Moravian College



Investigation of BuffersName Name of Lab Partner(s)Date of Experiment Date Report Submitted This page, Page 1, is your cover sheet.In what follows, the BLACK text is text you can keep or modify as you wish.The GREEN text contains instructions for you to follow in completing the report. There are tables where you must enter data. Feel free to change the length of the tables to accommodate all the data that you have. The ORANGE text xxxxx simulates text you need to write. Replace the orange with your own words. You may need more than one word!The YELLOW background indicates a place where you must enter numerical results into the text.REMOVE all the colors when you are done!AbstractOur investigation of buffers revealed that a buffer solution always contains significant concentrations of xxxxx, and that a buffer solution xxxxx changes in pH when a strong acid or base is added to it. Buffers can be prepared by mixing a weak acid with the salt if its xxxxx, and also by mixing a weak acid with xxxxx or by mixing a weak base with a xxxxx. For the acetate buffers we prepared, we observed pH changes of about xxxxx pH units when one drop of 0.5 M HCl or 0.5 M NaOH were added. We made buffer solutions by mixing acetic acid and sodium acetate to test the Henderson-Hasselbach equation, and found that xxxxx ….. We made buffer solutions using both graduated cylinders and micropipettes to measure the volumes of solutions to mix, and found that the two methods of measuring volume produced/ did not produce buffers with significantly different pH values.IntroductionThe purpose of this experiment is explore the chemistry of buffers …Explain your model of buffer behavior. You will want to talk about weak acids, conjugate bases, and of course insert the lovely H-H equation about the pH of buffers. Be sure to identify variables in the equation and provide a general explanation of buffer behavior based on the equation.Experimental MethodTo determine what kinds of mixtures could produce buffer solutions, we measured …..Describe Part 1 of the experiment! What was the buffer, and what was the control? How did you observe “Buffer Behavior”?Describe Part 2 of the experiment! How did you measure pH? How did you determine which mixtures were or were not buffers? What observations did you make to support the determination? Generally speaking, the passive voice is used to present the experimental method, so you say “the pH was measured using a pH probe” rather than “I used a pH probe to measure the pH”. This is the only section of a lab report that I believe should be adhere to the use of the passive voice. But then notice that I didn’t use the passive voice in my first sentence above! Also, the two sentence below!We tested the H-H equation by …..Describe Part 3 of the experiment! You should explain why only the volumes are needed to compute the log term.To determine the precision of making buffers, we tried making them by mixing together an acid and its conjugate base using both grad cylinders and micropipettes.Describe Part 4 of the experiment! Which target pH values were you assigned? What volumes did you try to mix?To assess the differences of these two methods we used a paired t test.How does that work? Explain what P-values would indicate that the methods are significantly different.Raw DataTable 1. pH Change after Addition of Strong Acid or Strong Base1ΔpH (HCl)ΔpH (NaOH)What buffer solution?What control solution?1what concentrations, how many drops?Describe the color changes that took place when you added acid or base. Be clear about the two different indicators your used.Table 2 presents the raw data you and your partners collected. Point out the pH of the original solutions as well; that will be important later!Table 2. pH: Original Solutions; Mixtures of Acids and BasespH Mixed w/ 0.2 M HOAcMixed w/ 0.2 M NaOAcMixed w/ 0.1 M NaOH0.1 M HCl0.2 M HOAc4.75b0.2 M NaOAc0.1 M NaOH b Possible acetate bufferCenter the pH values in the cells of the table.Place a superscript b in the cells where, in your opinion, the pH indicates a possible buffer.We chose two or three mixtures to test for buffer behavior by adding one drop of strong acid or strong base. Describe what you observed. Did it match what you saw in part 1? Table 3. Your Title—Only you and your partners’ data goes in this table!ml base/ml acid1Log [B]/[A]pH8/80.04.761What were those solutions that you used?Center the information in the cells. Note the sneaky way that both acid and base volumes are entered in the table!Fig 1. pH vs log [B]/[A]. Graph the data in Table 3. You will want to use markers only for the data, and draw a line using Trend line. Table 4 presents the results of attempting to make identical buffers solution using grad cylinders and micropipettes. We prepared acetate, ammonia, and phosphate buffers. Table 4. Your Title—Only you and your partners’ data goes in this table!Target pHml base/ml acid1pH (grad cyl)ml base/ml acidpH (micropip)4.758/84.763.5/3.54.741acetate 2ammonia 3phosphateCenter the information in the cells. Note the sneaky way that both acid and base volumes are entered in the table!Sample CalculationShow how you used the buffer equation to determine the volume to mix for one – just one! – of your target pH values! Yep – you’ve got to do some algebra. In p chem I would let you do this by hand. You can do that here too.Data AnalysisSignificance TestingTable 4 presents the pooled data from the entire class for all mixtures of buffers made using grad cylinders and micropipettes. Here’s where you mention the Paired t test again! Please don’t let the lovely table get broken across a page break! Table 4. Plain vs Plain M&M data analysis.studentspH (grad cyl)pH (micropip)WHOThis time you’re going to report the P-value from the test in the text of a paragraph! And you will immediately state whether the results indicate there is or isn’t a significant difference in the methods of preparing the buffers!DiscussionHere’s where you get down to the data. The first thing to do is use the data in Tables 1 and 2 to convince the reader that you know which mixtures created buffers and what “buffer behavior” is! Do the solutions you tested in Table 1 show buffer behavior? How? Then explain why the mixtures in Table 2 made buffers – some balanced reactions will help! The pH of the original solutions are important, for example, explain what happens when NaOAc and NaOH are mixed together. Who controls the pH of that mixture? How do you know based on pH? You should have an explanation for all six mixtures in this paragraph-either why they are a buffer or why they aren’t!Does the data in Table 3 and Figure 1 verify the H-H equation? What are the values of slope and y-intercept, and what do they mean chemically? How accurately can you make buffers to a given pH? How precisely can you make buffers using two different ways of measuring the volumes. Which question does the paired t test answer??ConclusionsWhat did you learn about buffer behavior from this experiment? Summarize buffer behavior, and how it comes about chemically. Why do you think that chemists usually make a buffer for a particular pH using a weak acid and a strong base with a pH probe at in hand? What advice would you give to someone trying to make a buffer, say yourself in the future?Literature CitedHarris,D,C.(2003). Quantitative Chemical.Analysis (6th Ed), p72-74, New York: W. H. Freeman Company.List any reference to the procedure that you followed.Cite a reference other than Harris for the statistical tests.Where did you get any other information you used? ................
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