Avogadro Report F21

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University Of Connecticut *

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Chemistry

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Dec 6, 2023

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Prof. Rugg and Prof Wood Avogadro Page 1 University of Connecticut Stamford Campus CHEM 1127 Laboratory Fall 2021 Determination of Avogadro’s Number from a Thin Film of Stearic Acid Introduction Atoms and understanding them are fundamental to the study of chemistry. Everything in chemistry depends on atomic theory, and in turn, understanding atomic theory requires understanding both atoms in both the qualitative and the quantitative senses. We have begun with some of the qualitative: that elements are composed of atoms, and that atoms form compounds when they combine. Atoms also have mass, which brings in the quantitative, and in that realm, requires a counting unit for atoms. Just as bakers count donuts by the dozen, chemists count atoms by the mole. The existence of the fundamental constant called Avogadro’s number is fundamental to the way we understand atoms as the foundation of chemistry. In this experiment, we will investigate the magnitu de of Avogadro’s number. We will do this by invoking a concept you have likely heard in the past: that oil and water do not mix. They don’t, due to the tendency of like atoms and molecules to associate with each other, and unlike molecules to repel each other. Some molecules are highly polar: there is a significant separation of positive and negative charge as part of their structure. Water is among these molecules. Other molecules have little or no separation of charge; these molecules (such as simple oils) are called nonpolar. In fact, as you have seen in everyday life, oil will spread out on the surface of water, often producing an interference pattern resembling a rainbow. Although oil and water will not mix on their own, there are plenty of molecules that have one end capable of associating with water and the other with oil – you likely used some of them within the last few hours, as they are fundamental to soaps and detergents – whether they’re laundry detergents like Tide or Persil, or soap or body wash like Dial or Axe. The molecule we will be using in this experiment is stearic acid dissolved in a solvent called cyclohexane. Cyclohexane is a nonpolar solvent with the formula C 6 H 12 , and stearic acid has the formula C 17 H 35 COOH. Notice that part of the stearic acid looks like cyclohexane; write water as H- OH and you’ll see that the other part looks just like water. Stearic acid is obtained from fats and oils, whether of plant, animal or petroleum origin. The long chain of carbons and hydrogens is strongly attracted to the cyclohexane solvent, while the polar COOH group easily forms intermolecular bonds with water. This means the molecules will line up readily at the interface between the cyclohexane and the water. The structure of stearic acid: We will make a few approximations for the system in this experiment: 1. That we can estimate the volume taken up by one molecule of stearic acid 2. That the molecule approximates the shape of a rectangular prism (think elongated “cube”). 3. That the molecule is about 4.55 Å on a side, and 24.2 Å long. (1 Å = 1 X 10 -10 m or 1 X 10 -8 cm). 4. That we can spread out the stearic acid to form a monolayer – a single layer of molecules on the surface of a circular dish of water. The long chain points away from the surface of water, while
Prof. Rugg and Prof Wood Avogadro Page 2 the portion of the molecule with COOH groups sits on the surface, interacting with the water molecules. From these points, we can estimate both the volume of one molecule and the volume of a film consisting of a monolayer of stearic acid. We can then determine the number of molecules on the surface of the water. Using the density of the stearic acid solution and the molar mass of stearic acid, we can calculate Avogadro’s number. Procedure – please watch the video 1. CAUTION: No flames during this experiment – cyclohexane is a flammable solvent. 2. Obtain about 5 mL of cyclohexane in a clean, dry 100 mL graduated cylinder. To calibrate the pipet, add cyclohexane drop by drop (with counting!) until you fill a 10 mL graduated cylinder to the one mL mark. Repeat this procedure two or three times, holding the pipet at a constant angle so the delivery of the drops is standardized. 3. You should obtain a count between 60 and 70 drops per milliliter. Record the average number of drops per mL. 4. Obtain a Petri dish from the stock bench and wash it thoroughly with detergent and water. Rinse afterward under cold tap water for at least two minutes to thoroughly flush away all traces of detergent. Rinse the dish with RO water and then fill the dish about ¾ full with RO water. The level must be a bit below the rim of the dish. Between runs, wash with water only, finishing up with an RO water rinse. DO NOT use any further detergent! 5. Measure and record the diameter of the dish to the nearest 0.1 cm. It is the inside diameter you need to measure. 6. Obtain a sample of stearic acid in cyclohexane (the concentration will be provided in lab). Keep the bottle closed when not using it to prevent the concentration from changing due to the evaporation of cyclohexane. 7. Rinse your calibrated pipet twice with the solution, discarding the rinse solution between rinses. Add the solution dropwise (observing the angle precaution from your calibration run) slowly. The drops will spread out over the surface of the water in the dish. As each drop spreads out, add another drop. The spreading out of the drops will slow as the surface reaches monolayer coverage. The final drop you add will not spread out but will form a lens-shaped droplet that will persist for 30 seconds. Stop adding solution when this happens; you will subtract that drop from your total count. Record the total number of drops you add. 8. Repeat the above process (starting with adding the water to the dish) twice more for a total of three runs. What you should find is that the total number of drops agrees to within two or three.
Prof. Rugg and Prof Wood Avogadro Page 3 Report for Avogadro Name__________________________________________ Section________________________ Data and Calculations Show your work! Remember Sig Figs! Remember 1 mL = 1 cm 3 1. Record the concentration of the stearic acid in cyclohexane: ____________________g/mL 2. Average # of drops per mL cyclohexane:___________drops/mL (ave of 3 runs) Run #1 = _______ Run #2 = __________ Run #3 = ____________ 3. Average # of drops needed to form a monolayer on the water surface: ____________drops Run #1 = _________ Run #2 = ___________ Run #3 = _____________ Measure the petri dish to the appropriate number of significant figures. 4. Diameter of dish: _____________ cm 5. Radius of the dish:_____________ cm Perform the following calculations with your lab partner. Show all your work! 6. Calculate the volume of one molecule of stearic acid in cm 3 . (The molecule is about 4.5 Å by 4.5 Å by 24.2 Å. ) Remember that one angstrom (Å ) is equal to 1 X 10 -8 cm). Show your work. Remember Significant Figures! ___________________________cm 3
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