REAGENTS 10 mL of fatty acid (e.g., stearic, lauric, oleic, palmitic, linoleic), 5 mL of fatty acid solution (0.05 g/L in hexane or pentane) SAFETY AND DISPOSAL Refer to the MSDS information available online when working with stearic,' lauric, oleic, palmitic, linoleic acids, pentane," or hexane." 3 Disposal for all compounds must be in accordance with local, state and federal regulations. Disposal of fatty acids should be into a labeled laboratory waste container for solid organic chemicals. Disposal for pentane, hexane and fatty acid solutions of the same should be into a labeled laboratory waste container for liquid organic chemicals. INTRODUCTION Avogadro's number, 6.0225 x 10 unit particles, is named after Amedeo Avogadro, whose observations of chemical reactions led to its discovery. This number, which defines the number of nit particles in a mole, can be estimated by measuring the number of molecples in a fatty acid film formed on the surface of water. Fatty acids are substances that contain a carboxyl group (-COOH) and a long chain aliphatic group. They are oily substances with the properties of acids. The structure of lauric acid, CH (CH) COOH, a typical fatty acid, is shown in Fig. 4-1. Such substances are insoluble in water but soluble in organic substances such as hexane or pentane amount of a fatty acid INTRODUCTION Avogadro's number, 6.0225 x 1023 unit particles, is named after Amedeo Avogadro, whose observations of chemical reactions led to its discovery. This number, which defines the number of nit particles in a mole, can be estimated by measuring the number of molecules in a fatty acid film formed on the surface of water. Fatty acids are substances that contain a carboxyl group (-COOH) and a long chain aliphatic group. They are oily substances with the properties of acids. The structure of lauric acid, CH3(CH2)10COOH, a typical fatty acid, is shown in Fig. 4-1. Such substances are insoluble in water but soluble in organic substances such as hexane or Structural formula Fig. 4-1 Lauric Acid Schematic representati pentane. In this experiment, a known amount of a fatty acid solution is deposited on a water surface of known area (a watch glass can be used to hold the water). As the solvent evaporates, the fatty acid spreads into a film whose structure is shown in Fig. 4-2. The acid end of the fatty acid molecule adheres to the water surface, and the aliphatic chain stands perpendicular to the surface. The film formed by such fatty acids is assumed to be one molecule thick and therefore is called a molecular monolayer. The boundary of the water surface, and therefore is called a molecular monolayer. The boundary of the water surface, and therefore the boundary Experiment 4 Monolayer betermination of Avogadro's Number Fig. 4-2. Compact fatty acid molecular monolayer of the monolayer, will be defined by the rim of the watch Fatty acid monolayer glass. Thus, both the mass and Water surface Watch glass the area of the film are measured in its formation. If both the volume of the fatty acid added to the water surface and the area occupied by the film are known, the film thickness can be determined. It should closely approximate the length of a fatty molecule. If one dimension of a fatty acid molecule is known, and if the molecules have a known shape, it is possible to estimate the volume occupied by a single molecule and thus to calculate the number of molecules in the film. From the known mass of the film, the number of molecules per gram can be calculated. Knowledge of the formula weight and the number of molecules per gram allows calculation of the number of molecules per mole - Avogadro's number. gram allows calculation of the number of molecules per mole - Avogadro's number. PROCEDURE Wear your safety goggles throughout the experiment. Keep fatty acid solutions away from al open flames. All equipment must be scrupulously clean and rinsed free of any detergent. Record the name and molecular weight of the assigned fatty acid on your report sheet. Density Determination If the assigned fatty acid is a liquid. Determine its density in the following manner. Weigh a dry 10.0-mL graduated cylinder and record its mass. Add 5 to 10 mL of the fatty acid, reweigh, record the volume of the fatty acid present, and calculate its density in grams per milliliter. Dispose of the fatty acid in the waste jar designated for that purpose. If the assigned fatty acid is a solid. Determine its density as follows. Mount a 250-mL beaker on a wire mesh on a ring stand above a Bunsen burner, Fill the beaker with approximately 200 mL of water and heat the water to about 85°C. While the water is heating, weigh a dry 10.0-mL graduated cylinder and record its mass. Add solid fatty acid to the graduated cylinder to a height of 3 to 4 cm, reweigh, and record the mass of the graduated cylinder and its contents. After the water in the beaker has reached a temperature of 85°C, stop heating and place the graduated cylinder inside the beaker of water. Do not let the inside of the cylinder or its contents get wet. The fatty acid should begin to melt within the cylinder. When all the fatty acid has melted, remove the cylinder from the water bath and dry the outside with a towel. Allow the liquid to condense once more into a solid. Record the column of fatty acid present and calculate the density in grams per mL. Dispose of the fatty acid as directed by your instructor. Dropper calibration Prepare a capillary dropping pipet from a piece of 7-mm glass tubing. The capillary should be at least 5 cm long and the wide portion at least 10 cm long. (Review the material on glass working in the Laboratory Equipment and Techniques section.) After it has cooled, fit it with a rubber dropper bulb. From your instructor obtain, in a covered beaker, 5 mL of a solution of the same fatty acid. The solution should contain an accurately known concentration of fatty acid in an easily vaporized solvent (e.g., hexane). Record the concentration of fatty acid in g/mL of solution. Calibrate the pipet by counting and recording the number of drops of fatty acid solution required to fill a 10-mL graduated cylinder to the 2-mL mark. Properties of the Film Measure and record the diameter of a "100-mm" watch glass to the nearest 0.2 mm in several directions. Calculate an Experiment 4 Monolayer Determination of Avogadro's Number Fig. 4-3. Watch glass mounted on ring stand average diameter. Place the 100-mm watch glass on a 3-in iron ring mounted on a ring stand and then fill it to the grim with distilled water as shown in Fig. 4-3. A truly clean watch glass will show no "dimples" about the glass-water interface. Equipment must be scrupulously clean. Using the calibrated pipet, slowly and gently add drops of fatty acid solution to the water surface at the center of the watch glass Be sure to record the number of drops of solution added to the surface of the water. As the solvent evaporates, a thin film of fatty acid will be deposited on the surface of the water. Allow about 10 seconds per drop for the solvent to evaporate. Continue to add the fatty acid solution until it is no longer dispersed across the water surface, that is, until the addition of a single drop forms a lens of fatty acid. Under these conditions the surface of the water is covered by a monolayer of fatty acid. Dispose of the excess fatty acid solution as directed by your instructor. Calculations. From the volume and known concentration of fatty acid solution added, calculate the mass of fatty acid present as a thin film. Mass of fatty acid = drops of fatty acid milliliters of grams of fatty acid X film solution added. fatty acid solution per drop x per milliliter of solution Calculate the average thickness of the thin film using the previously determined density and mass. Volume of fatty acid film = mass of fatty acid monolayer film density of fatty acid Calculate the average thickness of the thin film from its volume and area. Average film thickness= volume of fatty acid surface area of monolayer film If the assumption that a molecular monolayer of fatty acid is formed is correct, then the average film thickness should equal the apparent length of a fatty acid molecule. Perform the following calculations to the correct number of significant digits, Use a separate sheet and include it in the report on this experiment. Show all units used in arriving at an answer. Record the answers in the appropriate spaces on the report sheet As a rough approximation, consider that the fatty acid film is a molecular monolayer and the molecules are cubes. What is the length, I, of an edge? The volume of a cube is given by the formula 1=1 Determine the volume of one molecule. Since the volume of the film is known, the number of molecules in the film can be calculated. Number of molecules in film = volume of fatty acid film volume of one molecule Divide the number of molecules in the film by the mass of the monolayer to find the number of molecules per gram of fatty acid. Avogadro's number (No, the number of molecules in 1 mole) can be calculated from the number of molecules per gram and the molecular weight. No number of molecules per gram x mass per mole = number of molecules per gram x molecular weight = number of molecules per mole DATA AND RESULTS REPORT ON EXPERIMENT 4 Monolayer Determination of Avogadro's Number Assigned fatty acid Stearic Acid_ - Molecular weight 284.48 g/mol Density Determination (Volume Acid = 10 mL) Mass of graduated cylinder and fatty acid 37.48 g 29.57 Mass of dry graduated cylinder g Mass of fatty acid Density of fatty acid Dropper Calibration Number of drops per milliliter Fatty acid solution concentration Properties of the Film Diameter of watch glass 9.25cm - 84 0.00028 g/L Number of drops per milliliter Fatty acid solution concentration Properties of the Film Diameter of watch glass 9.25cm Average diameter, d Area of monolayer film, d²/4 84 0.00028 g/L 9.17cm Number of drops of solution required to make film 5 drops Calculations Mass of film Volume of film Average thickness of film Dimensions of one molecule == Volume of one molecule Number of molecules in film Molecules per gram of fatty acid Avogadro's Number Show the details of all calculations; use extra sheets if necessary. QUESTIONS 1. (Submit your answers on a separate sheet if necessary) Briefly discuss whether the following "mistakes" would increase, decrease, or have no effect on the value calculated for Avogadro's number. a) The solvent does not completely evaporate from the molecular film. b) The water surface is contaminated by small amounts of detergent. c) The density of the fatty acid was smaller than reported. d) The actual number of drops per milliliter of the pipet is small than reported. 2. An oil tanker whose cargo hold is in the shape of a rectangular solid discharges its oil into a suitable tank on shore. If the inner dimensions of the tank are 25' x 75' x 25', and a monomolecular film of oil cannot be removed from the inner wall surfaces, how many gallons of oil remain? Assume a 2.0 nm thick monolayer of oil is formed.