Lab1-Calibration - Chemistry 227 Fall 2008 Lab 1:...

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Chemistry 227 – Fall 2008 Lab 1: Calibration of Balances, Thermometers, and Glassware Objectives: Erroneous results are often obtained in chemical analyses because the analyst puts too much trust in instruments. You should never simply assume that an instrument is working correctly or that it has not drifted out of calibration. Furthermore, a scientist should know the limits of performance of the apparatus he or she uses. Just because a number is generated doesn’t mean it is reliable. In this lab, you will check the performance of your balances, calibrate your thermometers and volumetric glassware, and determine the uncertainties in measurements made with them. Although these procedures may appear to be tedious, the accuracy of much of your subsequent will work depend critically on the results . Chapter 2 of your text contains valuable information for this lab. Assessment of Balance Performance In this procedure you will check the calibration of your balance and estimate limits of error for weighings. The electronic balances work on the principles described on pp 22–23 of Harris , 7 th Ed. Treat the balances gently and keep them clean. Be sure to close the balance chamber doors after each use. It is not good practice to dispense a chemical to a container while the container is on a balance pan (however, we will need to make an exception to this rule for the calibration of the syringe buret described below). If you ever spill a substance inside the balance chamber, clean up the spill immediately–don’t leave your mess for someone else. Procedure for Analytical Balances Each member of the team should perform steps 1–5 individually. All members of the team should use the same standard weight because you will be pooling the results in your lab reports. 1. Select an analytical balance and check the bubble gauge to make sure that the balance is level. If the balance is not level, carefully adjust its feet. Check the bubble gauge as a matter of course each time you use a balance during the semester. 2. a) Make sure that the balance reading is zero with no load on the pan. Using the special tweezers provided, gently place a 40- to 100-g standard weight (or combination of standard weights) on the pan and record the reading. Good practice is to place any object being weighed on the center of the pan, rather than to rely on the automatic weight distribution compensator in the balance. Make sure that you note from which set you take the weights, and return the weights to that same set when you are finished. Record all information in your notebook, including which weight set and which balance you use. Keep in mind that for the standard weights to remain useful as references they must be kept free of finger grease and other contamination, and they must not be scratched. Observe whether the balance reading tends to drift over a period of four or five minutes.
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This note was uploaded on 03/15/2012 for the course CHEM 227 taught by Professor Stevengoates during the Fall '10 term at BYU.

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Lab1-Calibration - Chemistry 227 Fall 2008 Lab 1:...

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