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Lecture 18

CHEM 06101 Lecture Notes - Lecture 18: Flight Controller, Lab Notebook, Stopcock


Department
Chemical Engineering Biochemistry
Course Code
CHEM 06101
Professor
Lawrence Boni
Lecture
18

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Acid-Base Titrations
Lab Notebook and Lab Report
You must include a summary of lab procedures in your lab notebook for all parts of the lab. These should
not be a copy of the lab procedures, but detailed enough for you to follow in the lab as you will not be
permitted to bring these procedures into the lab. You need to copy into your lab notebook any tables that
may be useful for data collection as well as any tables you will need for data analysis. You should also
include equations that you need for calculations which will be needed for data analysis.
Your lab report should follow the format that is provided on the lab grading rubric. Be sure to include
your section number on your cover sheet. For this lab you will have one lab report sheet. The lab report
sheet should only be filled in after the lab has been completed and all calculations are performed in your
lab notebook.
OBJECTIVES
The objectives of this experiment are to:
1. prepare a solution of sodium hydroxide (NaOH) by dilution
2. accurately determine the concentration (molarity) of the NaOH dilution by titrating against a
standard solution of hydrochloric acid (HCl)
3. determine the molar mass of an unknown acid by titration with a standardized NaOH solution
OVERVIEW
In this experiment you will prepare a solution of NaOH and then accurately determine its concentration
by titrating it with a standard solution of HCl. You will then use the NaOH solution you prepared to
determine the molar mass of an unknown solid acid.
BACKGROUND
Acids are compounds that release hydrogen ion, H+, in a chemical reaction (i.e., they are H+ ion donors).
Bases are compounds that react with H+ (i.e., they are H+ ion acceptors). Thus, when mixed together in
solution, an acid will react with a base by transferring a H+ ion from the acid to the base.
A titration is a common technique used in the laboratory to determine the amount of an acid or a base
present in a sample. For example, in a titration, a standardized solution containing a base (i.e., one in
which the concentration of the base is accurately known) may be slowly added from a buret to a flask
containing an unknown amount of acid. (The acid and base could be reversed, of course.) By
measuring the amount of titrant (the solution in the buret) required to complete the reaction, and the
stoichiometry of the reaction (the ratio of moles of acid and base needed), the amount of the acid in the
sample can easily be determined.
The equivalence point of a titration is where the amount of added titrant is stoichiometrically equivalent
(i.e., in the exact ratio of moles predicted by the balanced chemical reaction) to the substance being titrated.
The end point of the titration is where some change is observed that indicates that the equivalence point has
been reached. The end point can be observed by using either a chemical indicator (if the solutions are not
colored) or a pH meter. A pH meter is a device that allows one to observe when a solution changes from
acidic to basic, or vice versa, as the solution is being titrated. An indicator is a chemical dye that has a

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certain color in acidic solution and a different color is basic solution. Ideally, the (theoretical) equivalence
point and the observed end point are exactly the same.
In this experiment, you will first prepare a dilute NaOH solution, and then determine its molarity by using
it to titrate a standardized (known concentration) solution of HCl. After you have prepared and
standardized the NaOH solution, you will use it to titrate an unknown acid to determine its molar mass.
In all titrations, phenolphthalein will be used as the chemical indicator. Phenolphthalein is colorless in
acid solution and red/pink in basic solution.
Prepare sodium Hydroxide solution
Standardizing a Base, NaOH, by Titrating with a Known Amount of Acid
A titration is used in this experiment to standardize an aqueous solution of NaOH (i.e., to accurately
determine its concentration). A known volume of an HCl solution that has a known concentration
(moles HCl
L soln ) is placed into a flask, and some water is added to provide enough volume that it is easy to
observe. An indicator is added that will change color when the solution turns basic. Then, the NaOH
solution (whose concentration is unknown) is slowly added from a buret, causing this reaction to occur:
HCl + NaOH → H2O + NaCl (all substances in aqueous solution) Rxn 1
The solution in the flask (HCl alone, initially) is of course acidic at first. As NaOH is added in small
increments, the solution remains acidic, but becomes less acidic as more NaOH is added and more of the
HCl is consumed by reacting with it. Eventually the HCl in the flask is used up completely, and the next
single drop of NaOH turns the solution basic, causing the indicator to change color. At the equivalence
point in a titration the amount of base added is exactly equal to the amount necessary to react with all of
the acid originally present in the flask.
# moles NaOH = # moles HCl at the equivalence point.
We can calculate the amount of base that reacted by first calculating the amount of acid that reacted. All
of the acid that reacted was present in the flask when the titration began. The number of moles of acid
can be calculated from the volume (# L) of it you put into the flask and its concentration (moles/L) in
that volume:
# moles HCl = (# moles HCl
1 L ) × # L HCl solution
The stoichiometry of Rxn 1 tells us that each mole of HCl reacts with exactly 1 mole of NaOH.
Therefore,
# moles NaOH = # mole HCl × 1 mole NaOH
1 mole HCl
The concentration of the NaOH solution can then be calculated from the number of moles of NaOH and
the volume that it was dissolved in (i.e. the volume of NaOH solution that you added from the buret):
Molarity of NaOH (M) = # moles NaOH
# L NaOH soln
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Titrating a Diprotic Acid
When a diprotic acid, for example H2SO4 , is titrated with NaOH, both hydrogen ions on the acid are
transferred during the neutralization reaction. Write the balanced reaction of H2A with NaOH for
yourself. What does the balanced reaction tell you about the amounts of acid and base required for the
reaction?
To find the molar mass of H2A (MM = the number of g H2A
mole H2A ), what two quantities must you know?
Techniques for Using Burets Accurately
1. Clean the buret by rinsing it with tap water and brushing it with a buret brush don’t use soap.
Water in the buret should drain freely, without leaving clinging drops on the inside.
2. After cleaning a buret, rinse it thoroughly with ~5-10 mL portions of the solution that goes into
the buret. . This prevents dilution of the solution with water left in the buret. Ask the instructor
to demonstrate this technique
3. The easiest way to fill/refill a buret is to use a small (100-250 mL) beaker and a liquid funnel.
4. Do not initially fill the buret to EXACTLY 0.00 mL. Just fill to within a couple of mL from the
top, and record the value* exactly in your notebook.
5. Be sure the tip of the buret is filled and free of air bubbles before you take the initial reading.
Do this by opening and closing the stopcock rapidly several times to allow the liquid running
through the tip to dislodge the bubbles.
6. Some people find it helpful to read a buret while holding a piece of white paper behind the buret
to blank out objects or people across the room. This also makes the meniscus darker and easier
to see.
7. Double-check your readings carefully. Burets can be tricky to read, because they are calibrated
from top to bottom. The buret reading is just the number of mL read to the one hundredths
place indicated by the bottom of the meniscus.
8. Read burets to the nearest 0.01 mL (hundredths)*. This requires interpolation between the
smallest calibration marks on the buret, and there is some uncertainty in the digit in the second
decimal place.
DO NOT just add a zero after the first decimal place. This introduces error.
9. Remove any drops hanging from the buret tip with a wash bottle immediately before measuring out
the liquid.
10. The buret reading you record is just the number of mL to the hundredths read from the buret.
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