1. Determining Solution Concentration

Transcription

1 In this exercise you will determine the concentration of salt solutions by measuring samples with known concentration and making a calibration curve. You will review units of concentration, and how to make a graph in Excel. You will also learn how to use a calibration curve to determine a physical property of an unknown. Prelaboratory Assignment Read this lab guide and Section 4.4 in your textbook. Then, complete the prelab assignment in Chem21. The prelab assignment is due at the beginning of lab on the day your lab meets. 1.1 Keeping a Laboratory Notebook In CHEM 139, you were taught to keep a laboratory notebook. You will continue with this practice throughout this course. As a reminder, the notebook is designed to be a permanent record. Therefore, it should have the following characteristics: 1. It should be a permanently-bound book (not spiral bound) so that original pages cannot be removed. 2. Pages should be numbered consecutively. 3. No pages should be skipped, even if you start a new project in the middle of doing another one. (Simply put Goto page... at the end of an entry if that work is continued somewhere else in the notebook.) 4. It should have a table of contents at the beginning. 5. All entries should be made in ink. 6. All work should be dated. 7. Any mistakes should simply have a line drawn through them so that they are still readable. Entries should never be obliterated so that they can t be read. 1

2 The general format of the lab notebook entries should be as follows: 1. Write your name and contact information (phone and/or address) somewhere on the cover of your notebook. Also, as you go, you should write your name and your lab partner s name on each page of your notebook. 2. Label the first page as the Table of Contents. As you make entries in the notebook, you should continually update the table of contents. (Use the cardboard insert to prevent your marks from transferring to the other carbon copies within the notebook.) 3. Skip the first few pages of the notebook (to leave space for the table of contents) and record the title of the experiment, and the date at the top of the page. (The title of this experiment is Determining Solution Concentration. Remember to use pen so you have a permanent record! 4. Create a section beneath the title called Objective. (This section is also sometimes labeled as the goal or introduction.) In this section, describe what you think the objective of the experiment is. This section should only be about 2 sentences long. (Hint: Look at the first paragraph of the lab guide for clues on what to write.) 5. Create a section entitled Procedure. You will write the procedure as a numbered or bulleted list of steps. 6. Finally, create a section entitled Observations. In this section, you should take notes throughout your experiment on (1) what you did and (2) what you saw or measured. (Both parts are critical!) All observations should be written in complete sentences and numerical values should be in a table whenever possible. When in doubt, write something down. It s always better to write too much information than too little! This guide to keeping a lab notebook is posted at the top of your Chem21 homepage, and you should refer back to it frequently for guidance on proper note-taking techniques. 1.2 Introduction A calibration curve is a plot of some physical parameter of a substance (color intensity, density, etc.) as a function of that substance s concentration. In this exercise, we will make several sodium chloride solutions with different concentrations and measure their densities. Then we will plot density vs. concentration to make a calibration curve. Once a mathematical expression has been fit to the data, we will use it to determine the concentration of an unknown sample of which the density has been measured. You will be graded on the accuracy of your results, so use care when making and recording measurements. 2

3 Figure 1.1: Sketch of a volumetric flask. The unit used for concentration in this experiment is molarity. molarity = moles of solute volume of solution in liters Solutions will be made using volumetric flasks (see Figure 1.1). The solute (NaCl in this case) with a known mass is added to the flask first, and then enough water is added to ensure the solid dissolves completely. Then, additional water is added using a pipet or dropper to bring the meniscus of the solution to the line marked on the neck of the flask. Finally, the solution is mixed completely by inverting the stoppered flask several times. (1.1) 1.3 Making Solutions To make the sodium chloride (NaCl) solutions for this lab, you will make a stock solution for the highest concentration first, then perform dilutions to create the less concentrated solutions. Stock Solutions Make the 3 M stock solution as described below and record your data in your notebook. Show all your work for calculations in your notebook as well. In your notebook, make a heading that says theoretical mass of NaCl. Beneath the heading calculate how many grams of NaCl you would need to use to make 100 ml of a 3 M NaCl solution. (This problem is identical to what you did in the prelab assignment.) Place a weigh boat on the balance and press the TARE or ZERO button. The balance should now read g. Remove the boat from the balance, carefully pour some of the NaCl into the boat and place it back on the balance. The amount does not have to be exactly equal to the calculated amount, but try not to go over the calculated amount as you may have trouble getting all your salt to dissolve. If you ve 3

4 added too much, you can dispose of some excess in the trash can NOT back into the original container. If you don t have enough, remove the boat, add some more salt and check the weight again. Record the ACTUAL mass (whatever that turns out to be) in your notebook and label it the actual mass of NaCl. Be sure to record all the decimal places from the balance (even if the last number is a zero). Transfer the salt to the 100 ml volumetric flask by slightly bending the boat into a funnel shape. It s a good idea put another weigh boat or clean piece of paper under the flask to catch any spills. These can then be added to the flask as well. Obtain a wash bottle with deionized (DI) water, and add some water to the flask. Swirl the solution in the flask until all the salt has dissolved. (The solution may remain slightly cloudy due to an insoluble additive used in edible commercial salt to prevent caking.) Once dissolved, use a plastic pipet to add water so that the bottom of the meniscus is at the mark on the neck of the flask. This volume is EXACTLY ml and it should be recorded in your notebook as the volume of solution. Label a clean, dry 200 ml beaker Soln 1 and transfer your new solution to this beaker. Set this solution aside for later. In your notebook, calculate the ACTUAL concentration of your solution using the actual mass of NaCl and the volume of the solution. Label this the Soln 1 concentration. Note that this concentration may be slightly different than 3 M. Just be sure to use this value (not 3 M) as C 1 for all future calculations. Dilutions Solutions of lower concentration will be made by diluting portions of the higher concentration stock solutions. To calculate the amount of stock solution you need to use, you will use the formula C 1 V 1 = C 2 V 2 (1.2) where C stands for concentration and V is the volume of the solution. The indices (subscripts) 1 and 2 stand for the higher and lower concentration solutions, respectively. Say, for example, you have a 3 M stock solution and you want to make 25 ml of a 2.5 M solution. To find out how much of the 3 M stock solution to use, set up your equation like this: (3 M)(V 1 ) = (2.5 M)(25 ml) (1.3) The left-hand-side of the equation refers to the stock solution, and the right-hand-side refers to the new, lower concentration solution. Solving for V 1 gives ml, meaning you should take ml of the stock solution and dilute it to 25 ml. Obviously, ml will be difficult to measure exactly using a graduated cylinder, so you can safely round this result. You will recalculate the actual concentration anyway. Prepare a data table in your lab notebook that looks like Table 1.1 (next page). Then, follow the instructions below to make solutions with approximate concentrations of 2.5 M, 4

5 2 M, 1.5 M and 1 M. Label and record any calculations you perform in your notebook below the table, and complete the table as you go. Table 1.1: Sample table for the lab notebook. Abbreviations are as follows: Calc. = Calculated, Act. = Actual, and Theor. = Theoretical. Soln. # Calc. V 1 (ml) Act. V 1 (ml) V 2 (ml) Theor. C 2 (M) Act. C 2 (M) ml ml ml ml 1 In your notebook, use Equation 1.2 to calculate the volume of Soln 1 you will need to make 25 ml of a 2.5 M NaCl solution. This is Calc. V 1. (Remember to use the actual concentration of your stock solution for C 1 in your calculation.) Add the calculated volume of Soln 1 to a 100 ml graduated cylinder. You shouldn t worry about being exact; just record the actual volume in your data table under Act. V 1. Transfer the solution from the graduated cylinder into a 25 ml volumetric flask. Add some deionized water to the flask, swirl to mix, then add more water (using a pipet) until the solution is at the mark on the neck of the flask. Add a stopper, and invert the flask several times to ensure complete mixing. Then, transfer the solution to a beaker labeled Soln 2. Rinse the graduated cylinder and dry it. Rinse the flask thoroughly with DI water. (There is no need to dry the flask since you will be adding more water when you make the next solution.) Calculate the ACTUAL concentration of your new solution (C 2 ) using Equation 1.2 and the Act. V 1, V 2 and C 1. Record this actual concentration in your data table under Act. C 2. Repeat the above steps to make three more 25 ml solutions with the following concentrations: 2 M (Soln 3), 1.5 M (Soln 4) and 1 M (Soln 5). For every solution, use Soln 1 as your stock solution, and don t throw your solutions away! 1.4 Density Density (D) is defined as the mass (m) of a substance divided by its volume (V ): D = m V (1.4) 5

6 Determine the density of your five solutions and two unknowns as described below, and record your data in your lab notebook. (Make a table on your own this time!) Show your work for any calculations. At the balance, tare a 10 ml graduated cylinder. Remove the cylinder from the balance. Use a plastic transfer pipet to transfer approximately 8 ml of Soln 1 to the cylinder. Return the cylinder to the balance. Record the actual mass and actual volume of the solution in your data table. Calculate the actual density of the solution and enter it in your table. Pour the solution from the graduated cylinder into the sink. Then, rinse and dry the graduated cylinder. Repeat this process for your four remaining solutions and two unknown samples from the front of the lab. Once you have all your data, you can begin cleaning up. 1.5 Hazardous Waste and Clean-up Hazardous Waste There is no hazardous waste generated in this lab. You can pour the salt water you used down the drain Clean-up Remove all label tape from your glassware. Rinse and dry your glassware and return it to your station. Then, wipe down your benchtop with a wet paper towel and dry it. Have your instructor check your station before your leave. BEFORE YOU LEAVE LAB: Tear out the carbon-copy pages of your notebook. Make sure your name, your partner s name and your section number are on each page. Staple these pages together and turn them in to your instructor Lab Assignment Enter all of your data, calculations and answers to questions in the Lab 1 Assignment in Chem21. You will need to refer to your notebook for this. The Assignment is due approximately 15 minutes before your next lab meeting. The Assignment includes a hard copy of the graph described below. This graph can be turned in at the beginning of your next lab meeting. 6

7 1.6 Calculations The information in this section will help you complete the lab assignment in Chem21. You are encouraged to stay and work on this part before you leave the lab.when you click on the assignment in Chem21, you will be asked to enter all your data before proceeding with the calculations. Once your data has been entered, you will be asked to calculate the following: Concentration of solutions 1-5 Density of solutions 1-5 Density of your first unknown Density of your second unknown Then, you will be asked to prepare the calibration curve. There are specific instructions in Chem21 for doing this. The x-axis is always the dependent variable, the one you can control. The y-axis is the independent variable, the value that you measured. When constructing your graph, ask yourself: Which value did I have control over, the density or the concentration? Which value did I measure, the density or the concentration? This will help you determine which axis is which. For this graph, you will add a trendline. Be sure to display the equation of the line on the graph. Also, by clicking in the box of the equation, you can change the default variables of x and y to variables that correspond to what you plotted. Use C for concentration and D for density. The slope and intercept of your equation should also have units. The units for the intercept are the same as the units of the y-axis. For the units of the slope, recall rise over run. The slope has some y-value divided by some x-value, so the units should be the units of y divided by the units of x. Add these units into your equation as well. When you re finished with the graph, sketch its appearance and the equation of the line in your notebook. When you are satisfied with your graph, you should have your instructor look it over. Finally, use the equation from your graph and the densities of your unknowns to determine Concentration of your first unknown Concentration of your second unknown 7