenolphthalein indicator to each flask. Titrate. If the values of milliliters of
titrant per milliliter of sample do not agree within 1%, repeat with one or more
additional samples until you obtain reproducible results.
Titration of unknown acid
You will be assigned a HCl solution. Titrate duplicate samples by the procedure of Step
3. Repeat with one or more samples if the values of milliliters of titrant per milliliter of
HCl do not agree within 1%.
Cleanup
Since solutions of strong base can etch glass, it is imperative that you empty your buret
and volumetric flask and thoroughly rinse them with tap (and distilled) water before you
leave. Be sure to let the water flush out the stopcock and the top of the buret. When
you are finished, clean your lab area and glassware before being signed out.
Calculations
Calculate the following to three significant figures. For each type of titration use the results of
your best two trials (those which agree by 1%).
1.
Calculate the molarity of the NaOH solution.
2.
Calculate the molarity and percent by mass of the acetic acid in vinegar. Use a value of
1.005 g/mL for the density of vinegar for conversions.
3.
Calculate the molarity of the unknown HCl solution.
Post Lab Questions
1.
2.
What effect would spilling a small portion of the KHP in Step 2 (after weighing) have on
the calculated molarity of the acetic acid in the vinegar?
The minimum legal content of acetic acid in vinegar is 4% by mass. Does the vinegar
that you titrated comply?
3
Introduction to Acid-Base Titrations
Report: Introduction to
Acid-Base Titrations
Name
Lab Partner(s)
Section
Data
Date performed
Vinegar:
Trials ______ & _______ below agree within 1%? 
Titration 1
Titration 2
Titration 3
Sample volume
mL
mL
mL
(NaOH) titrant delivered
mL
mL
mL
Unknown Acid
Code:
Trials ______ & _______ below agree within 1%? 
Volume of HCl solution
mL
mL
mL
Titrant delivered
mL
mL
mL
Standardization of NaOH titrant:
Trials ______ & _______ below agree within 1%? 
Mass of KHP
g
g
g
Titrant (NaOH) delivered
mL
mL
mL
M
M
M
Calculations
(detailed calculations are attached)
NaOH M
average:
Acetic Acid M in Vinegar
(using avg. NaOH M)
M
average:
% by Mass Acetic Acid in Vinegar
M
M
M
M
%
(using avg. HOAc M)
Unknown HCl Molarity
(using avg. NaOH M)
4
M
average:
M
M
M
Introduction to Acid-Base Titrations
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