Scientific Measurement: Glassware, Precision, and Accuracy

Scientific Measurement: Glassware, Precision, and Accuracy Prior Knowledge: • Basic algebra skills. • How to use a spreadsheet program. • No lab experience required. Learning Objectives Students will be able to: Content: • Correctly name common laboratory glassware. • Become familiar with the precision associated with specific glassware & equipment. Process: • Work together to choose reasonable equipment to complete a procedure. (Teamwork) • Practice data manipulation using a spreadsheet program. Technique: • Choose glassware and use it correctly to execute simple lab procedures. Safety: • Wear safety goggles always in the laboratory. • HANDLE ALL SOLUTIONS WITH CAUTION. If acid and/or base is spilled on the countertop, clean it up immediately. IF A CHEMICAL IS SPILLED ON THE SKIN, IMMEDIATELY RINSE Background Chemistry labs contain many pieces of equipment with which you may not be familiar. In order to design and execute experiments, you need to know the names of the equipment and how to properly use them. Appendix A has a listing and image of common glassware. In this exercise, we are focusing on measuring volumes of liquids using the appropriate glassware. When liquids are placed into containers a meniscus may form. A meniscus is a phase boundary that is curved because of surface tension. To accurately measure volume, read the center of the meniscus at eye level. In the case of water, the meniscus is concave, so the center is at the bottom (see Figure 1). Figure 1: Determining the meniscus in a glass container Information statement: All scientific measurements will have some uncertainty (precision) associated with them. This is not the same as you making a mistake, either experimentally or in your calculations. Mistakes or inexperience are not “sources of error”. Information statement: Accuracy—refers to how close a measured value is to the actual value Precision—refers to how close a series of measurements are to one another or how reproducible they are There are two broad classes of errors you should look for when addressing sources of error in your experiment: systematic and random. Systematic: Errors from definite causes that bias the data in one direction. These arise from a flaw in the equipment or the experimental design and are reproducible. They are often caused by poorly or incorrectly calibrated instruments and can usually be discovered and corrected. • Examples: weighing a sample that absorbs water from the air; the absorbed water makes the mass read too high. A balance is calibrated incorrectly and all the values it reports are 0.50 g too low. Random: Errors from uncontrolled variables in the experiment. These types of errors cause high as well as low results, are usually small, and cannot be completely eliminated. Technique skills are a major player. Taking multiple readings limits the effects of random errors. Every measuring instrument will have some uncertainty. • Examples: Analog devices require estimation between the markings on them. Calibrations are performed under certain conditions that may be different from those the data is collected under. Noise in an instrument. Laboratory Activities Two laboratory activities will be performed in the lab today. First is the determination of an unknown container’s volume and the second investigating the relationship between Fahrenheit and Celsius temperature scales. Laboratory Activity #1: Determination of an Unknown Container’s Volume The first laboratory activity you will perform is determining the volume of an unknown object given to you by your instructor. 1. 2. 3. 4. Sketch a picture or describe in words the shape of the unknown container. Using a buret, determine the volume of your unknown container. a. Fill buret with water as demonstrated by your laboratory instructor. b. Read the initial reading (i.e., 0.00 mL) of water in the buret. c. Fill unknown container with water dispensed by the buret. i. Record measurement (i.e., final volume) to the appropriate precision. d. Empty water from unknown container and dry using paper towel the container. e. Repeat steps b-d two more times. f. Calculate the average volume for your unknown container. Using a graduated cylinder determine the volume of your unknown container. a. Fill unknown container with water and then carefully pour water in container into graduated cylinder. b. Record volume of water in graduated cylinder to the appropriate precision. c. Dry unknown container with paper toweling. d. Repeat steps a-c two more times. e. Calculate the average volume of your unknown container. Using beaker determine the volume of your unknown container. a. Fill unknown container with water and then carefully pour water in container into graduated cylinder. b. Record volume of water in graduated cylinder to the appropriate precision. c. Dry unknown container with paper toweling. d. Repeat steps a-c two more times. e. Calculate the average volume of your unknown container. Laboratory Activity #2: Determining the volume of unknown container via density 1. Using a balance, measure the mass of the clean, dry unknown container and record its mass to the correct precision. 2. Fill container with water to the top so that the water is “flat” not rounded upwards. 3. Place container with water on balance and measure the mass and record value to the correct precision. 4. Repeat steps 1 - 3 until you have collected two sets of data. *Upon completion of laboratory activities 1 & 2, return the unknown container to your instructor. Laboratory #3: Investigating the Mathematical relationship between the Fahrenheit and Celsius Temperature Scales. The third activity will be investigating the linear relationship between the temperature scales Fahrenheit ( oF) and Celsius (oC). The data that you collect will be used to construct a graph. 1. Gather the following pieces of equipment: 250- and 400-mL beaker, a hot plate, two thermometers – one Celsius & one Fahrenheit, a stir bar, two one-hole split stoppers, two clamps and a ring stand. 2. Set up a table in your lab notebook Readings ever 5 degrees oC Starting: o C o F 3. Using the 250 mL beaker obtain ice from the ice maker. Add the stir bar to the 400 mL beaker, and then fill it ½ way with tap water. Next add the ice. 4. Assemble set-up as demonstrated by your laboratory instructor. 5. Place the 400 mL beaker containing the ice-water on the hot plate. Arrange your set-up such that the two thermometers are placed within the ice-water mixture and will not be hit when the stir bar is moving. 6. Once the apparatus has been set-up, turn on the stir bar (you do not want a tornadic vortex). Take the temperature of the ice-water mixture from both thermometers. Record data to the proper precision. 7. Turn on the hot plate to a medium setting and record the temperatures (oF & oC) every FIVE degrees in change on the centigrade scale until the final temperature in the beaker is 60-65oC. 8. Using the data gathered, prepare a Celsius vs. Fahrenheit and a Fahrenheit vs. Celsius on the same plot. Scale each axis from (-) 150 to (+) 150 degrees. You are making a four-quadrant plot. 9. Place the 400 mL beaker containing the ice-water on the hot plate. Arrange your set-up such that the two thermometers are placed within the ice-water mixture and will not be hit when the stir bar is moving. 10. Once the apparatus has been set-up, turn on the stir bar (you do not want a tornadic vortex). Take the temperature of the ice-water mixture from both thermometers. Record data to the proper precision. 11. Turn on the hot plate to a medium setting and record the temperatures (oF & oC) every FIVE degrees in change on the centigrade scale until the final temperature in the beaker is 50-60oC. 12. Using the data gathered, prepare a Celsius vs. Fahrenheit and a Fahrenheit vs. Celsius on the same plot. Scale each axis from (-) 150 to (+) 150 degrees. a. You will construct one graph from the data collected. i. Note: You are making a four-quadrant plot. b. There will be two trend lines generated: i. Celsius (y-axis) vs. Fahrenheit (x-axis) ii. Fahrenheit (y-axis) vs. Celsius (x-axis). Clean-up: 1. Carefully disassemble the set-up. THE WATER IS HOT and so is the HOT PLATE. Return and/or store equipment. Do not wrap cord around hotplate. 2. Excess water and ice are to be poured into the large sink in the laboratory classroom. 3. Return all glassware and equipment used to their appropriate places. 4. Wipe down the bench. 5. Wash hands prior to leaving the laboratory classroom.