CHEM 132 Lab 11
Western Carolina University
Name __________________________
Chemistry 132 Lab 11
Prelaboratory Exercise
Lab Section______________
How Effective is Your Antacid?
1. How many grams of NaOH will you need to make 250 mL of 0.5 M NaOH solution?
2. What is the purpose of the first, quick titration you will do? How is it different from the three
subsequent titrations you will do?
Laboratory Summary
1. Prepare 250 mL of NaOH solution with a concentration of about 0.5 M.
2. Standardize the sodium hydroxide solution with hydrochloric acid solution (four titrations).
3. React an antacid with the hydrochloric acid solution to dissolve it, and use your standardized
sodium hydroxide solution to determine the ability of the antacid to neutralize acid.
Introduction
This experiment consists of three parts. In the first part, you will prepare a solution of sodium
hydroxide (NaOH), a strong base. As you may recall, a solution is a homogeneous mixture. In
aqueous solutions, water is called the solvent and the other compound is called the solute. One
common way of describing how much solute is dissolved in a solution is molarity or the molar
concentration. Molarity is defined as moles of solute divided by the volume of solution in liters, and
the equation is as follows:
moles of solute
(1)
Molarity =
liters of solution
Molarity is abbreviated as M, and has units of moles/liter. Hence a 1 M hydrochloric acid (HCl)
solution contains one mole of HCl per 1 L solution. In this experiment you will have to make 250
mL of an NaOH solution whose molarity is approximately 0.5 M. You need to calculate the mass of
sodium hydroxide such that when it is diluted to 250 mL it has a molar concentration of 0.5 M.
Solid sodium hydroxide will absorb moisture from the air, so it is not possible to measure the mass of
NaOH with high accuracy. It is therefore difficult to prepare sodium hydroxide solutions of precisely
known concentration. Instead, in part 2 of the experiment, you will carefully add some of your
NaOH solution using a burette to an HCl solution whose concentration is known precisely using a
procedure called a titration. A solution whose concentration is known precisely is called a
standardized solution. The reaction between the NaOH and HCl is given by:
NaOH (aq) + HCl(aq) → H2O (l) + NaCl (aq)
(2)
We will be able to determine the unknown concentration of NaOH because a known quantity of HCl
will be the limiting reagent. It will be completely consumed by an equivalent amount of NaOH.
How will we know when we have added the same amount of NaOH as the amount of HCl initially
present? We will add a chemical compound that changes color depending on whether it is in an
acidic or basic solution. This compound is called an indicator and the indicator we are going to use
is phenolphthalein. An acid-base indicator is a material that changes color when the pH of a solution
changes from acid to base or from base to acid. Phenolphthalein is colorless in acidic solutions but is
pink in basic solutions. Hence we will be able to tell when we have added a slight excess of base (we
have consumed all of the HCl) when the color change is observed. A total of four titrations will be
performed.
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CHEM 132 Lab 11
Western Carolina University
The third part of the experiment is to determine the amount of acid that an antacid can neutralize.
Although antacids do not readily dissolve in water, they react with acids and the reaction products
will dissolve. In order to determine the neutralizing power of the antacids, a known excess volume of
hydrochloric acid will be added to the antacid. The amount of excess acid can then be determined by
titrating the solution with the standardized base in order to find the amount of acid that was
neutralized by the antacid.
Experimental
1. Preparation of the ~0.5 M NaOH Solution
The first step of the experiment is the preparation of 250 mL of NaOH solution with a concentration
near 0.5 M. In order to prepare the solution, it is necessary to calculate the mass of solid NaOH that
must be added to 250 mL of water. This calculation employs the following formula:
mass of NaOH in grams = VNaOH × MNaOH × MMNaOH
(3)
where VNaOH = volume of NaOH solution in liters, MNaOH = molarity of NaOH solution, and MMNaOH
= molar mass of NaOH in g/mol. Fill in the information on your data sheet and calculate the mass of
NaOH required. Check the result with your instructor before beginning with the solution
preparation.
→ Volume of NaOH solution
L
→ Molarity of NaOH solution
M
→ Molar Mass of NaOH
g/mol
→ Mass of NaOH needed
g
After your calculation has been checked by your instructor, weigh out the mass of NaOH required
into a 250 mL flask. CAUTION: Solid sodium hydroxide is a corrosive material that may cause
serious skin and eye damage. Wear your safety glasses at all times. Add 250 mL of water to the
solution, stopper, and mix well. Label the flask with the approximate concentration, the solute
(NaOH), and your name.
2. Standardization of NaOH solution with HCl solution
You will be provided with an HCl solution whose concentration is 0.500 M. Obtain approximately
220 mL of this solution in a clean, labeled 250 mL beaker. Use a graduated cylinder to dispense
exactly 25.0 mL of this solution into four labeled 125 mL Erlenmeyer flasks. Add four drops of
phenolphthalein indicator to each flask.
In order to perform the titration, you will use a piece of glassware called a burette (Figure 1). First,
add about 10 mL of NaOH solution to the burette and allow it to drain out, in order to rinse the
burette. With the stopcock closed, fill the burette to near the top of the graduated region with the
NaOH solution. (You do not have to hit the 0.00 mL mark exactly.)
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CHEM 132 Lab 11
Western Carolina University
Figure 1. Titration apparatus.
A quick titration is first done in order to estimate the volume of NaOH needed to exactly neutralize
the acid. Record the initial volume of NaOH solution in the burette (to the hundredths place). Add
20 mL of NaOH solution from the burette to Erlenmeyer flask #1, while swirling. The solution
should appear pink momentarily, and then return to colorless with swirling. Next add NaOH to the
flask 1 mL at a time until the solution stays pink. Record the final burette reading to the hundredths
place. Subtraction of the final burette reading from the initial burette reading gives you the total
volume of NaOH added.
Three careful titrations will be performed in order to find the exact concentration of the NaOH. For
these titriations, the volume of NaOH you can add quickly without going past the endpoint is 2 mL
less than the volume that caused a color change in titration #1. For example, if your solution turned
color after addition of 27 mL, then 25 mL NaOH solution can be quickly run into to the flask. (Any
additional NaOH will be added very slowly.) Fill in the data from quick titration #1 on your data
sheet.
Titration #1 – approximate endpoint:
→ Initial burette reading
mL
→ Final burette reading
mL
→ Volume to go past the endpoint
mL
→ Volume you can add quickly
without exceeding the endpoint
mL
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CHEM 132 Lab 11
Western Carolina University
Now you will perform the three careful titrations. Refill the burette with sodium hydroxide solution,
record the initial burette reading to hundredths place in Table I below, and add the amount of NaOH
you can add quickly without exceeding the endpoint to Erlenmeyer flask #2. Then slowly and
carefully add NaOH solution one drop at a time with swirling, until the proper endpoint is reached.
At the endpoint, the solution should turn faint pink, and this color should last for about 30 seconds
then fade. (If your solution is a bright pink color that persists, you have gone past the endpoint.) At
the endpoint of the titration, the added base will exactly neutralize the acid that was in the flask.
Once you reach the endpoint, record the final burette reading (to hundredths place) in the table under
“Titration #2. Repeat this procedure for Erlenmeyer flasks #3 and #4.
Table I – Exact Titration Data
Titration #2
(flask #2)
Use Data Sheet
Titration #3
(flask #3)
Titration #4
(flask #4)
Initial burette reading
mL
mL
mL
Final burette reading
mL
mL
mL
Volume of NaOH
mL
mL
mL
Molarity of HCl solution
0.500 M
Molarity of NaOH solution
0.500 M
M
0.500 M
M
M
M
Average NaOH Molarity
from three titrations
(transfer this number to Table II)
The concentration of NaOH (MNaOH) is determined for each titration from the following equation:
MNaOH =
MHCl × VHCl ×
1
VNaOH
(4)
where VHCl is the volume of standardized HCl solution, MHCl is the molarity of the standardized HCl
solution, and VNaOH is the volume of NaOH solution added. Perform this calculation three times and
calculate the average molarity of the NaOH solution.
1. Why do you have to perform this titration three times?
3. Rolaids versus Tums
Both Rolaids and Tums neutralize excess stomach acid (HCl) and both make various advertising
claims. Let's find out which is more effective.
Although antacids do not readily dissolve in water, they react with acids and the reaction products
will dissolve. In order to determine the neutralizing power of the antacids, a known excess volume of
hydrochloric acid will be added to the antacid. The amount of excess acid can then be determined by
titrating the solution with the standardized base. Calculation of the moles of acid neutralized per
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CHEM 132 Lab 11
Western Carolina University
antacid tablet (molant) is achieved by use of the following equation, which simply states that at the
endpoint the moles of acid are equal to the moles of base:
(VHCl × MHCl) = (VNaOH × MNaOH) + molant
(5)
where VHCl is the volume of HCl solution added in liters, MHCl is the molarity of the HCl solution
used in the previous portion of the experiment, VNaOH is the volume of NaOH solution added in
liters, and MNaOH is the molarity of the NaOH solution standardized in the previous portion of the
experiment. Equation (5) is rearranged to solve for the moles of antacid which gives:
molant = (VHCl × MHCl) – (VNaOH × MNaOH)
(6)
Procedure. Separately, crush a tablet of each kind using the mortar and pestle designated for each
antacid. Weigh the Rolaids and Tums powders and place each in a separate, clean 125 mL flask.
Use your graduated cylinder to add exactly 50.0 mL of 0.500 M HCl to each flask and stir until the
reaction has stopped. Add 4 or 5 drops of phenolphthalein indicator. Titrate with your standardized
NaOH solution from the previous section and calculate the number of moles of acid neutralized by
each of the tablets using the equation above. Record results in Table II of your data sheet.
Table II
Tums
mass of powder
Rolaids
g
G
Initial burette reading
mL
mL
Final burette reading
mL
mL
Volume of NaOH
mL
mL
L
L
M
M
Volume of NaOH
Molarity of NaOH (average
from table in section #2)
Molarity of HCl solution
0.500 M
0.500 M
Volume of HCl
Moles of antacid per tablet
(molant)
0.0500 L
0.0500 L
mole
tablet
mole
g
Moles of antacid per gram
of tablet
5
mole
tablet
mole
g
CHEM 132 Lab 11
Western Carolina University
Questions - answer on-line
1. Which antacid is more effective per tablet?
2. Which antacid is more effective per gram of tablet?
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