Mass of Stearic Acid in Monolayer 2

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					Estimating Avogadro’s Number

         Chemistry 141
          Lab Handout
                            Stearic Acid
  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




         water surface                                                         non-polar hydrophobic tails




                      C    C      C      C           C
                    HO O HO O   HO O   HO O HO C O HO O     C
                                                          HO O HO C O
                                                                          C
                                                                        HO O
                                                                                polar hydrophyllic head group



                Monolayer of stearic acid molecules
                      Stearic Acid
                            Thickness = 5.44 x m



       H H H H H H H H H H H H H H H H H O

xm   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


              Volume of 1 stearic acid molecule
              = length x width x height
              = (x m) (x m) (5.44 x m)
              =5.44 x3 m3
       Part 1: Calibration of the pipet.
• Wash and dry a 10 mL beaker.
   –   3x ammonia wash
   –   3x deionized water rinse
   –   1x acetone rinse
   –   Dry well with paper towel
   –   3x hexane rinse (1/2 mL per rinse) ALL WASTE HEXANE MUST BE
       DISPOSED OF IN THE WASTE CONTAINERS. DO NOT DUMP HEXANE
       IN THE SINK.
• Fill 10 mL beaker with hexane.
• Transfer hexane drop-wise to a clean dry 10-mL graduated cylinder
  to reach a volume of 1 ml. Record the number of drops.
• Repeat until 2 trials are within 10% of each other.
• Dispose of excess hexane in the appropriate waste container.
• Calculate average volume per drop of hexane.
                Sample Data
Trial   Drops of    Average Drops   Average
        hexane/mL   hexane/mL       mL/drop
                                    hexane
1       18          19              0.052 mL
2       20
3       19
                Part II: The Monolayer.
• Clean a watchglass with soap and water, ammonia, and deionized water.
  Hold the glass by the edges to avoid putting fingerprints on it.
• Place the watch glass on a ring and make sure that the watch glass is
  parallel to the floor. Fill the watch glass to the brim with deionized water.
• Fill clean 10 mL beaker with stearic acid/hexane solution from under the
  hood.
• Add stearic acid/hexane dropwise to the watchglass. Count the drops.
    – The solution should spread out rapidly across the surface of the water and
      disappear within a few seconds. 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. If the first few drops
      disappear rapidly, continue adding the solution drop-wise, counting the drops.
      After approximately 20 drops, as the monolayer nears completion, the drop of
      solution forms a circular pattern rather than flowering out. The circular film of
      solution contracts as it evaporates and disappears in a relatively short time.
      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. 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 “lens”.
                     Sample Calculations
                             Trial 1

Drops hexane/mL              19

Number of drops of           42
solution used for
monolayer


Stearic Acid concentration   0.10 g/mL




Mass of Stearic Acid in      2.2 x 10-4 g SA
Monolayer
                     Sample Calculations
                     Trial 1
Mass of Stearic      2.2 x 10-4 g SA
Acid in Monolayer
Density of Stearic   0.870 g/mL
Acid
Volume of Stearic    2.5 x 10-4 mL SA
Acid in Monolayer
                    Sample Calculations
                    Trial 1
Diameter of water   15.3 cm
surface
Area of water       184 cm2
surface
                    Sample Calculations
                    Trial 1
Volume of Stearic   2.5 x 10-4 mL SA
Acid in Monolayer
Area of water       184 cm2
surface
Thickness of        1.4 x 10-6 cm
monolayer
               Sample Calculations
               Trial 1
Thickness of   1.4 x 10-6 cm
monolayer
Height of SA   1.4 x 10-6 cm
molecule
Width of SA    2.5 x 10-7 cm
molecule
Volume SA      8.7 x 10-20 cm3
molecule
                    Sample Calculations
                    Trial 1
Volume of Stearic   2.5 x 10-4 mL SA
Acid in Monolayer
Volume SA           8.7 x 10-20 mL
molecule
Number SA           2.9 x 1015
molecules in
monolayer
                    Sample Calculations
                    Trial 1
Mass of SA in       2.2 x 10-4 g SA
Monolayer
Molar Mass SA       284 g/mol
Moles of Stearic    7.7 x 10-7 mol SA
Acid in Monolayer
                     Sample Calculations
                     Trial 1
Number SA            2.9 x 1015          Not bad for random
molecules in         molecules SA            numbers!!
monolayer
Moles of Stearic     7.7 x 10-7 mol SA
Acid in Monolayer
Experimental value   3.8 x 1021
of Avogadro’s        molec/mol
number

				
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posted:4/2/2013
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