Dallas College Dilute NaOH Solution Preparation Pre Lab Questions

QUANTITATIVE ANALYSIS
Acid Base Titrations with Volumetric Dilution
CHEM 1405 Laboratory Experiment
Introduction
Much of the testing done in Chemistry is “qualitative”: trying to find out “what” is present in a given sample.
Once the identity is known, “how much” of a substance is present is answered using Quantitative Analysis.
One of the basic techniques in quantitative analysis is the Titration. A titration uses a buret to deliver, in a
controlled fashion, measurable quantities of a liquid to a flask containing a substance being reacted. The
liquid contained in the buret is called the titrant. The substance in the flask reacts chemically with the titrant
until all of the substance in the reaction flask is consumed. Figure 1, shows the general setup of equipment to
perform a titration.
Titration Apparatus and its Parts
One of the most common titrations involves a neutralization reaction – the reaction between an acid and a
base. For this experiment, the ultimate task is to determine the concentration of an assigned aqueous Oxalic
Acid solution. In order to do this, a base of known concentration (NaOH) is reacted with the acid solution.
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Oxalic Acid is an organic compound that can be represented chemically as H2C2O4 or structurally as
Oxalic Acid is diprotic which means it has two acidic hydrogens. Its reaction with sodium hydroxide can be
represented as:
2NaOH(aq) + H2C2O4(aq) → Na2C2O4(aq) + 2H2O(l)
Note that it requires two moles of NaOH (base) to neutralize one mole of H2C2O4 (acid). When all of the acid is
neutralized, the moles of base added equal the moles of acid that were present. This is defined as the
equivalence point.
The equivalence point can be determined visually using an indicator. An indicator is a chemical which has a
distinct color change occurring as closely to the equivalence point as possible. The point at which the color
change occurs is known as the “endpoint” of the titration. In an acid base titration, the indicator color change
is pH dependent. There are many indicators that are pH dependent with color changes occurring over the
entire range of the pH scale. The indicator of choice will be the one which changes color as closely to the
equivalence point as possible.
The equivalence point at which all of the Oxalic acid and Sodium hydroxide reactants are consumed occurs at
a pH ≈ 8. The indicator being used in this titration is Phenolphthalein which changes from COLORLESS in acidic
solutions
(pH ≤ 7) to PINK at about pH ≥ 8. Since the endpoint of the titration (signaled by the change in color of the
indicator) and the equivalence point of the titration occur at approximately the same pH, the choice of
phenolphthalein is a very good one. Once the equivalence point is reached, the next small addition of NaOH
titrant will cause the pH of the solution to increase dramatically and the color of the indicator will become
more intense. The fainter the pink color, the better the results of the titration since the end point will be
closer to the actual equivalence point.
Given the mole basis of the chemical equation, one of the most convenient concentration units to describe
the solutions is called Molarity. It is assigned the symbol, M, and is defined as the number of moles of solute
per liter of solution.
Molarity = M =
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molsolute
Lsolution
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Many times in an analysis, the titrant is too concentrated to allow accurate volumetric results. The smaller the
volume of titrant used, the less accurate the results because the error in measurement becomes greater
through the estimation of the meniscus. That is the reason that most titrations target endpoints that use 20 –
30 milliliters of titrant as a goal.
If the NaOH standard is too concentrated to allow a large enough volume of titrant in the analysis of the acid
solution, the amount of titrant can be increased by accurately diluting the stock standard base with volumetric
glassware. The two pieces of glassware that would be used are a Volumetric Flask and a Volumetric Pipet.
VOLUMETRIC FLASK
The Volumetric Flask is a pear-shaped flat-bottomed container with a long straight narrow neck that is
calibrated to hold a specific amount of liquid. At a particular point on the neck is an inscribed line ringing the
neck. Figure 2, shows examples of volumetric flasks and their closures.
Examples of Volumetric Flasks
The inscribed line is the fill line that indicates the position of the meniscus when the container is holding the
amount of liquid it is calibrated to hold at a specific temperature. Figure 3, shows the fill line and its use. The
line circles the neck so that error due to parallax can be reduced. This is done by holding the flask at eye-level
so that the inscribed circle appears as a single line. The lowest point of the meniscus should rest on the top of
this line. This piece of glassware is very accurate and is manufactured with the narrow neck so that small
changes in volume show dramatically which decreases error due to meniscus estimation.
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The Meniscus and the Volumetric Flask
Figure 3
The volumetric flask will always be labeled with defining information such as the volume at a specified
temperature, tolerances (statistically accepted variation in measured volume), and the type of flask. See
Figure 4 for these items.
Information Typically Found on a Volumetric Flask
Figure 4
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VOLUMETRIC PIPET
There are different types of volumetric pipets, those which contain a specific volume of liquid (“To Contain” =
TC) and those which deliver a specific volume of liquid (“To Deliver” = TD) at specified temperatures. The
lower portion of the pipet has a long pulled section of glass designed to deliver the liquid at a controlled rate.
The upper thin neck of the pipet functions the same way as the narrow neck of the Volumetric Flask. It has a
ring inscribed to show the position of the meniscus and because of the thinness of the tube, it is very accurate.
There are several ways to pull the liquid into the pipet. Typically, a pipet bulb is used. NEVER mouth pipet.
Figure 5 will show the pipet and typical pipet bulbs.
Pipet and Pipet Bulbs
Figure 5
Every pipet is labeled with important information specific to that pipet. Refer to Figure 6 to see that labeling.
Pipet Labeling
Figure 6
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TECHNIQUE – Using Pipet Bulbs
With the tip of the pipet submerged in the liquid, touch the bulb to the top of the pipet holding it to make a
seal (DO NOT force the bulb onto the end of the pipet). Slowly draw up liquid. When sufficient liquid is filling
the pipet, quickly remove the bulb and cover the top of the pipet with your finger. (Lifting and replacing your
finger on the pipet allows you to adjust the liquid level in the pipet. Lifting your finger completely allows the
liquid to drain from the pipet.) Look closely at Figure 7.
Figure 7
Experimental Overview
A stock NaOH solution will be provided which is too concentrated to yield good analytical results. It will need
volumetric dilution to function as a good analytical standard. The diluted NaOH solution will be used to titrate
an oxalic acid solution of unknown concentration. From the balanced equation, the Molarity of NaOH used,
and the volume of the titrant; the molarity of the acid solution will be determined.
To summarize, during this lab you will:
•
Prepare an NaOH solution by the volumetric dilution of a stock solution
•
Calculate the concentration of the diluted NaOH solution
•
Normalize the Buret with the diluted NaOH solution
•
Titrate the diluted NaOH against your “Unknown” oxalic acid solution
•
Calculate the concentration of the oxalic acid solution
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Experimental Procedure
A. Preparation of the diluted NaOH Solution
Preparing the glassware
Technique Note: The volumetric flask and the pipet must be clean. They do not need to be dry.
1. Rinse the 250 mL Volumetric Flask three times with DI water. (Based upon its tolerances, the volume
of the flask is 250.00mL.) See Figure 8.
Figure 8
2. Rinse the 10.00 mL Volumetric Pipet three times with DI water by drawing a few milliliters of water
into the pipet and then rocking it back and forth while turning it to cover all inner surfaces with the
water. Allow the water to completely drain from the pipet and repeat the rinsing process for a total of
three times.
3. Place about 50mL of the Stock Standard NaOH in a clean, dry 100mL beaker. Using this Stock Standard
NaOH, normalize the water-rinsed 10mL volumetric pipet as follows:
Technique Note: Normalize the pipet with the NaOH solution by drawing up a small amount of the stock
sodium hydroxide solution into the pipet and then rock the pipet back and forth while turning the pipet to
wash the entire surface with the solution to be tested. (This is the same procedure that was used to water
rinse the volumetric pipet.) Allow the solution to drain from the pipet into a waste beaker and repeat the
normalization two more times.
4. Draw liquid into the pipet above the fill line. Dry the outside of the pipet down
to the tip with a
paper towel and then set the meniscus of the liquid on the fill line. Drain the contents of the pipet into
the Volumetric flask. When the liquid has stopped draining, continue to hold the pipet vertically and
wait about 15 seconds to allow any remaining solution to settle into the tip. There will be a drop
hanging from the pipet. Touch the tip to the side of the flask and allow the solution to leave the pipet
(see Figure 9). Any liquid remaining in the pipet is calibrated to be there and should remain. Do not
blow out this pipet. At this point you will have added 10.00mL of the Stock NaOH Solution.
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Figure 9
5. Rinse the walls of the Volumetric Flask with DI Water to wash all of the pipetted solution into the flask.
Fill the 250mL Volumetric Flask to the fill line with DI water. Place the meniscus carefully. Over-dilute
and you must start over. Cap the flask and mix by inversion at least ten times. See Figure 10.
Figure 10
6. Label the volumetric flask with your name and the contents. Keep the standard closed except when
using the solution to minimize the absorption of CO2 from the atmosphere. Over time, the CO2
absorption will change the molarity of the NaOH solution.
7. Calculate the molarity of the secondary standard and enter it on the Data Sheet. (See Calculation Help
Sheet Example 1.)
B. Normalization of the buret with the diluted NaOH solution
Technique Note: Once a solution is prepared, the resulting solution must be protected from unwanted
dilution; for example, a wet buret. This is why the buret is normalized. In normalization, the buret is rinsed
with the titrant so that the buret will have the same concentration of solution as that in the bottle. This is
similar to the technique used to normalize the pipet.
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8. Make sure that the buret stopcock is closed. See Figure 11.
Figure 11
9. Pour about 3mL of the diluted NaOH solution into the buret
10. Rinse the walls of the buret with the solution by gently rocking the buret back and forth while turning
the buret so that the entire interior surface of the buret is bathed in the solution.
11. Open the stopcock and allow the NaOH to drain from the buret into a waste beaker.
12. Close the stopcock and repeat the steps 9) through 11) twice more. The buret is now normalized.
Technique Note: The buret that will be used is a 50.00 mL buret. Each marked milliliter is subdivided into 0.1
mL markings making the estimation between the marks to the nearest 0.01 mL. The buret reads from 0.00 mL
at the top to 50.00 mL at the bottom. The only readable area on the barrel of the buret is between the 0.00
mL line and the 50.00 mL. Always work between these values. If the volume goes below the 50.00 mL mark,
the entire titration must be redone.
Figure 12
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Technique Note: The milliliter lines ring the barrel to help lessen reading error due to parallax. This error is
due to reading position. The meniscus should be read at eye level. At this point, the closest milliliter marking
should appear as a single line and not as a ring. Then, read the meniscus correctly by estimating the value
crossing at the bottom most part of the meniscus curve. See Figure 13.
Reading the Buret
Figure 13
C. Titration of the Acid Unknown with the Diluted NaOH Solution
13. Record the number of the Unknown on the data sheet.
14. Label a 250mL Erlenmeyer flask with your unknown number. It does not need to be dry but must be
clean. This single flask will be used for the multiple titrations you will complete.
15. Preparation of the sample. Rinse the volumetric pipet at least three times with DI water to remove any
traces of NaOH. This pipet will be used to dispense the unknown acid sample. Normalize the 10mL
pipet by quantitatively rinsing the pipet three times with the unknown acid solution.
16. Using the Volumetric Pipet, transfer 10.00mL of sample into a labeled 250mL Erlenmeyer flask.
17. Add about 50mL of DI water to the Erlenmeyer flask. Add three drops of phenolphthalein indicator to
the sample and swirl to mix. The sample is ready to titrate.
18. Fill the NaOH normalized buret with the diluted NaOH solution above the 0.00mL line.
19. Place a 50mL beaker under the buret. Open the stop cock and allow the solution to fill the buret tip.
The solution meniscus should be below the 0.00mL line. Close the stopcock. There should be no
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bubbles in the tip. The solution in the buret should be continuous from top through tip. DO NOT try to
set the meniscus on 0.00mL.
20. Record the initial buret reading on the data page to the nearest 0.01mL. (Remember, you are
estimating the last digit!)
21. Begin the titration by adding small amounts of titrant to the Erlenmeyer flask while swirling the flask to
promote complete mixing.
Technique Note – Quantitative Technique during Titration:
a. Make sure that the titrant goes into the solution and doesn’t just remain on the walls of the flask.
b. Always swirl the flask to promote complete mixing of the solution.
c. As you approach the endpoint, the pink color will persist for a longer period of time before
disappearing. Be sure to rinse the walls of the flask completely so that all of the titrant is in the solution.
i. In a titration, you are titrating a certain number of moles of one substance with the required number
of moles of the other substance. The amount of water added to the titration flask does not change
the number of moles of the substance in the flask. Therefore adding water does not affect the
results of the titration by dilution.
d. As you approach the endpoint, half drops can be added. If it is believed that the endpoint has been
reached and a very pale pink solution persists, take a volume reading before adding the next half drop just
in case that this is the endpoint and the next half drop is past the endpoint.
22. Titrate the first sample to the pink endpoint.
23. Record the final buret reading to the nearest 0.01 mL. (The difference between the final reading and
the initial reading represents the volume of titrant used in the titration.)
24. After recording the final buret reading, refill the buret with the diluted NaOH solution so that the next
titration can be completed without refilling during the titration. Refilling the buret during a single
titration adds error to the determination by requiring more than two meniscus estimations in a single
titration.
25. Rinse the Erlenmeyer flask with tap water three times followed by three DI water rinses. Using this
clean Erlenmeyer flask, repeat steps 16 – 23 of the titration for samples number two and three.
26. Two of the samples should have a titrant volume difference of no more than 0.2mL.
27. If two samples do not agree, titrate two more samples and recheck agreement.
28. Calculate the Molarity of the acid solutions. (See Calculation Help Sheet Example 2.)
29. For the two (or three) runs that are acceptable, calculate the average Molarity.
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D. Clean Up
30. Dispose of the diluted NaOH solution by pouring down the sink with water running. Rinse out the
volumetric flask. Remove and dispose of the label.
31. Rinse the Erlenmeyer flask with water and remove any labels.
32. Rinse the pipet three times with portions of DI water.
33. Rinse the buret three times with portions of DI water and return the buret to the buret cart.
34. Make sure that your bench is wiped up and that no acid or base solution spills are left on the bench.
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QUANTITATIVE ANALYSIS
Acid Base Titrations with Volumetric Dilution
CALCULATION HELP-SHEET
EXAMPLE 1:
A 3.062M stock solution of NaOH is used to make a diluted NaOH solution by pipeting 10.00mL of the stock
solution into a volumetric flask and then diluting the solution to the total volume of the 250mL volumetric
flask (250.00mL). Calculate the Molarity of the diluted solution. [Procedure Step A.7, and Pre-Lab Question 2.]
Dilutions utilize the equation:
M1V1 = M2V2
where:
M1 = Molarity of the stock solution
V1 = Volume of the stock solution (volume pipeted)
M2 = Molarity of the diluted stock solution (what is determined)
V2 = Total Volume of the dilution (volume of the volumetric flask)
STEP 1) Determine what is known and what is needed from the statement of the problem:
M2 = ? (value to calculate)
V2 = 250.00mL (could also be represented as 0.25000L)
M1 = 3.062M
V1 = 10.00mL (could also be represented as 0.01000L)
STEP 2) Algebraically solve for the unknown variable, M2:
M2 =
M1 V1
V2
STEP 3) Substitute the values into the equation. The volumes on both sides of the equation must have the
same units. Therefore, Volume in L can be used as long as liter is the unit used on both sides of the equation.
Similarly, the Volume in mL can be used as long as milliliter is the unit used on both sides of the equation:
M2 =
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(3.062 M)(10.00 mL)
= 0.1225 M
(250.00 mL)
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EXAMPLE 2:
If a 10.00 mL sample of H2C2O4(aq) requires 20.35 mL of 0.1354M NaOH(aq) to completely neutralize it in a
titration, calculate the molarity of the H2C2O4(aq). [Procedure Step C.28 and Pre-Lab Question 3.]
This is a three-step calculation:
STEP 1) Determine the number of moles of H2C2O4 in the reaction:
Solution map: mL NaOH → L NaOH → mol NaOH → mol H2C2O4
Chemical Equation: 2NaOH(aq) + H2C2O4 (aq) → Na2C2O4(aq) + 2H2O(l)
Conversion factors: 1000 mL  1L
Molarity of NaOH = 0.1354 M
2 mol NaOH  1 mol H2C2O4 (molar ratio from chemical equation)
Calculation:
1L NaOH
0.1354 mol NaOH 1 mol H2 C2 O4
)(
)(
) = 1.378 x 10−3 mol H2 C2 O4
20.35 mL NaOH (
1000 mL NaOH
1 L NaOH
2 mol NaOH
STEP 2) Determine the volume, in L, of H2C2O4 required in the titration:
Solution map: mL H2C2O4 → L H2C2O4
Conversion factor: 1 L  1000 mL
Calculation:
10.00 mL x
1L
1000 mL
= 0.01000 L H2C2O4
STEP 3) Calculate the molarity of the solution by using the number of moles of H2C2O4 (aq) , determined in
step 1, and the volume of H2C2O4 (aq) in L, determined in step 2:
Equation:
Molarity =
molsolute
Lsolution
Calculation:
1.378 x 10−3 mol
0.01000 L
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= 0.1378 M
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Name: ___________________________________________ Date: _______________ Section: __________
QUANTITATIVE ANALYSIS
Acid Base Titrations with Volumetric Dilution
PRE-LAB QUESTIONS
1. In an acid-base titration, an indicator is used to visually determine the equivalence point.
a. What indicator will be used in today’s experiment? __________________________
b. What color is this indicator at pH > 8? _____________________________________
2. To accomplish the titration, a NaOH stock standard solution of concentration 3.065M is diluted by
pipetting 25.00mL of the stock solution into a 500mL volumetric flask and then diluting the stock to a
total volume of 500.00mL with DI water. Calculate the concentration of the new solution.
Hint: This problem is an example of the calculation you will use to determine the molarity of your
diluted NaOH solution. Example (1) on the “calculation help-sheet” can be used to help solve this
problem.
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3. Calculate the molarity of an oxalic acid solution, H2C2O4(aq), if a 10.00 mL sample of the oxalic acid
solution requires 25.80 mL of 0.1573M NaOH to neutralize it in a titration.
Hint: This problem is an example of the calculation you will use to determine the molarity of the acid.
Example (2) on the “calculation help-sheet” can be used to help solve this problem.
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Name: ___________________________________________ Date: _______________ Section: __________
QUANTITATIVE ANALYSIS
Acid Base Titrations with Volumetric Dilution
DATA AND RESULTS SHEET
Determination of the Molarity of the Oxalic Acid Solution
Molarity of the Stock Standard NaOH:_________________ M
Calculated Molarity of the Diluted NaOH solution: _____________M
Show your calculation (Hint: Refer to Calculation Help Sheet Example 1)
Unknown Number: ___________
Run #
Pipetted Volume
of Oxalic Acid
Sample
(mL)
Initial Buret
Reading
(mL)
Final Buret
Reading
(mL)
Volume of Titrant
(NaOH)
added
(mL)
Molarity of
Oxalic Acid
Solution
(M)
1
2
3
4
Average Molarity of Oxalic Acid Solution: ________________ M
Show Your Calculations on the back of this page. (Hint: Refer to Calculation Help Sheet Example 2.)
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