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EXPERIMENT 2.3 SOLUTION CONCENTRATION INTRODUCTION In this experiment, you will prepare a sodium chloride solution and compare its boiling point to that of pure water. The objectives of the experiment are to practice the calculation of concentration using several unit systems and to observe the colligative property of boiling point elevation. Colligative properties are properties of solutions that vary with the numerical amount of solute particle concentration but do not depend on the identity or chemical nature of the solute. The solution will be prepared by mass. That is, the amounts of salt and water added together will be weighed. The density of the resulting solution will then be determined, using a pycnometer. Pycnometers are glass vessels designed so that when they are filled with liquid samples, the samples will consistently have the same volume. By finding the mass and volume of a sample of your solution, you will be able to calculate its density. This, along with the original masses of solute and solvent will provide sufficient information for you to calculate the concentration of the salt in units of percent by mass, molality, molarity and mole fractions. Each of these units is defined in the class text book. By measuring the boiling point temperature of the solution and comparing it to that of pure water, you will observe whether or not the boiling point will have risen due to the addition of the solute to water. Further, you can comment on the apparent accuracy of predicting the boiling point of your solution using the boiling point elevation equation Tb = iKbm where Tb is the amount of increase in the boiling point over that of the pure solvent, i is the effective number of independent solute particles per formula unit dissolved, Kb is a constant for the solvent, and m is the molality of the solute formula units. The mathematical laws for the colligative properties of solutions are all estimates, making some idealized assumptions about solution behavior. Our experimental method will have some uncertainties due to instrument and operator limitations. You will be asked to consider how each of these may contribute to imprecision between the calculated and measured boiling point values. MATERIALS AND SAFETY The materials used are limited to sodium chloride and water so there are no special hazards involved. Nevertheless, wear goggles at all times to avoid the high salt concentration and other materials in the lab environment. The pycnometers are expensive and very fragile. Handle one with care and do not lose the small cap associated with it. At the end of the procedure, rinse the pycnometer with distilled water and return it to the instructor or to the equipment cart. PROCEDURE Clean and dry a 100 mL beaker. Weigh the beaker on the electronic balance and record the mass in the data section of the report. Use your graduated cylinder to measure about 60 mL of distilled water and add it to the beaker. NEVER ADD LIQUIDS TO CONTAIN-ERS WHILE THEY ARE ON THE BALANCE. Weigh and record the mass of the beaker and water. Next, measure about 12 grams of sodium chloride in a plastic “weighing boat” and add it to the beaker (away from the balance). Weigh and record the mass of the beaker and solution. Obtain a 50 mL pycnometer from the equipment cart. Please treat it with extreme care. If it appears to be wet inside, use a very small portion of acetone (red squirt bottles) to rinse the interior and allow the acetone to evaporate. When it appears dry inside and out, weigh the pycnometer and record its mass. Use a distilled water squirt bottle to fill the pycno-meter to near the top of its neck. Have a lab tissue at hand and insert the holed cap into the neck. The level of water should be high enough and the insertion done carefully so that excess water comes out the hole and no air is trapped below the cap. If you see substantial air bubbles or the water does not rise to the top of the hole, take the cap out, add water and try again. Use the tissue or paper towel to dry the outside and briefly pass the tissue over the top of the hole to take off excess water bulging out above the hole. Weigh and record the mass of the pycnometer and water. The ground glass joint should make it possible to insert the cap to the same depth any time the pycnometer is filled. Thus, if liquid fills to the same point at the top of the hole in the cap, the volume will be consistent from sample to sample. Pour the water out of the pycnometer and into a beaker and measure the water temperature. Before leaving the lab, refer to the water density chart posted on the wall near the barometer between the storeroom windows that face into the two labs. Record the density of water at the measured temperature. Rinse the pycnometer with one or two very small portions of the salt solution you have prepared. Then fill it with salt solution and insert the cap as before. The rinsing is done to substitute traces of your solution for the water so that water from the previous filling will not change the concentration of the sample of your solution. There should be more than enough solution to fill the pycnometer. Save the excess in the beaker. Weigh and record the mass of the pycnometer and salt solution. After weighing, pour the solution sample back into the 100 mL beaker with the excess. While one team member is carrying out some of the weighings, the other should set up an iron ring and insulated wire mesh pad to support a beaker over a Bunsen burner. Place about 60 mL of distilled water in a different 100mL beaker and heat to a moderate steady boil. Use a thermometer to measure an estimate of the boiling point temperature. Support the thermometer by hand as opposed to clamping or standing in the beaker, and hold it well above the bottom of the beaker so it does not contact the glass from which heat is being transferred to the water. Record the boiling point of distilled water. After the pycnometer weighing of the salt solution have been completed and the solution, less the amount used for rinsing, has been returned to its 100 mL beaker, place this beaker over the burner and measure its boiling point. Try to match the extent of boiling as closely as possible to that of the distilled water sample boiled earlier. Record the solution boiling point. When the data section is complete, rinse all of the glassware exposed to the salt and return all items to your drawer or the equipment cart as appropriate. DATA: Create the following data table in your lab notebook as part of you prelab. a) Mass of empty 100 mL beaker________________ b) Mass of beaker plus distilled water________________ c) Mass of beaker plus solution of salt and water________________ d) Mass of empty pycnometer________________ e) Mass of pycnometer plus distilled water________________ f) Temperature of distilled water________________ g) Density of distilled water________________ h) Mass of pycnometer plus solution of salt and water________________ i) Approximate boiling point of distilled water________________ j) Approximate boiling point of solution of salt and water________________ Prelab Questions 1. A solution made to be 20.2% HCl by mass in water has a density of 1.096 g/mL. Calculate the molarity and molality of the HCl in the solution. 2. Calculate the expected boiling point of a solution of 238 g of KBr added to 3.00 kg of water. For water, Kb is equal to 0.512 °C/m. 3. As a solution of KBr and water boils, only the water molecules will vaporize. What should happen to the boiling point temperature over time as such a solution boils? 4. As a KBr and water solution begins to freeze, water molecules will tend to become organized into the pure ice crystal structure excluding the K+ and Br- ions which stay mixed with the remaining liquid water. Eventually, the remaining liquid water will be saturated with KBr and solid KBr will precipitate. Thus the freezing process, if performed slowly, may separate the water from the KBr. What should happen to the freezing point temperature during the early stages of the process? Postlab For each calculation, show detailed work steps in the space between the statement of the objective and the blank result line. In contrast to many of the experiment reports of Chemistry 151, the results have not been broken down into as many intermediate reported steps. This is intended to give you more flexibility as well as responsibility to determine an appropriate calculation method for each objective. 1. Find the Percent by mass of NaCl in solution 2. Find the Molality of NaCl in solution 3. Find the Density of solution 4. Find the Molarity of NaCl in solution 5. Consider the solution to consist of three distinct particles: water molecules, sodium ions and chloride ions. Find mole fraction of water in the solution. 6. Difference in measured boiling points of water and solution 7. Expected T from calculation 8. Learning and Errors CONCLUSIONS Was boiling point elevation observed in the measurements? Comment on the agreement between the measured and expected boiling point differences. State the main trend in accuracy of the calculation as solute concentration varies. Also identify the greatest source or sources of uncertainty in the measurement.
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