Avogadro, Alginates and All That 1.9 2008/09/04 00:27:21 GMT-5 2008/09/05 16:03:16.591 GMT-5 Mary Ellen Rose McHale mmchale@rice.edu Mary Ellen Rose McHale mmchale@rice.edu Alginates Avogadro Estimation of Avogadro's number and fruit spheres Experiment 1: Avogadro, Alginates and All That Mandatory Safety talk by Kathryn Cavender, Director of Environmental Health and Safety at Rice. Objective The purpose of this laboratory exercise is to help you familiarize yourself with the layout of the laboratory including safety aids and the equipment that you will be using this year. Then, to make an order-of-magnitude estimate of the size of a carbon atom and of the number of atoms in a mole of carbon based on simple assumptions about the spreading of a thin film of stearic acid on a water surface Finally, to make fruit spheres Grading Pre-lab – not required for the first lab Lab Report (90%) TA points (10%) Before coming to LabRead the following: Lab instructions Background Information Concepts of the experiment Print out the lab instructions and report form. Read and sign the equipment responsibility form and the safety rules and email Ms Duval at nduval@rice.edu to confirm completing this requirement by noon on September 12 th size 12{ {} rSup { size 8{"th"} } } {}. Introduction Since chemistry is an empirical (experimental) quantitative science, most of the experiments you will do involve measurement. Over the two semesters, you will measure many different types of quantities – temperature, pH, absorbance, etc. – but the most common quantity you will measure will be the amount of a substance. The amount may be measured by (1) weight or mass (grams), (2) volume (milliliters or liters), or (3) determining the number of moles. In this experiment we will review the methods of measuring mass and volume and the calculations whereby number of moles are determined. Experimental Procedure 1. Identification of Apparatus On this tray (in DBH 214) we have a number of different pieces of common equipment. We will, identify and sketch each - I know this may sound a trivial exercise but it is necessary so that we are all on the same page. beaker erlenmeyer flask (conical flask) side-arm Erlenmeyer flask (conical flask) graduated (measuring) cylinder pipettes, both types graduated and bulb burette Bunsen burner test tubes watch glass volumetric flask 2. Balance Use In these general chemistry laboratories, we only use easy-to-read electronic balances – saving you a lot of time and the TA’s a lot of headaches. However, it is important that you become adept at the use of them. Three aspects of a balance are important: The on/off switch. This is either on the front of the balance or on the back. The "Zero" or "Tare" button. This resets the reading to zero. CLEANLINESS. Before and after using a balance, ensure that the entire assembly is spotless. Dirt on the weighing pan can cause erroneous measurements, and chemicals inside the machine can damage it. Balance Measurements: Turn the balance on. After the display reads zero, place a piece of weighing paper on the pan. Read and record the mass. (2) With a spatula, weigh approximately 0.2 g of a solid, common salt NaCl, the excess salt is discarded, since returning the excess salt may contaminate the rest of the salt - in this exercise, this is not a big deal but in strict analytical procedures it is. Record the mass (1). To determine how much solid you actually have, simply subtract the mass of the weighing paper (2) from the mass of the weighing paper and solid (1). Record this mass (3).You have just determined the mass of an "unknown amount of solid." Now place another piece of weighing paper on the balance and press the Zero or Tare button then weigh out approximately 0.2 g of the salt (4). Thus, the zero/tare button eliminates the need for subtraction.  3. Measuring the volume of liquids When working with liquids, we usually describe the quantity of the liquid in terms of volume, usual units being milliliters (mL). We use three types of glassware to measure volume – (1) burette, (2) volumetric pipette, and (3) graduated cylinder. A volumetric flask will also allow for a high degree of accuracy and precision (more on that in future weeks) in the measurements of any liquids, so a 100 mL volumetric flask will contain precisely 100.0 mL of solution when filled to the line marked on the neck of the flask. Examine each piece of equipment. Note that the sides of each are graduated for the graduated cylinder and the burette. Since the pipette will deliver a specific volume, in your case, 10 mL and will be precise to two decimal places. The burette will be used to deliver a variable volume of solution and will also be precise to two decimal places. Another type of burette is a bulb burette which is only graduated to deliver a set volume of solution, say 25 mL. Put some water into the graduated cylinder. Bend down and examine the side of the water level. Note that it has a "curved shape." This is due to the water clinging to the glass sides and is called the meniscus. When reading any liquid level, use the center of the meniscus as your reference point.  Graduated cylinder Look at the graduations on the side of the cylinder. Note that they go from 0 on the bottom and increase upwards. Since volumes in graduated cylinders are only precise to one decimal place, a graduated cylinder is generally only used when a high degree of precision is not required. Thus, to get the mass of 10 mL of a liquid from a graduated cylinder, do the following: Add water up to the 10 mL line as accurately as possible. Dry a small beaker and weigh it (2). Pour the 10 mL of water from the cylinder into the beaker. Reweigh (1). Subtract the appropriate values to get the weight of the water (3).  Pipette You may find either that 0 is at the spout end or at the top of the pipette. You should be aware of how these graduations go when using each pipette. Thus, to get the mass of 10 mL of a liquid from a pipette, do the following: Half-fill a beaker with water. Squeeze the pipette bulb and attach to the top of the pipette. Put the spout of the pipette under water and release the bulb. It should expand, drawing the water into the pipette, do not let the water be drawn into the bulb. When the water level is past the last graduation, remove the bulb, replace with your finger, and then remove the pipette from the water. Removal of your finger will allow liquid to leave the pipette. Always run some liquid into a waste container in order to leave the level at an easy-to-read mark. Add 10 mL of water to a pre-weighed dry beaker (5). Weigh (4). Subtract to get the weight of the water (6).  Burette Examine the graduations. Note that 0 is at the top. Using a funnel, add about 10 mL of water. To do this, first lower the burette so that the top is easy to reach. Run a little water from the burette into a waste container. Then turn the burette upside down and allow the rest of the water to run into the container (you will have to open the top to equalize the pressure). You have just "rinsed your burette." This should be done every time before using a burette – first rinse with water, then repeat the process using whatever liquid is needed in the experiment. Fill the burette to any convenient level (half-way is fine). It is a good technique to "overfill" and then allow liquid to run into a waste container until you reach the appropriate level so that you fill the space from the top to the tip of the burette. Dry a beaker and weigh (8). Add 10 mL of water to a pre-weighed dry beaker (7). Subtract to get the weight of the water (9). 4. Estimation of Avogadro's number Briefly, as a group with your TA, you will make an approximate (order of magnitude) estimate of Avogadro's number by determining the amount of stearic acid that it takes to form a single layer (called a monolayer) on the surface of water. By making simple assumptions about the way the stearic acid molecules pack together to form the monolayer, we can determine its thickness, and from that thickness we can estimate the size of a carbon atom. Knowing the size of a carbon atom, we can compute its volume; and if we know the volume occupied by a mole of carbon (in the form of a diamond), we can divide the volume of a mole of carbon by the volume of an atom of carbon to get an estimate of Avogadro's number. Dr Hennessy went through this in lecture last week. Procedure Special Supplies: 14 cm watch glass; cm ruler; polyethylene transfer pipettes; 1-mL syringes; pure distilled water free of surface active materials; disposable rubber gloves (for cleaning own watch glasses in 0.1 M NaOH in 50:50 methanol/water): 13 size 12{ times } {}100 mm test tubes with rubber stoppers to fit. Chemicals: pure hexane, 0.108 g/L stearic acid (purified grade) solution in hexane. 0.1 M NaOH in 50:50 methanol/water used for washing the watch glasses. SAFETY PRECAUTIONS: Hexane is flammable! There must be no open flames in the laboratory while hexane is being used. WASTE COLLECTION: At the end of the experiment, unused hexane solvent and stearic acid in hexane solution should be placed in a waste container, marked "Waste hexane/stearic acid solution in hexane." Measurement of the volume of stearic acid solution required to cover the water surface Your TA will do this as a group demonstration: Fill the clean watch glass to the brim with deionized water. One recommended way to do this is to set up your 25 mL burette on a ring stand. Wash and drain the burette with deionized water (the deionized water comes from the white handled spouts at each sink). Seal the edges of the watch glass with deionized water to make sure the surface of the watch glass is completely covered, add a drop of food coloring, swirl to mix (facilitates seeing the monolayer easier). With careful dispensing, the surface tension of the water should allow you to fill the entire watch glass with relative ease. Using a transfer pipette, obtain about 3-4 mL 0.108 g/L stearic acid solution in hexane in a clean, dry 13 size 12{ times } {}100 mm test tube. Keep the tube corked when not in use. Carefully measure the diameter of the water surface with a centimeter ruler. It should be close to 14 cm, + or - a couple of millimeters. Next, rinse and fill your 1 mL syringe with stearic acid solution, taking care to eliminate bubbles in the solution inside the syringe. Read and record the initial volume of the syringe (1 mL is always a good place to start.) Then add the stearic acid solution drop by drop to the water surface. Initially, the solution will spread across the entire surface, and it will continue to do so until a complete monolayer of stearic acid has been formed. If your first few drops do not spread and evaporate quickly, either your water or watch glass is still dirty. As this point is approached, the spreading will become slower and slower, until finally a drop will not spread out but will instead sit on the surface of the water (looking like a little contact lens). If this "lens" persists for at least 30 s, you can safely conclude that you have added 1 drop more than is required to form a complete monolayer. Now, read and record the final volume reading of the syringe. Thoroughly clean the watch glass (or get another one), and repeat the experiment. Repeat until the results agree to within 2 or 3 drops (0.04 ml). When you have completed all of your measurements, rinse your syringe with pure hexane, and dispose of all the hexane-containing solutions in the waste collection bottle provided. Calculation Of Avogadro's Number The calculation proceeds in several steps. We calculate the volume of stearic acid solution in hexane required to deliver enough stearic acid to form a monolayer. All of the hexane evaporates, leaving only the thin monolayer film of stearic acid, so we next calculate the actual mass of pure stearic acid in the monolayer. We calculate the thickness of the stearic acid monolayer, using the known density of stearic acid and the area of the monolayer. Assuming the stearic acid molecules are stacked on end and are tightly packed, and knowing that there are 18 carbon atoms linked together in the stearic acid molecule, calculate the diameter and volume of a carbon atom. Calculate the volume of a mole of carbon atoms in diamond; divide the molar volume of carbon (diamond) by the volume of a single carbon atom to obtain an estimate of Avogadro's number. Remember that the units of Avogadro’s number are mol −1 size 12{ {} rSup { size 8{ - 1} } } {}, so you can use unit analysis to check your answer. Alginate spheres Materials used:2.0 g sodium alginate200 g water (with low calcium content!)50 g blueberry syrup Xanthan 2.5 g calcium chloride Plastic syringes (sharp cannula) Strainer 500 g water Procedure: 2 g sodium alginate and 200 g water have been mixed vigorously in blender for you. The mixture has then left to stand for some hours in order to get rid of the air bubbles. 50 g blueberry syrup is added to the sodium alginate solution. A calcium chloride bath is prepared by dissolving 2.5 g calcium chloride in 500 g water. The sodium alginate/blueberry mixture can then be dripped into the calcium chloride bath using a plastic syringe with a steel cannula. After 1-3 min the pearls can be removed and rinsed with water. Enjoy!