Chemistry 142 Laboratory Manual

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EXPERIMENT 8            Estimating Avogadro's

        To estimate the number of stearic acid molecules in a mole of stearic acid based on an assumption
        about a thin film of stearic acid on a water surface. A monolayer of stearic acid can be created in
        which we can estimate the number of molecules. To do this we make assumptions of the size and
        shape of the stearic acid molecule.

        In the International System (SI) of units, the fundamental measure of quantity is the mole. A mole
        of carbon is 12.0000 grams of carbon-12, and Avogadro's number, NA, is the number of carbon
        atoms in this mass. A mole of any substance contains Avogadro's number of particles.

        In this experiment you will estimate Avogadro's number by calculating the amount of stearic acid
        necessary to form a single layer on the surface of water. This single layer is called a monolayer. We
        will be making some assumptions about how stearic acid molecules arrange themselves to form the
        monolayer. From these assumptions we can determine the thickness of the layer, and from the
        thickness we can estimate the number of the stearic acid molecules. If we know the volume occu-
        pied by a mole of stearic acid molecules we can use the volume of one stearic acid molecule to esti-
        mate Avogadro's number. We simply divide the volume of a mole of stearic acid by the volume of
        one stearic acid molecule.

        Note  the number calculated will not be accurate, but should be within a power of 10 of the
        accepted value of 6.02214 x 1023 particles/mol.

        Theory and Basis of the Experiment

        One of the properties displayed by water is its polarity. For covalent polar molecules, it is a partial
        charge separation denoted by a delta, . These polar ends are referred to as dipole moments. Polar

        Chemistry 120 Grossmont College                                                              8127
Estimating Avogadro's Number

molecules attract each other in much the same way magnets do. The negative end of the dipole
moment of one polar molecule is attracted to the positive end of the dipole moment of another polar
molecule. Because of this, acetic acid, CH3COOH, will dissolve in water. Whereas, pentane,
CH3CH2CH2CH2CH3, a non-polar molecule is not water soluble. It stands to reason that a molecule
that possesses both a nonpolar and polar end will have the polar end attracted to water and the non-
polar end be repelled by water.

Stearic acid, CH3(CH2)16CO2H, has the structure shown below. The acid has a polar portion and a
non-polar portion. The large non-polar, hydrophobic portion, of the molecule makes it immiscible
with water.

        FIGURE 8.1

           H H H H H H H H H H H H H H H H H O
    H C         C C     C    C C       C    C C        C    C C        C     C C        C    C C       OH
           H H H H H H H H H H H H H H H H H

                     nonpolar end                                                              polar end

We will create a layer of stearic acid one molecule thick on the surface of a watch glass filled with
water. The polar head group will stay near the water and the non-polar tail will orient themselves
away from the water.

        FIGURE 8.2

water surface                                                              non-polar hydrophobic tails

                 C    C                         C
                            HOCO     C
               HO O HO O           HO O H OCO HO O   HO CO HO CO   HO CO
                                                                             pola r hy drophyllic hea d group

We will create our monolayer by using a solution of the stearic acid in hexane. Hexane is also
hydrophobic and will not mix with water. The hexane will evaporate leaving just the thin layer of
stearic acid.

If we know the area of the monolayer and we know the volume of the monolayer, we can measure
the thickness, t, of the monolayer. This thickness will correspond to the length of the stearic acid
molecule. If we make the assumption that the stearic acid molecule is a rectangular solid, with a
width and length equal to 1/5.44 of the height, where the height equals the thickness of the mono-
layer, we can calculate the volume of one stearic acid molecule. Knowing the volume of the mono-
layer and the volume of one molecule we can calculate the number of molecules in the monolayer.
By using the density of stearic acid we can determine the number of moles in the monolayer.
Finally, we can use the molar volume of stearic acid and divide by the volume of the stearic acid
molecule to get Avogadro's number, NA!

8128                                                                      Chemistry 120 Grossmont College

There are two parts to this experiment. In part 1, we will calibrate a dropping pipet so that we can
determine the number of drops in 1 mL. This will require a Pasteur pipet for optimum results. In
part 2, we will slowly add the stearic acid/hexane solution to a watch glass of water. We will allow
the hexane to evaporate leaving just the stearic acid on the water.

Any impurities will ruin this experiment so be very careful not to put soap, water or dust where it
does not belong. Do not use dirty glassware!


Part 1: Calibration of the Pipet
1.   Obtain a Pasteur pipet from your instructor
2.   Obtain a 10 mL beaker from the stock room. Wash it well. Wash the inside 3 times with approx-
     imately 1 mL of aqueous ammonia. Rinse the beaker 3 times with distilled water. Rinse the bea-
     ker 1 time with acetone and then dry the beaker thoroughly with a paper towel. Rinse the beaker
     three times with approximately 1/2-mL of hexane and then add 3 mL of hexane to the beaker.

             HEXANE IN THE SINK.

3.   Obtain a clean dry 10 mL graduated cylinder from your drawer. If your graduated cylinder is not
     clean and dry, clean it and then dry it with acetone and a tissue. Add hexane drop-wise to a vol-
     ume of 1 mL. Record the number of drops.
4.   Repeat this procedure.
5.   If the two values are relatively close (within 10% of each other), then find the average. If not,
     repeat step 3 again.
6.   Dispose of excess hexane in the appropriate waste container and save the beaker for part 2.

Part 2: The Monolayer
1.   Obtain a watch glass from your drawer. Measure the diameter. Thoroughly clean the watch
     glass by scrubbing with soap for 2 minutes. Rinse well with tap water and then scrub with aque-
     ous ammonia. Rinse 3 times with tap water and then three times with distilled water. Hold the
     glass by the edges to avoid putting fingerprints on it.
2.   Place the watch glass on a beaker; making sure that the watch glass is parallel to the floor. Fill
     the watch glass to the brim with distilled water.
3.   Take your clean 10 mL beaker and pour into it the stearic acid/hexane solution from under the
4.   Draw some of the stearic acid/hexane solution into your Pasteur pipet, hold the dropper perpen-
     dicular to the surface of the water in the watch glass, and add one drop of the stearic acid solu-
        a.   The solution should spread out rapidly across the surface of the water and disappear
             within a few seconds.
        b.   If the watch glass is not properly cleaned then an oily residue may appear after only a
             few drops of solution. In this case it will be necessary to clean the watch glass again.

Chemistry 120 Grossmont College                                                              8129
Estimating Avogadro's Number

        c.   If the first few drops disappear rapidly, continue adding the solution drop-wise, counting
             the drops.
        d.   As the monolayer nears completion, the drop of solution forms a circular pattern rather
             than flowering out.
        e.   The circular film of solution contracts as it evaporates and disappears in a relatively
             short time.
        f.   This pattern will be observed for a few drops until finally, one drop strikes the surface
             and remains as a lens or globule that requires a prolonged period of time to disappear.
5.   Record the number of drops. At this point, the surface of the water is covered with a monolayer
     of stearic acid and one more drop placed at a different point on the water surface forms a second
6.   Fill out the following table. You must show your calculations for all starred rows. Clearly show
     these calculations using dimensional analysis.

8130                                                               Chemistry 120 Grossmont College
Prelaboratory Exercise

Prelaboratory Exercise                       Lab Grade
                                             Prelab Questions                                20
Estimating Avogadro's Number                 General Format (Signature, ink,                 10
                                             no obliterations, etc.)
Name: _____________________                  Data and Analysis (observations,                20
                                             questions, units, significant fig-
Section:____________________                 ures, sample calculations, etc.)
                                             Accuracy                                        30
                                             Post Lab Questions                              20
                                             Total                                           100

1.   What substance will you use to make a monolayer? ___ ___ ___ ___ ___ ___ ___

2.   How close should your experimental value be to the actual value of Avogadro's number?
        a.   Within 1% of actual value
        b.   Within 5% of actual value
        c.   Within a factor of 10 of the actual value
        d.   Within a factor of 100 of the actual value

3.   What do polar molecules behave like? ___ ___ ___ ___ ___ ___ ___.

4.   Where do you dispose of the waste hexane?

5.   What is the procedure for cleaning the watch glass?

6.   How do you know when the stearic acid has formed a monolayer covering the entire surface of
     the water?

Chemistry 120 Grossmont College                                                        8131
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