of Density Liquids and Solids
By Yiliu (Peter) Wang Teaching Assistant: Josie Bodle September 7, 2007
Results and Discussion:
Controlled Experiment:
Distilled water was used first as a control to check the accuracy and precision of the balance. Water was used since the density of water was known to be 1.00 g / mL. It is then possible to check the balance by seeing whether experimental mass over volume correlated with actual density, which is 1.00 g / mL. A graduated cylinder was weighted several times on the balance. Its mass was always 69.586 grams. This entails that the balance is precise. 25.0 mL of distilled water was then poured into the graduated cylinder, and the weight of the water and the graduated cylinder was measured to be 94.552 grams. Subtracting the two numbers gives the change in mass, which was measured out to be 24.966 grams. Dividing that by the change in volume, or 25.0 mL, gives distilled water's experimental density of .998 g/mL. Percent error: |( .998 1.00 / 1.00 )| X 100 % = .002 % The balance was determined, due to the minimal error of .002%, to be accurate.
Liquid Density Experiment:
#4 Liquid Unknown was acquired. It was observed as clear, not viscous, and having an alcohol-like odor. In order to determine the density of the liquid unknown, the mass of the graduated cylinder was measured. Then after pouring in 10.0 mL of liquid unknown by use of the 10 mL pipette, the final mass was measured. Three experimental trials were performed. The data was recorded table 1. Table 1: Liquid Unknown Volume and Mass Trial Number 1 2 3 Volume of liquid unknown 10.0 mL 10.0 mL 10.0 mL Mass of graduated cylinder 67.620 grams 67.105 grams 67.634 grams Mass of graduated cylinder with liquid unknown 76.564 grams 76.058 grams 76.595 grams
After pouring the liquid unknown in and out of the graduated cylinder, not all the liquid unknown comes out. This gives the data set another means of variation. Thus, in order to be more accurate and precise, three different and dry graduated cylinders were used. Liquid Unknown Calculations: Subtracting the masses of graduated cylinder from the mass of graduated cylinder and liquid unknown gives the change in mass, or the mass of the liquid unknown, and dividing that by the change in volume,
or the volume of the liquid unknown, which was always 10.0 mL, gives the experimental density of the liquid unknown. Trial 1 Calculations: 76.564 g 67.620 g = 8.944 g D = m/V 8.944 g / 10.0 mL = .894 g/mL Trial 2 Calculations: 76.058 g 67.105 g = 8.953 D = m/V 8.953 g/ 10.0 mL = .895 g/ mL Trial 3 Calculations: 76.595 g 67.634 g = 8.961 g D = m/V 8.961 g/ 10.0 mL = .896 g/mL Precision: Precision was calculated using the formula for relative average deviation. Average of three trials: .894 g/mL + .895 g/mL + .896 g/mL = .895 g/mL |.894 g/mL - .895 g/mL| + |.895 g/mL - .895 g/mL| + |.896 g/mL - .895 g/mL| / (3) = 6.67 X 10 ^ -4 g/mL (6.67 X 10 ^ -4 g/mL / .895 g/mL) X 100% = .0745% relative average deviation. The results, then, were determined to be adequately precise. Accuracy: Only after matching the calculated density with the actual density of the liquid can the calculation of accuracy be made possible. The nearest value to .895 g/mL was .9003 g/mL, which was the density of ethyl acetate. error: Percent |(.895 g/mL - .9003 g/mL) / .9003 g/mL| X 100% = .589% Because the result of the calculation of percent error was determined to be adequately accurate, it was safe to assume the liquid unknown #4 was ethyl acetate.
Conclusion: Because the experimental results were adequately precise, and that they correlated, with the small percentage error of .589%, to the density of ethyl acetate, liquid unknown #4 was determined to be ethyl acetate.
Solid Density Experiment:
#6 solid was acquired. It was observed as metallic, gray with rust and black spots, light, and lustrous. In order to determine the density of the solid unknown, the mass of the metal was first measured on the balance. Then, the volume of distilled water poured inside a graduated cylinder was measured. Then after dropping the solid unknown into the graduated cylinder, the final volume was measured. Three experimental trials were performed. The data was recorded table 2. Table 2: Solid Unknown Volume and Mass Trial Number 1 2 3 Mass of solid unknown 2.847 g 2.842 g 2.850 g Volume of water inside graduated cylinder 10.0 mL 10.0 mL 10.0 mL Mass of water and solid unknown inside graduated cylinder 11.50 mL 11.60 mL 11.60 mL
The mass of the solid unknown were different each time as to minimize the variation that impurities in the metals might possess. Solid Unknown Calculations: Subtracting the volumes of water inside the graduated cylinder from the water and solid unknown inside the graduated cylinder gives the change in volume, or the volume of the solid unknown, and dividing that from the mass of the solid unknown gives the experimental density of the solid unknown. Trial 1 Calculations: 11.50 mL 10.0 mL = 1.50 mL D = m/V 2.847 g/ 1.50 mL = 1.90 g/mL Trial 2 Calculations: 11.60 mL 10.0 mL = 1.60 mL D = m/V 2.842 g/ 1.60 mL = 1.78 g/mL
Trial 3 Calculations: 11.60 mL 10.0 mL = 1.60 mL D = m/V 2.850 g/ 1.60 mL = 1.78 g/mL Precision: Precision was calculated using the formula for relative average deviation. Average of three trials: 1.90 g/mL + 1.78 g/mL + 1.78 g/mL = 1.82 g/mL |1.90 g/mL 1.82 g/mL| + |1.78 g/mL - 1.82 g/mL| + |1.78 g/mL - 1.82 g/mL| / (3) = .053 g/mL (.053 g/mL / 1.82 g/mL ) X 100% = 2.91 % relative average deviation. The results, then, were determined to be adequately precise. Accuracy: Only after matching the calculated density with the actual density of the solid can the calculation of accuracy be made possible. The nearest value to 1.82 g/mL was 1.80 g/mL, which was the density of magnesium. Percent error: |(1.78 g/mL 1.80 g/mL) / 1.80 g/mL| X 100% = 1.11% Because the result of the calculation of percent error was determined to be adequately accurate, it was safe to assume the solid unknown #6 was magnesium. Conclusion: Because the experimental results were adequately precise, and that they correlated, with the small percentage error of 1.11% to the density of magnesium, solid unknown #6 was determined to be magnesium.
Sources of Error:
Possible sources of error include the inaccuracy of the balance scale, the inadequacy of the human eye at judging where the meniscus is at, the loss of liquid through transferring from pipette into graduated cylinder, and there being impurities in the unknown metal, the liquid and distilled water.