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Density Introduction: can be defined by the mass of the substance over the volume which this substance occupies. This is described by the well known equation D=m/ V, where D is the density of the substance, m is the mass of the substance and V is the volume occupied by the substance. The typical units for density are g/mL. Density is an intensive property of matter. This means that it does not depend on the mass of the substance present but rather on the nature of the substance. Each pure substance has a defined, unchanging density. As a result, density is very useful in determining the identity of an unknown liquid. The purpose of this experiment was to determine the identity of an unknown liquid based on its density using a procedure that we designed in order to maximize the precision and accuracy of our measurements of the liquids density. Experimental Procedure: Part I: Distilled Water: The procedure was first tested on samples of distilled water, which is known to have a density of about 0.997 g/ mL. 1. Distilled water was obtained in a 50mL beaker 2. A 30 mL beaker was cleaned, dried and massed on an analytical balance. The analytical balance was chosen because provided a greater degree of precision because it provided the mass to three decimal places. This mass was recorded. 3. 10.00mL0.04mL were measured in a clean calibrated pipet and placed in the 30mL beaker. The pipet was used because it provided a greater degree of precision than the graduated cylinder, which provides an uncertain measurement in the tenths place. (Before actually using the pipet for the procedure, the amount delivered by the pipet was initially tested by measuring the amount of distilled water in a graduated cylinder. The result was always very close to the 10.0 mL mark.) 4. The water and the beaker were massed in the analytical balance. This mass was recorded. 5. Steps 2-4 were repeated three times to yield three sets of data. Part II: The Unknown Liquid: 1. The glassware was cleaned and the pipet was rinsed with two washes of acetone and one of the unknown. 2. Steps 2 through 4 of the distilled water procedure were repeated but the distilled water was replaced with the unknown. Five trials were performed to yield five sets of results in order to increase the precision of our density data. Results and Discussion: Part I: Distilled Water The procedure for determining the density of 10.00mL 0.04mL distilled water was repeated three times. The data for the three trials follow. Table 1: Data and Results for the Distilled Water Procedure. Trial # 1 2 3 Mass of Beaker (g) 29.678 29.681 29.678 Mass of Beaker and Distilled Water (g) 39.617 39.672 39.619 Mass of Distilled Water Alone (g) 9.939 9.991 9.941 Density of Distilled Water (g/mL) 0.9939 0.9961 0.9941 The density of the distilled water for each trial was calculated by subtracting the mass of the empty beaker from the total mass of the beaker and the 10.00mL 0.04mL of water. This yielded the mass of the distilled water alone. This value was then divided by 10.00mL which is the average amount of water delivered by the pipet, yielding the density of the sample of distilled water. A sample calculation follows using the data from Trial 1. Sample Calculation for Density of Distilled Water: 30.617 g Beaker Distilled and Water -29.678g Empty Beaker=9.939 g Distilled Water 9.939 g Distilled Water/ 10.00mL Distilled Water = 0.9939 g/mL The average of the densities obtained in the three trials was calculated in the following manner. The average deviation for the densities obtained in the three trials was calculated in the following manner: g/mL Given the relatively small average deviation of the calculated densities for the distilled water samples (0.0938% of the average density for the distilled water), the results were determined to have an adequate degree of precision. Based on the given accepted value of .997g/mL for the density of distilled water, the percent error for our average density for distilled water was calculated in the following manner. Based on this relatively small percentage error, it was determined that our procedure produced adequately accurate results. Our conclusion on this part of the experiment was that our procedure could be safely used on the unknown in order to find its density with a great degree of precision and accuracy, meaning that our results would be reproducible as well as close to the actual accepted value. Sources of error may be contamination of the distilled water with substances that may have been present in the beaker or slight errors in pipet usage that may have allowed too little volume to be delivered into the empty beaker accounting for our slightly low average density. Part II: The Unknown Liquid: The procedure for determining the density of 10.00mL0.04mL of the unknown liquid was reproduced five times. The data for the five trials is in the following table. The mass of the unknown liquid and the density of the unknown liquid were found using the same calculation methods used in the distilled water section. Table 2: Data and Results for the Unknown Liquid Procedure Trial # Mass of Beaker (g) 32.072 29.697 32.064 29.680 29.679 Mass of Beaker and Unknown Liquid (g) 38.686 36.256 38.684 36.265 36.268 Mass of Unknown Liquid Alone (g) 6.614 6.577 6.620 6.585 6.589 Density of Unknown Liquid (g/mL) 0.6614 0.6577 0.6620 0.6585 0.6589 1 2 3 4 5 The average density of the unknown liquid was obtained in the same manner as in the distilled water section and was calculated to be 0.6597g/mL. The average deviation was calculated to be 0.0016. This relatively small average deviation (about 0.2425% of the average calculated density) led us to the conclusion that our procedure produced sufficiently precise results. The average value for the density of the unknown liquid was very close to the 0.6603 g/mL value for hexane. Using this information, the percent error was calculated to be about 0.09089%. This relatively small percent error suggests that our results were highly accurate and that it was most likely that our unknown liquid was hexane. Sources of error may be contamination of the Hexane with substances that may have been present in the beaker (acetone or distilled water for example) or slight errors in pipet usage that may have allowed too little volume to be delivered into the empty beaker accounting for our slightly low average density. Conclusion: It was determined with a high degree of precision and accuracy that the unknown liquid was hexane with an average density of 0.6597 g/ mL. ... View Full Document

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