Laboratory Exercise: The Precision of Volume Measurements

Transcription

1 CHEM 109 Introduction to Chemistry Revision 3.0 Laboratory Exercise: The Precision of Volume Measurements The accurate measurement of the volume of liquids tends to be difficult and chemists have a number of different types of devices for makin this measurement; beakers, raduated cylinders, volumetric flasks, etc. Each of these devices has a specific purpose and a distinct level of accuracy and precision associated with it. Beakers are used primarily for crudely measurin out lare volumes of liquids, mixin reaents and carryin out chemical reactions. The raduated markins on a beaker are only approximate, so a beaker should never be used for accurate volume measurements. Graduated cylinders are specifically desined to measure out liquid volumes. Their tall narrow desin makes for a more precise readin of the liquid level. If more precision is needed, a volumetric flask can be used. These flasks typically measure a sinle volume via a sinle mark on a tall narrow neck. These flasks are enerally used for solution preparation. Beaker Graduated Cylinder In accurate work it is never safe to assume that the volume delivered by or contained in any volumetric instrument is exactly the amount indicated by the calibration mark. Manufacturers only uarantee the device is manufactured to within some prescribed tolerance. Instead, recalibration is usually performed by weihin the amount of water delivered by or contained in the volumetric apparatus. The mass measurement is then converted into a volume measurement usin the known density of Water. Mass measurements can be made much more accurately and precisely than can volume measurements. Volume = Mass / Density

2 The density of Water is well known and tabulated values are provided in the Appendix. In addition to recalibratin the volumetric device for accuracy, we can make replicate measurements of the volume contained in or delivered by the device in order to ain an understandin of the precision of the device. The precision can be estimated as the Standard Deviation for the replicate measurements, althouh other more conventional measures, such as the Confidence Interval, based on the standard deviation determination, are more widely used. In our case, we will calibrate two raduated cylinders. The precision of these cylinders will then be compared.

3 Procedure 1. Obtain about 700 of Distilled Water in an 800 Beaker. This Water will be used to calibrate our Graduated Cylinders. In each case, we will measure the mass of Water, as measured by the device, five times. 2. Measure the temperature of the Water. This is needed because the density of Water is temperature dependent. 3. Obtain a 10 and a 50 Graduated Cylinder. Thorouhly clean each with a test tube brush and some Alconox cleanin solution. (Cleanliness of volumetric lassware is critical to ensure its correct performance.) Water will wet the sides of clean lassware but will not bead-up in drops. 4. Make a note as to whether the Cylinder is a To Contain (TC) or a To Deliver (TD) device. To Contain devices, as the name implies, contain the stated amount of liquid when filled to the mark. To Deliver devices deliver the stated amount of liquid into another container. Account is taken for any liquid remainin wetted on the walls of the device after the delivery. If the Cylinder is a To Contain device: 5. Completely dry the Cylinder and weih it empty usin an Analytical Balance. (Your laboratory instructor will demonstrate the use of the balance. Do not use the balance until you have been shown how to properly use it.) 6. Fill the Cylinder with Distilled Water to the top mark. Make sure no droplets of Water are stickin to the walls above the Water level. If this is the case, use a piece of paper towel to blot them up. Your laboratory instructor will demonstrate how to do this effectively. Make sure your eye is on a level with the mark. A Paralax Error will result, otherwise. This will lead to a readin that is either too hih or too low. Always read the meniscus from the bottom.

4 Measure the mass of the Cylinder filled with Water. 7. Completely dry the Cylinder and repeat the measurements four more times. 8. Determine the mass of Water contained in the Cylinder for each trial and then use the density to convert this to a volume readin. If the Cylinder is a To Deliver device: 5. Obtain and completely dry a 50 Plastic Beaker and weih it empty usin an Analytical Balance. (Your laboratory instructor will demonstrate the use of the balance. Do not use the balance until you have been shown how to properly use it.) 6. Fill the Cylinder with Distilled Water to the top mark. Make sure no droplets of Water are stickin to the walls above the Water level. If this is the case, use a piece of paper towel to blot them up. Your laboratory instructor will demonstrate how to do this effectively. Make sure your eye is on a level with the mark. A Paralax Error will result, otherwise. This will lead to a readin that is either too hih or too low. Always read the meniscus from the bottom. 7. Transfer the measured amount of Water from the Cylinder to the Plastic Beaker. Measure the mass of the Beaker filled with Water. 8. Completely dry the Cylinder and repeat the measurements four more times. 9. Determine the mass of Water delivered by the Cylinder for each trial and then use the density to convert this to a volume readin.

5 Appendix - Density of Water Temperature ( o C) Density (/)

6 Appendix - Tolerances for Class A Graduated Cylinders at 20 o C Capacity () Tolerances () The Tolerances for Class B cylinder is typically twice that of a Class A device. (ASTM E )

7 Volume Measurements Water Temperature Water Density o C / 10 Graduated Cylinder TC or TD Class Mass Empty Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Mass Filled Mass Water Volume Averae Vol. Per. Error % Std. Dev.* Within Tol. 50 Graduated Cylinder TC or TD Class Mass Empty

8 Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Mass Filled Mass Water Volume Averae Vol. Per. Error % Std. Dev.* Within Tol. Name: Date: Sinature:

9 Additional Points to Consider You do not need to submit answers to these questions. However, you should seriously ponder the answers to these questions as they could reappear on an exam. If you are always readin from below the mark, will your volume measurements be infected with a systematic or random error? In this case, will the accuracy or the precision of your measurement suffer? Explain. Most precise volume measurin devices have markins alon a narrow neck? Why do you think this is important?