Use of Volumetric Glassware: The Floating Egg Problem. By: Danielle Corgliano. Performed: August 24 th Lab Partners: Moe and Berry

Transcription

1 Use of Volumetric Glassware: The Floating Egg Problem By: Danielle Corgliano Performed: August 24 th 2004 Lab Partners: Moe and Berry September 1 st 2004 Chemistry 112 Section Unknown: January 1 st 2004

2 Introduction Every solution, whether it is water or motor oil, has a density associated with it. Density is a measurement of a physical property of solutions. For any volume of a solution with a known mass, density can be derived by using Equation 1. Even with unknown solutions, a density can be derived by knowing the mass of the solution at a particular volume by using Equation 1. Equation 1: Density = (grams of solution)/(ml of solution) A salt solution, with an unknown density, was prepared. By using several different delivery techniques, one can determine the average density and a density for each technique used. Procedure An aqueous sodium chloride solution was prepared by adding solid sodium chloride to 1500 ml of distilled water while stirring until an uncooked egg floated. The amount of salt added had an unknown mass; however the density was determined indirectly by the use of four different techniques: burette, Mohr pipette, volumetric pipette, and a volumetric flask. For each technique, a system was put in place to determine how much a volume of solution weighed. An empty flask was used to contain a delivered amount of solution. The mass of the flask was taken before the addition of the solution and then again after the addition. By difference, it was determined how much the solution weighed for a certain volume. This technique was used for the burette, Mohr pipette, and for the volumetric pipette. The volumes for each technique varied. The volumes of the salt solution that were delivered were approximately ml for the burette, ml for the Mohr pipette, and ml for the volumetric pipette. The use of the volumetric flask required a different experimental scheme. The mass of an empty ml volumetric flask was determined. Then the flask was filled with solution and reweighed. The solution was then delivered to a pre-weighed beaker and weighed a second time. Density calculations were completed to determine the value for each technique used. Data Table 1: Experimental masses of the unknown salt solution for each technique Burette Mohr Pipette Volumetric Pipette Trial g g g Trial g g g Trial g g g Average ± 0.10 g ± 0.25 g ± 0.32 g

3 Table 2: Experimental volumes used of the unknown salt solution for each technique Burette Mohr Pipette Volumetric Pipette Trial ml ml ml Trial ml ml ml Trial ml ml ml Average ± 0.02 ml ± 0.02 ml ± 0.00 ml Table 3: Experimental data for the volumetric flask technique Flask empty Flask full Beaker empty Beaker full Trial g g g g Trial g g g g Trial g g g g Average ± 0.02 g ± 0.31 g ± 0.02 g ± 0.27 g Calculations The mass of the solution was determined by using Equation 2. Equation 2: Mass solution = (Mass beaker filled Mass empty beaker) For example: Mass solution = (68.14 g g) = g From the mass of the solution listed in Table 1 and the corresponding volume listed in Table 2, one could determine the density of the unknown solution by using Equation 1. Recall that Equation was: Equation 1: Density = (grams of solution)/(ml of solution) A sample calculation is as follows: Density = (26.42 g)/(24.02 ml) Density = 1.10 g/ml A standard deviation was performed on all trials to determine the precision, displayed by Equation 3. Equation 3: Standard Deviation = = (((26.34g-26.32g) 2 +(26.34g-26.45g) 2 +(26.34g 26.26g) 2 )/(3-1)) = 0.10 grams

4 A propagation of error calculation was performed on the data to determine the uncertainty. Equation 4 was used to determine the error when subtracting two data values. Equation 5 shows the error calculation for the density. Equation 4: mass = full + empty = 0.31 g g = 0.33 g Equation 5: density = mass + volume density mass volume density = 0.10 g ml 1.10 g/ml g ml density = 0.01 g/ml Results and Discussion It was observed that the calculated densities were similar for each technique used. For example, the density of the sodium chloride solution was calculated to be 1.10 ± 0.01 g/ml for the burette, 1.16 ± 0.01 g/ml for the Mohr pipette, 1.07 ± 0.01 g/ml for the volumetric pipette, and 1.12 ± 0.01 g/ml. The overall average density was 1.11 ± 0.01 g/ml. The density calculated for the volumetric pipette delivery method was less than that of the other methods. All techniques displayed low standard deviation. However, the standard deviation for the recorded mass was larger than that of the standard deviation for the recorded values of the volume. One source of error that would contribute to this could be the beaker was not weighed correctly and the possibility of solution in the clean beaker. Furthermore, inaccurate readings of the burette and Mohr pipette would have contributed to the error. Advantages and disadvantages were observed for each technique used. The burette was a simple tool to use because of the presence of the stopcock. However, initial air pockets in the tip of the burette could have skewed the correct delivery volume. The Mohr pipette was useful to dispense a smaller amount of liquid as compared to the burette. Since the Mohr pipette along with the volumetric pipette were hand held and suction driven, this made the use more difficult. The volumetric pipette was limited by only delivering a calibrated amount; however this calibrated amount was accurate to ml. The volumetric flask was similar to the volumetric pipette, but this glassware was used to contain a known volume of liquid rather than to dispense a volume. If the volumetric flask was used to measure a known volume, then used to dispense the solution, possible error could occur. The mass of the liquid contained by the volumetric flask was calculated by difference to be ± 0.33 g. The mass of the liquid delivered from the volumetric flask to an empty pre-weighed beaker was only ± 0.29 g. This difference would be the result of liquid remaining in the volumetric flask. Since the flask was calibrated to contain a known amount, that was its purpose.

5 The best and most accurate technique to determine the density of an unknown solution was to use the volumetric flask to contain the liquid. Conclusion The averaged density of a salt solution, in which an egg floated, was determined to be 1.11 ± 0.01 g/ml. Each technique used resulted in different values for the density. However, low standard deviations occurred for each trial. It was determined that the volumetric flask was the best and most accurate piece of equipment to use to determine the density of an unknown solution.