Es#ma#ng 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
polar end
nonpolar end
non-polar hydrophobic tails
water surface
C
C
HO O HO O
C
HO O
C
C
HO O HO CO HO O
C
HO O HO CO
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
x m 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
Volume of 1 stearic acid molecule = length x width x height = (x m) (x m) (5.44 x m) =5.44 x3 m3 polar end
•
Part 1: Calibra,on 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 un#l 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 hexane/mL Average Drops Average mL/
hexane/mL drop hexane 1 18 19 2 20 3 19 0.052 mL •
Part II: The Monolayer. Clean a watchglass with soap and w ater, ammonia, and deionized water. Hold the glass by the edges to avoid puZng 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 solu#on from under the hood. •  Add stearic acid/hexane dropwise to the watchglass. Count the drops. –  The solu#on 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 a_er only a few drops of solu#on. In this case it will be necessary to clean the watch glass again. If the first few drops disappear rapidly, con#nue adding the solu#on drop-­‐wise, coun#ng the drops. A_er approximately 20 drops, as the monolayer nears comple#on, the drop of solu#on forms a circular pa`ern rather than flowering out. The circular film of solu#on contracts as it evaporates and disappears in a rela#vely short #me. This pa`ern will be observed for a few drops un#l finally, one drop strikes the surface and remains as a lens or globule that requires a prolonged period of #me 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 Calcula#ons Trial 1 Drops hexane/mL 19 Number of drops of solu#on used for monolayer 42 Stearic Acid concentra#on 0.10 g/mL Mass of Stearic Acid in Monolayer 2.2 x 10-­‐4 g SA Sample Calcula#ons Trial 1 Mass of Stearic Acid in Monolayer 2.2 x 10-­‐4 g SA Density of Stearic Acid 0.870 g/mL Volume of Stearic Acid in Monolayer 2.5 x 10-­‐4 mL SA Sample Calcula#ons Trial 1 Diameter of water surface 15.3 cm Area of water surface 184 cm2 Sample Calcula#ons Trial 1 Volume of Stearic Acid in Monolayer 2.5 x 10-­‐4 mL SA Area of water surface 184 cm2 Thickness of monolayer 1.4 x 10-­‐6 cm Sample Calcula#ons Trial 1 Thickness of monolayer 1.4 x 10-­‐6 cm Height of SA molecule 1.4 x 10-­‐6 cm Width of SA molecule 2.5 x 10-­‐7 cm Volume SA molecule 8.7 x 10-­‐20 cm3 Sample Calcula#ons Trial 1 Volume of Stearic Acid in Monolayer 2.5 x 10-­‐4 mL SA Volume SA molecule 8.7 x 10-­‐20 mL Number SA molecules in monolayer 2.9 x 1015 Sample Calcula#ons Trial 1 Mass of SA in Monolayer 2.2 x 10-­‐4 g SA Molar Mass SA 284 g/mol Moles of Stearic Acid in Monolayer 7.7 x 10-­‐7 mol SA Sample Calcula#ons Trial 1 Number SA molecules in monolayer 2.9 x 1015 molecules SA Moles of Stearic Acid in Monolayer 7.7 x 10-­‐7 mol SA Experimental value 3.8 x 1021 molec/
of Avogadro’s mol number Not bad for random numbers!!