Density and Laboratory Measurements

Transcription

1 Density and Laboratory Measurements This exercise, which covers density, unit conversions, and dimensional analysis, is intended to review basic laboratory techniques and facilitate the development of problem solving and critical thinking skills through collaborative learning and interactive application. Please perform thoughtful discussions with your laboratory group while performing the requested measurements and calculations. Although this lab activity is designed to be a group exercise, each person will submit their own Data and Question Sheet for assessment. You must show all of your work to receive full credit! Common Laboratory Equipment Locate your assigned glassware and hardware drawers. Many of the items contained in these drawers are pictured below in Figure 1. Select laboratory equipment (such as pipets, burets, volumetric flasks, filter flasks, Büchner funnels, etc.) will be provided as needed. Ring stands and Bunsen burners are stored in the laboratory; however their location varies from room to room. Locate where these items are stored in your lab. You will be using them next lab period! Figure 1. Common laboratory glassware and equipment Beakers 2. Erlenmeyer Flasks 3. Volumetric Flask 4. Filter Flask 5. Graduated Cylinders 6. Wash Bottle 7. Test Tubes 8. Test Tube Brushes 9. Test Tube Holder 10. Watch Glasses 11. Funnels 12. Büchner Funnel 13. Evaporating Dish 14. Crucible and Lid 15. Crucible Tongs 16. Spatula 17. Scoopula 18. Buret 19. Volumetric Pipet 20. Graduated Pipet 21. Stirring Rod 22. Ring Stand 23. Double Buret Clamp 24. Iron ring 25. Clay Triangle 26. Extension Clamp 27. Utility Clamp 28. Clamp Holder 29. Wire Gauze 30. Bunsen Burner 31. Flint Striker

2 Chemistry is an experimental science that relies on careful measurements and the proper recording, evaluation, and analysis of the collected data. Making accurate and meaningful scientific measurements requires an understanding of basic laboratory techniques and the proper use of common laboratory equipment and instrumentation. The following activities are meant to help you become familiar with the use of some of the laboratory equipment shown in Figure 1. The experiments included in this exercise emphasize volume measurements, mass measurements, and the concept of density. The data collected during these experiments should be recorded in the accompanying Data and This sheet must be completed and submitted at the beginning of the next laboratory period. Determining Densities from Laboratory Measurements As indicated in Equation 1, the density of a substance is its mass divided by its volume. Mass Density = (1) Volume Density is an intensive property, which means that this property is independent of the quantity or amount of material present. The density of pure water (H 2 O) is g/ml at 4 C. It is slightly less than this at room temperature ( at 25 C). In the first part of this activity (Part A), you will determine the density of deuterated (or heavy) water and compare it with the density of normal water at the same temperature. In Part B, you will determine the density of a 40% w/w sugar solution and then use your experimental results to predict whether or not a golf ball should float in this solution. In Part C of this experiment, you will determine the identity of an unknown metal bar by measuring its density. In this case, two different methods will be used to determine the volume of the metal sample. In the final part of this activity (Part D), you will be asked to estimate the thickness of two types of aluminum foil using the density of aluminum metal and the equipment and materials available in the laboratory. After each experimental activity, the resulting density measurements will be evaluated and, in some cases, compared with literature values. A series of questions will also follow each experiment. All data, calculations, and question responses should be included on your Data and Part A: Comparing the Densities of Normal Water and Deuturated (or Heavy) Water The chemical formula of water is H 2 O. This means that each molecule of water consists of 2 hydrogen atoms and a single oxygen atom. In deuterated water (D 2 O), the hydrogen atoms are replaced with deuterium atoms. Deuterium is an isotope of hydrogen. As discussed in Section 2.3 of your textbook, isotopes of a given element have the same number of protons, but different numbers of neutrons. The nucleus of a hydrogen atom consists of only a single proton. A deuterium atom, on the other hand, has one proton and one neutron in the nucleus. Thus, deuterium atoms have the same number of protons as hydrogen atoms, but they are twice as heavy due to an added neutron. The density of a substance can be determined by a variety of methods. In this experiment, the densities of normal water and deuterated water will be determined from a series of mass and volume measurements. First, the mass of a clean, dry graduated cylinder will be determined using a laboratory balance. Graduated cylinders are used for measuring approximate volumes of liquids. A specific volume of distilled water will be added to the cylinder, and the mass of the cylinder/water system will be measured. The mass of the water will be calculated by subtracting the mass of the empty cylinder from the mass of the cylinder and water. The measured volume and the calculated mass will then be used to determine the density of water at a particular 2

3 temperature. This procedure will be repeated for deuterated water. A step by step procedure is given below. Please follow this procedure and complete the Data and Question Sheet as prompted. Procedure: Part A 1. Obtain approximately 15 ml of distilled water. Measure the temperature of this water using the thermometers or temperature probes that are provided. Record this temperature on your Data and 2. Measure the mass of an empty, dry 10 ml graduated cylinder. Please read the instructions shown below before using the electronic balances that are available in the lab. Your TA may also provide you with additional instructions regarding the proper use of these balances. Record the cylinder mass on your Data and Using an Electronic Balance to Measure the Mass of the Graduated Cylinder 1. With the balance pan empty and all doors completely closed, press the On/Off/Tare bar. 2. When the display reads g, gently place the graduated cylinder on the center of the balance pan. 3. Close all balance doors. 4. The mass reading will likely fluctuate briefly before stabilizing. When the mass reading stabilizes, record this value. 3. Carefully remove your graduated cylinder from the balance. 4. Add 10 ml of distilled water to the cylinder. Instructions for reading the volume of a liquid in a graduated cylinder are provided below. In this case, the very bottom of the meniscus should just touch the 10 ml line. Record the water volume on your Data and This measurement should be written with the appropriate number of significant figures. Measuring Volume with a Graduated Cylinder The surface of a liquid in a glass cylinder exhibits a definite curvature, called a meniscus. For most liquids, this curvature is concave (downward), and the bottom of the meniscus is used for reading the volume (as shown in the picture on the right). However, the surfaces of certain liquids, such as mercury, form convex (upward) curves. In these cases, you must read the top of the meniscus. When reading volumes on a graduated cylinder, your eye must be level with the bottom of the meniscus in order to observe the correct volume. Errors in measurement (called parallax error) will result if your eye is above or below the meniscus. Volume measurement should be recorded with the appropriate number of significant figures. 3

4 5. Weigh the cylinder + 10 ml of water. Record this mass on your Data and 6. Calculate the mass of water in the graduated cylinder. Record this mass (with the appropriate significant figures) on your Data and Show your work in the Calculations column. 7. Determine the density of water using the volume and mass data. Record this density (with the appropriate significant figures) on your Data and Show your calculations in the Calculations column. 8. Repeat this procedure with deuterated water. IMPORTANT: When you are finished with the deuterated water, please pour all of it back into the original reagent bottle. DO NOT pour normal water into the deuterated water container as this will contaminate the sample and lead to anomalous results in future investigations. Part B: Determining the Density of a 40% w/w Sugar Solution In this part of the exercise, you will make a 40% w/w sugar solution and determine the density of this solution using a procedure similar to that used in Part A. Note: The term w/w is an abbreviation for by weight. Therefore, the % w/w of a substance (A) in a mixture is given by: mass of A in the mixture % w/w A in a mixture = 100 total mass of mixture In Part A, volume was measured with a graduated cylinder. In this activity, volume will be measured using a volumetric pipet. Volumetric (or transfer) pipets are used for delivering a single, fixed volume of liquid. Volumetric pipets are calibrated with only one mark and are available with capacities ranging from ml. Your TA will show you how to properly use a volumetric pipet (the diagram in Figure 2 may also prove useful). Figure 2. Using a volumetric pipet. 4

5 A step by step procedure for this part of the exercise is given below. Please follow this procedure and complete the Data and Question Sheet as prompted. Procedure: Part B 1. Make a 40% w/w sugar solution. Detailed instructions for preparing this solution are given below. a. Weigh out 80 g of sugar. Set this sample aside. Note: The analytical balances in the balance room have a sensitivity of ± g and measure a maximum weight of 151 g. The electronic balance at the front of the lab room has 0.01 g precision and a 1500 g weight capacity. b. Use a graduated cylinder to measure out 120 ml of distilled water. c. Transfer the water to a 400 ml beaker. d. Slowly add sugar to the beaker while stirring the solution with a glass stirring rod or a magnetic stirrer (recommended). Your TA will give you instructions on how to use this device. Some general guidelines are given below. Continue stirring the solution until all of the sugar has dissolved. General Guidelines for Using a Magnetic Stirrer Take care when adding a stir bar to your solution to avoid cracking or breaking the glass container. This can be accomplished by tilting the beaker and gently sliding the stir bar along the beaker wall. This technique should also minimize splashing and prevent sample loss. The stirrer should not be turned on until after you have placed the beaker on top of it. Once you turn on the stirrer, carefully increase the stirring rate so that the solution is being thoroughly mixed. Do not stir the solution too rapidly as splashing and poor mixing may result. Please return the magnetic stir bars when you are finished with them. DO NOT POUR THEM DOWN THE SINK OR PUT THEM IN THE TRASH!! 2. Measure the mass of an empty, dry 100 ml beaker. Record the beaker mass on your Data and 3. Using a 25 ml volumetric pipet, transfer 25 ml of the 40% sugar solution into the pre weighed 100 ml beaker. (Figure 2 outlines how to properly use a volumetric pipet.) Record the sugar solution volume on your Data and This measurement should be written with the appropriate number of significant figures. 4. Weigh the beaker + 25 ml of sugar solution. Record this mass on your Data and Pour the sugar solution back into the 400 ml beaker. 5. Calculate the mass of sugar solution in the beaker. Record this value on your Data and Show the associated calculation in the space provided. 5

6 6. Determine the density of the sugar solution using your volume and mass data. Record this value on your Data and Show the associated calculation in the space provided. 7. Repeat Steps 2 6 two more times. 8. Gently place a golf ball in the 40% w/w sugar solution and record your observations on the Data and 9. Use your density values from trials 1 3 to determine an average. Record this value on your Data and Part C: Determining the Identity of an Unknown Metal Each group (or each person if time permits) will be given an unknown metal sample. The purpose of this exercise is to determine the identity of this metal from its measured density. The density of the metal bar will be determined by measuring its mass and volume. In this experiment, two different techniques will be used to measure the volume of the bar. A technique called liquid displacement (or volume displacement) will be used first. In this method, the volume of the metal sample will be determined by immersing the bar in water and accurately measuring the volume of water that is displaced. In this case, the volume of the solid will equal the volume of water that is displaced. Since the unknown metal bar has a cylindrical shape, its volume can also be determined from its height (h) and radius (r). Recall: V πr h. Thus, a ruler will be used to measure the height and diameter (d = 2r) of the metal cylinder, and the volume of the bar will be determined by substituting these values into the equation shown above. A step by step procedure for this part of the exercise is given below. Please follow this procedure and complete the Data and Question Sheet as prompted. Procedure: Part C cylinder = 1. Obtain an unknown metal cylinder from your TA. Each unknown is marked with a different letter. Record this letter on your Data and 2. Measure the mass of your unknown metal bar. Record this mass on your Data and 3. Determine the volume of the metal bar using the volume displacement method. Specific instructions are given below. a. Add approximately ml of water to a 100 ml graduated cylinder. Record the initial volume of the water in the space provided on your Data and This volume should be recorded with the appropriate number of significant figures. b. Slightly tilt the graduated cylinder and carefully slide the metal sample down the inside surface of the cylinder. Take care when doing this to avoid breaking the glassware and/or splashing any of the water out of the cylinder. c. When the metal sample is completely immersed in the water, tilt the cylinder upright and set it on the lab bench. Record the final water volume in the space provided on your Data and Question Sheet. This volume should be recorded with the appropriate number of significant figures. d. Calculate the volume (in ml) of the metal bar by subtracting the initial water volume from the final water volume. Show your calculation and record your findings on your Data and Question Sheet. 6 2

7 e. Convert your volume from ml to cm 3. Recall: 1 ml = 1 cm 3. Record this volume on your Data and 4. Determine the volume of the metal cylinder using ruler measurements and the formula for the volume of a cylinder ( V πr h ). cylinder = 2 a. Use a ruler to measure the height (h) and diameter of the bar in inches. Record these dimensions on your Data and Make sure that your data is reported with the appropriate number of significant figures. b. Calculate the radius (r) of the bar from your diameter measurements. Record this information on your Data and c. Calculate the volume of the metal bar in units of in 3. Show your work in the Calculations section and record your results on your Data and d. Convert your volume from in 3 to cm 3. Recall: 1 in = 2.54 cm. Show your work in the Calculations section and record your results on your Data and 5. Calculate the density (in g/cm 3 ) of the metal bar based on your mass measurements and your two different volume measurements. Show your work in the Calculations section and record your results on your Data and 6. Return your unknown metal to your TA. Part D: Determining the Thickness of Aluminum Foil A variety of aluminum foils are available in the laboratory. Devise a method to determine the thickness (in mm) of two different types of aluminum foil using the density of aluminum (d Al = 2.70 g/cm 3 ) and the equipment and materials that are available to you in the laboratory. Conduct your proposed experiments. Please perform two trials for each foil sample. A description of your approach and all associated measurements and calculations must be included on your Data and Make sure that that you record the types of aluminum foil (e.g., standard Reynolds Wrap and Heavy Duty Reynolds Wrap ) that you investigated. All data should be reported with the appropriate number of significant figures. The proper units should be included on all of your measurements and calculated quantities. 7