Dallas College Dilute NaOH Solution Preparation Pre Lab Questions

  • Read the experiment and outline the general steps of the procedure in your own handwriting.
    Acid Base Titrations with Volumetric Dilution
    CHEM 1405 Laboratory Experiment
    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.
    Brookhaven College Chemistry – v3
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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
    (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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    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.
    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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    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
    Brookhaven College Chemistry – v3
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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.
    Brookhaven College Chemistry – v3
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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
    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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    Acid Base Titrations with Volumetric Dilution
    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
    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
    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 =
    Brookhaven College Chemistry – v3
    (3.062 M)(10.00 mL)
    = 0.1225 M
    (250.00 mL)
    Page 13 of 17
    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)
    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
    10.00 mL x
    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:
    Molarity =
    1.378 x 10−3 mol
    0.01000 L
    Brookhaven College Chemistry – v3
    = 0.1378 M
    Page 14 of 17
    Name: ___________________________________________ Date: _______________ Section: __________
    Acid Base Titrations with Volumetric Dilution
    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
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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.
    Brookhaven College Chemistry – v3
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    Name: ___________________________________________ Date: _______________ Section: __________
    Acid Base Titrations with Volumetric Dilution
    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
    Initial Buret
    Final Buret
    Volume of Titrant
    Molarity of
    Oxalic Acid
    Average Molarity of Oxalic Acid Solution: ________________ M
    Show Your Calculations on the back of this page. (Hint: Refer to Calculation Help Sheet Example 2.)
    Brookhaven College Chemistry – v3
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