The Sneaky Ethanol Molecules Lab

Experiment 2 Is volume conserved? “The sneaky ethanol molecules” Introduction Matter can neither be destroyed nor created. It is conserved. This concept relates to mass but what about volume? Is it conserved? The two factors that affect volume are intermolecular forces and molecular size. Intermolecular forces are forces between molecules – a topic you will learn more about later on this semester. Oil and water are not miscible due to different intermolecular forces. On the contrary, substances with similar intermolecular forces, such as alcohol (ethanol) and water, are miscible. Intermolecular forces also determine the structure of the intermolecular lattice, which is related to the density of the mixture. To understand how molecular size affects volume, think about mixing small and big spheres, e.g. apples and peas. The smaller peas can fill the holes between the bigger apples and in this way reduce the overall volume of the mixture. Similarly, molecules with different sizes might pack better in a mixture than in the pure liquid. In general, depending on the substances that are mixed, volume can decrease, increase or remain the same. Focus question: When you mix alcohol and water is the volume conserved? Why or why not? Explain using your data and think about molecular packing mentioned in the introduction above. __________________________________________________________________________________________ ____________________________________________________________________________________________________________ _ In today’s lab you will first determine the densities of alcohol and water. Then you will make mixtures of alcohol and water with different ratios and determine their densities. Procedure: Each group will have to perform all 3 parts of the experiment. Group size: you are working in groups of 2 Part 1: Determine the densities of water and ethanol All masses must be recorded to two decimal places. Determine the volume contained in a graduated cylinder by reading the bottom of the meniscus at eye level. Read the volume using all certain digits and one uncertain digit. Certain digits are determined from the calibration marks on the cylinder. The uncertain digit (the last digit of the reading) is estimated. Depending on the size of the graduated cylinder and the graduations, the uncertain digit may be the tenth place or the hundredth place. You will have to look at the graduated cylinder to determine how many digits to record. Refer to skills and knowledge section at the end of this laboratory. When using the 10-mL graduated cylinder, record the volume to two decimal places. When using the 25-mL graduated cylinder, record the volume to one decimal place. 1. Wash a 10-mL graduated cylinder with soap. Rinse it 3 times with water and then 3 times with about 1 mL acetone. Shake it to dry (carefully!). Remove the remaining acetone with a paper towel. 2. Weigh the empty cylinder and record the mass. 3. Fill a clean beaker with tap water. 4. Use a Pasteur pipet to transfer 8 to 10 mL into the graduated cylinder making sure that no drops keep sticking to the inside of the cylinder. Record the volume. 5. Weigh the cylinder with water. 6. Calculate the density of water. 7. Show your results to your instructor before you move on to step 8. 8. Repeat steps 1-6 using now ethanol and determine the density of ethanol. Show your results to your instructor before you move to part 2. Part 2: Competition Competition time! What is the total volume when 10.0 mL of water is mixed with 10.0 mL of ethanol? Write your predicted value on the board. Let’s do the experiment now and see who made the best prediction. 1. Wash and dry a 25-mL graduated cylinder. 2. Using a Pasteur pipet fill exactly 10.0 mL ethanol into the 25-mL graduated cylinder. 3. Fill 10.00 mL water into the 10-mL graduated cylinder. 4. Transfer the water to the 25-mL graduated cylinder containing ethanol. Mix well by shaking but make sure you don’t spill. Mix until cloudiness caused by initial water/ethanol contact goes away and solution is clear. Record the volume and write it on the board. The data of all groups should be recorded on the board in class. 5. Determine the average value. 6. The winner is the group with a predicted value closest to the average value. Part 3: Determine the density of water/ethanol mixtures 1. Weigh your clean and empty 25-mL graduated cylinder and record the mass. 2. Now make five experiments according to the table below. Fill the smaller amount of liquid (either water or ethanol) into the 10-mL graduated cylinder and the larger amount into a 25-mL graduated cylinder. Pour the content of the 10-mL graduated cylinder into the 25-mL graduated cylinder. Mix well by shaking but make sure you don’t spill. Mix until cloudiness caused by initial water/ethanol contact goes away and solution is clear. 3. Weigh the cylinder with the water/ethanol mixture. 4. Wait a moment until the drops sticking to the inside of the cylinder combine with the mixture and record the volume. 5. Run two trials for each experiment, record the data in the table and review it with other groups. Calculate the density for each trial and average density for each experiment, i.e. each mixture. 6. Calculate the volume % ethanol for each mixture (= Vethanol/Vsolution x 100%) and the pure liquids. Vsolution is the average value of the two trials. 7. Using Excel, plot density vs. volume % ethanol (there should be seven data pointsmake sure you include the data from part 1). Note that the dependent variable (density) is always plotted on the vertical axis, while the independent variable (volume % ethanol) is always plotted on the horizontal axis. 8. Add a trendline that fits your data best (you might have to try different trend types). 9. Calculate the expected density of each mixture based on your experimental densities of water (𝑑𝑤𝑎𝑡𝑒𝑟 ) and ethanol (𝑑𝑒𝑡ℎ𝑎𝑛𝑜𝑙 ). Add these points to your graph from step 7 and connect them. 𝑒𝑥𝑝𝑒𝑐𝑡𝑒𝑑 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 = 𝑣𝑤𝑎𝑡𝑒𝑟 𝑑𝑤𝑎𝑡𝑒𝑟 + 𝑣𝑒𝑡ℎ𝑎𝑛𝑜𝑙 𝑑𝑒𝑡ℎ𝑎𝑛𝑜𝑙 𝑒𝑥𝑝𝑒𝑐𝑡𝑒𝑑 𝑡𝑜𝑡𝑎𝑙 𝑣𝑜𝑙𝑢𝑚𝑒 Data Table 2: Volume % Ethanol Volume % Water Experimental density, g/mL Expected Density, g/mL Trial 2 Density, g/mL Trial 1 4.0 8.0 10.0 12.0 16.0 Volume solution, mL Trial 2 16.0 12.0 10.0 8.0 4.0 Mass solution, g Trial 1 1 2 3 4 5 Mass cylinder + solution, g Trial 2 Volume ethanol, mL Trial 1 Volume water, mL Trial 2 Exp. Trial 1 Data Table 1: Average density, g/mL Post-Lab Assessment Questions (20 points): 1. (3 pts) Based on what you did and learned in today’s lab what can you predict if 1propanol is mixed with distilled water? Are the mass and volume conserved? 2. (3 pts) When you mix two gases what can you predict about the conservation of volume in this case? 3. (4 pts) Hydrogen gas burns in the presence of oxygen gas to form liquid water. What can you say about the volume and mass conservation in this chemical reaction? 4. (5 pts) Based on your graph from part 3, predict the density of a 43% solution of ethanol in water. 5. (5 pts) Using the data you collected and the graphs you constructed compare the experimental density of the water/alcohol mixtures with the expected densities. Explain your results from a molecular perspective. SAFETY CONSIDERATIONS AND WASTE DISPOSAL: Often when scientists work in chemistry laboratory they are handling chemicals that are potentially hazardous and can be dangerous. In order to ensure their safety, the scientists always follow certain safety protocols. Two of the most important safety protocols that scientists always follow are: 1. The wearing of Laboratory Safety Goggles to protect your eyes, and 2. Proper disposal of all chemical waste. In our lab experiment for today we are not working with any hazardous or dangerous chemicals, but in order to be good scientists we will ALWAYS practice the proper safety protocols. This means that you must wear your safety goggles at all times when you are in the laboratory. Even though we may not be working with hazardous chemicals at the moment, later this semester we will use some chemicals that can be harmful. It is important that we begin practicing proper safety procedures now, so that when we do need them later on we are already experienced in the practice. Ethanol-flammable, may cause skin and eye irritation Acetone-flammable, volatile, may cause skin and eye irritation SKILLS AND KNOWLEDGE NEEDED 1. MEASURING MASS You will measure mass in this chemistry laboratory using the electronic balance. The electronic balance available to you in this laboratory can measure masses to the second decimal place (+ 0.01 g). Steps for using an electronic balance: 1. Place a weighing tray or a piece of weighing paper on the balance. If using weighing paper, it is important to fold the weighing paper diagonally down the middle before placing it on the balance. 2. Zero the balance by pushing the “ZERO” or “TARE” button on the bottom right corner. When the balance reads zero you are ready to load your sample. 3. Use a spatula to slowly measure out small quantities of your sample into the weighing tray. Do NOT pour chemicals directly out of the bottle onto the weighing paper. 4. When you have the desired quantity of material measured you are ready to remove your sample from the balance. Slowly and carefully lift the weighing tray off the balance and carry it to your bench top. If using weighing paper, it is a good idea to fold the edges of the paper around your sample so that it does not spill while you are walking. 2. MEASURING VOLUME In the laboratory you will encounter a variety of graduated glassware that can be used to accurately measure volumes. Accurately measuring volumes is often difficult for students. The type of glassware you choose for your measurement will depend on the volume you wish to measure. The graduations on the glassware will not always be the same. Before you begin any volume measurement you should carefully consider the divisions of measurement that are provided on your glassware. Regardless of the glassware you choose, it is imperative that you learn to read the volume accurately from the provided graduations. Reading volumes accurately: When a liquid (pure liquid or solution) is poured into a cylindrical container there will be a force of attraction between the liquid molecules and those that make up the container. The force of attraction between the liquid molecules and the container causes some of the liquid molecules to “stick” to the sides of the container. This results in a visible curve in the surface of the liquid. The curved surface of the liquid inside the container is called a meniscus. To read the volume of a liquid accurately you must place your eye level at the surface of the liquid and read the volume at the bottom of the curve of the meniscus. In the picture to the left, the volume should be read as 36.5 mL. Notice that you have to estimate the last digit, in this case the tenths decimal place. The Graduated Cylinder: A graduated cylinder is a cylindrical piece of glassware (or sometimes plastic) that has graduations in units of milliliters (mL) marked down one side. A graduated cylinder is one of the most common pieces of laboratory equipment used to measure milliliter volumes of liquids. You have 2 graduated cylinders available to you this semester: a 10-mL graduated cylinder and a 50-mL graduated cylinder. When measuring volumes smaller than 10 mL, you should ALWAYS use the 10-mL graduated cylinder because it has smaller units of measurement and thus allows for more accurate readings. Steps for using a graduated cylinder: 1. Choose the graduated cylinder of the appropriate size. 2. Place the graduated cylinder on a flat surface with your eye directly opposite the volume graduation that you want to measure. 3. Slowly pour the liquid you are measuring into the graduated cylinder until you are just below the volume you want to measure. 4. Use a Pasteur pipette to carefully fill the graduated cylinder to the exact volume. REMEMBER to read the volume at the bottom of the meniscus.