Lab #2 Accuracy and Precision

Transcription

1 Chem ID # Lab #2 Accuracy and Precision Millie Mole (even) Marvin Mole (odd) February 22, 2222

2 Academic Honesty Statement I, Print name attest that this lab report is my own work and that I received help from no one except Miss Stevens. I understand that any appearance of working with another person or rewording or copying will result in forfeiture of my lab. I am aware that additional violations will result in an unsatisfactory citizenship grade. I understand that repeated violations can affect future letters of recommendation and my college acceptance prospects. Student Signature This statement is to be printed out and attached to every lab report. It is to be placed after the cover page. Its absence will constitute an incomplete lab report and will not be accepted.

3 Calculations: 1. Calculate the volume of the cylinder to the 50.0 ml graduation. V = πr 2 x h V = volume Π= 3.14 r = radius h = height V = (3.14) (1.5 cm) 2 (9.05 cm) 68.3 cm 3 2. Assume the accepted value is 50.0 cm 3, Calculate the percent error. % E = TV EV X 100 % E = Percent Error TV TV = Theoretical Value EV = Experimental Value % Error = 50.0 cm cm 3 X 100 % Error = 36.6% 50.0 cm 3 3. Calculate the mass of water as measured by the balance. Mw = (c + w) - c Mw = mass of water c = cylinder w = water Mw = 33.82g 28.50g = Mw = 5.32 g

4 4. Calculate the mass of the water from its measured volume and its density. (Volume was from water in grad cyl. About 5 ml. You use your actual/measured value.) D = M/V M = DV D = density M = mass V = volume M = ( g/ml) (5.40 ml) M = 5.39 g 5. Using the mass of the water determined by the balance as the accepted value, calculate the percent error of the mass determined using the volume and density. % E = TV EV X 100 % E = Percent Error TV TV = Theoretical Value EV = Experimental Value % Error = 5.32 g 5.39 g X 100 % Error = 1.32% 5.32g 6. Determine the volume of the metal using your measurement of the volume of water displaced by the metal. Vm = (M + Vf ) Vi Vm = volume of metal M= metal Vf = final volume water Vi = initial volume of water Vm = 8.09 ml 5.08 ml = Vm = 3.01 ml

5 7. Determine the mass of the metal using your measurement of the mass of the cylinder with the metal. Mm = (w + m + c) (c + w) Mm = mass of metal c = cylinder w = water m = metal Mm = 36.34g g = Mm = g 8. Calculate the density of the metal Dm = M/V Dm = density metal M = mass V = volume Dm = g 3.01 Dm= 5.22 g/ml 9. Look up the density of the metal you used. Calculate the percent error for the density of the metal you determined in number 8 above. % E = TV EV X 100 % E = Percent Error TV TV = Theoretical Value EV = Experimental Value % Error = 8.94 g/ml 5.22 g/ml X 100 % Error = 41.6% 8.94 g/ml

6 Table of Results: METAL : COPPER Measured Value Accepted Value Percent Error Volume of Cylinder 68.3 cm ml 36.6 % Mass of Water 5.39 g 5.32 g 1.32 % Volume of Metal 4.60 ml X X Mass of Metal g X X Density of Metal 5.22 g/ml 8.93 g/ml 41.6 % OR YOUR TABLE MIGHT LOOK LIKE THIS: Mass (g) Volume (ml) Density (g/ml) Percent Error Cylinder to 50.0 ml xxxxxxxxxxxxxxx 68.3 xxxxxxxxxxxxxxx 36.6 % Water by balance 5.32 xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx 1.32 % Water by density 5.39 xxxxxxxxxxxxxxx xxxxxxxxxxxxxxx xxxxxxxxxxxxxx Metal: Copper % Discussion: This lab was pretty boring, especially when compared to last weeks lab. We basically had to measure a lot of different lab equipment and determine the volume and mass of empty and full graduated cylinders with and without metal. Sources of Error: 1) We had a lot of trouble measuring the graduated cylinder to the 50.0 ml mark. It would either go too high or too low due to the attached base. Suggested improvement: we should share the cylinders with the detachable base. 2) My partner and I took turns taking measurements so our precision may have been off resulting in the high percent error we calculated. Suggested improvement: One person should take all measurements for the same piece of equipment to be consistent. 3) Our measurements may have been off due to the angle of viewing the measuring device. This could also be a reason why our percent error was so high. Suggested improvement read the measuring device or equipment at eye level, and straight-on. If measuring a liquid volume, read the graduated cylinder at eye level from the base of the meniscus.

7 Conclusion: My hypothesis was correct, the density of the metal is greater than water. During both the lab and the time it took for me to complete this lab report I learned how to find the volume of different objects by displacement. I also learned how to find the mass of water by massing the empty container and then massing the full container and taking the difference. By doing this. I could calculate the density of the metal. I am sure that this method will come in handy for the rest of the school year. Questions: 1. What value of a measurement must be known if the accuracy of an experimental measurement is to be determined? The measurement that must be known if the accuracy of an experimental measurement is to be determined is the accepted value. 2. Sarah and John determined the density of a liquid three times. The values obtained were 2.84g/cm3, 2.85g/cm3, and 2.80g/cm3. The accepted value is known to be 2.40g/cm3. a) Are the values determined precise? Explain. b) Are the values accurate? Explain. The values are precise, because they are all very close to one another. They are all within the hundredths place away from each other. If the accepted value is 2.40 g/cm 3 then the values obtained were not accurate. In order for a value to be accurate it needs to be very close to the accepted value, and the values obtained by Sarah and John were not very close to the accepted value.