Titrations Practice Worksheet



Honors Chemistry Name:Acid-Base Titration Dry Lab (30 points)Part 1: Background:An acid-base titration is a neutralization reaction that is performed in the lab in order to determine an unknown concentration (Molarity) of acid or base. As long as the concentration of one of the solutions is known, the concentration of the other reaction can be obtained through titration. In a titration reaction, the moles of acid equal the moles of base at the equivalence point. Once the equivalence point is reached the concentration of the unknown solution can be determined because:MaVa = MbVbWhereMa = molarity of acid Va = molarity of base Mb = molarity of base Vb = volume of baseWhen you carry out a simple acid-base titration, an indicator is used to indicate when the acid and base have mixed in exactly the right proportion to neutralize each other. When the indicator changes color the end point of the titration has been reached. In an ideal world, the color change would happen when the two solutions are mixed in equal proportions. This is not always the case, but when it is, this point is known as the equivalence point. For strong acid/strong base reactions, the equivalence point occurs at pH =7.4166234832272If the pH of an acid is plotted against the amount of base added during a titration, the shape of the graph is called a titration curve. All acids titration curves follow the same basic shape. At the beginning, the solution has a low pH and climbs as the strong base is added. As the solution nears the point where all of the H+ are neutralized, the pH rises sharply and then levels out again as the solution becomes more basic as more OH- ions are added. The titration curve in Figure 1 shows a strong acid being titrated by a strong base. There is the initial slowrise in pH until the reaction nears the point where justFenough base is added to neutralize all the initial acid.iThis point is the equivalence point.guPrelabUse the graph and reading to answer the questionsrbelow on a separate sheet of paper.eDefine the following terms: titration, equivalence1point, end point, titration curve.Based on Figure 1, where does the pH begin and where does the pH end? Based on figure one, what is the independent variable? What is the dependent variable? Solve the neutralization problems below using the equation MaVa = MbVb. SHOW ALL WORK:If it takes 54 mL of 0.1 M NaOH to neutralize 125 mL of an HCl solution, what is the concentration of the HCl?If it takes 25 mL of 0.05 M HCl to neutralize 345 mL of NaOH solution, what is the concentration of the NaOH solution?If it takes 50 mL of 0.5 M Ca(OH)2 solution to completely neutralize 125 mL of sulfuric acid solution (H2SO4), what is the concentration of the H2SO4 solution?How many milliliters of 0.360 M H2SO4 are required to neutralize 25.0 mL of 0.100 M Ba(OH)2?What is the molarity of a 30.0mL hydrochloric acid solution (HCl) which is just neutralized by48.0 mL of 0.100 M sodium hydroxide (NaOH)?Part 2: Titration Experiment BackgroundA titration is an experimental laboratory procedure where one solution with an unknown concentration is reacted with a second solution whose concentration is known. By comparing the quantities involved the experimenter can determine the concentration of the unknown solution.The titration you will be doing in this lab involves and acid and a base. The reaction carried out in this lab involves vinegar (a diluted acetic acid solution) and sodium hydroxide. The reaction we will observe is shown in the equation:HC2H3O2 + NaOH {---} H2O + NaC2H3O2You may recall from our earlier study of acids and bases (Unknown Solutions Lab), that this reaction is called a neutralization reaction because the acid and base, when mixed in the proper proportions, will neutralize each other, creating water and an ionic salt.The point at which neutralization occurs (what we call the equivalence point) will be made visible through the use of an indicator (color change called “endpoint”) and pH sensor. The indicator phenolphthalein, which you used earlier this year, will be used. You may recall that phenolphthalein is colorless in acids but turns magenta as the solution becomes basic.The concentration of acetic acid HC2H3O2 in the vinegar is unknown. In this lab you will titrate a sample of vinegar against a 0.20 M NaOH solution. You will then calculate the concentration (molarity) of the acetic acid in the vinegar.Procedure (Read through this)Note: When using the buret you must make sure that you do not fill it higher than the 0.00 ml mark or let it drain lower than the 50.00 ml mark. If you find that you are running out, record your volume and refill the buret with more solution.1. Rinse one buret with distilled water, and then rinse with small amounts of NaOH. Fill the buret (again, not above the 0.00 ml mark) with the 0.20 M NaOH. Using label tape, label the buret “base” (NaOH). Record as the initial reading of base in the data table.2. Do the same with the second buret, this time rinsing with a small amount of vinegar. Label this buret “acid” (HC2H3O2). Record as the initial reading of acid in the data table. 3. Using the “acid” (vinegar) buret, transfer exactly 5.00 ml of vinegar into a clean, dry beaker. You will need to calculate from your initial buret reading where to stop to dispense exactly 5.00 ml. Use the pH sensor to measure the initial pH, and record as initial pH in the data table. Add 2-3 drops phenolphthalein to the beaker. Swirl gently until the contents are well mixed. (NOTE: IT IS VERY IMPORTANT THAT THE PHENOLPHTHALEIN ONLY BE PLACED INTO THE FLASK. AT NO TIME SHOULD THE PHENOLPHTHALEIN BE PLACED INTO THE BURET ITSELF). 4. Clamp the “base” (NaOH) buret so that its tip is directly above the beaker, above the surface of the solution.5. Begin the titration by adding successive portions of the NaOH solution. Initially, you will need to add large amounts of NaOH, until you get close to the end point, after which your amounts will be much smaller. After each addition, swirl the beaker to mix. 6. As you near the endpoint, the color will stay pink for a longer period of time before turning colorless when swirling. As you approach the endpoint, you should add NaOH drop by drop, until the solution remains the slightest shade of pink. Record as the final reading of base in the data table.7. If you go past the slight pink endpoint, change back over to your “acid” (vinegar) buret and add, drop by drop, vinegar, until you are back to the final endpoint (colorless). Record the final reading of acid. Use the pH sensor to measure the final pH at the end point.8. It may be necessary to go back and forth between each buret more than once to achieve the best endpoint. Just be sure that when you reach the desired endpoint, you note and record the final readings on each buret and the final pH. The difference in the initial and final buret readings is the calculated volume of acid and/or base.9. Dispose of the contents of your beaker down the drain and rinse well. Repeat this procedure, steps 3 through 8 a second time (i.e. two trials) and record.10. When you have finished, empty the NaOH and vinegar from the burets according to your teacher, and rinse each well with distilled water, filling them and allowing them to drain (through the tip). Store both burets, upside down, with the valve open.11. Clean up your area and wash your hands.PRELAB QUESTIONS1. What must you be careful of when filling and using your buret?2. What acid is found in vinegar?3. Why is the reaction in this experiment called a neutralization reaction?4. How will we known when neutralization has occurred?5. What specifically are we trying to determine in this lab?DATA TABLE(S): This has been completed for you as though you had done this in the lab.Trial #Molarity of BaseInitial Reading (base)End Reading (base)Total Volume (base)Initial reading (acid)End Reading (acid)Total Volume (acid)10.20 M19.33 mL 44.23 mL24.90 mL10.4 mL30.32 mL19.85 mL20.20 M13.68 mL47.28 mL33.60 mL13.9 mL40.78 mL26.88 mLACID BASE TITRATION LAB WRITEUP1. Write the balanced chemical equation for the reaction which occurred in this experiment.2. What was the total volume of NaOH used? Show for both trials.Trial 1:Trial 2:3. Using the molarity of NaOH, determine the moles of NaOH used. Show for both trials.Trial 1:Trial 2:4. According to the balanced chemical equation, what is the mole to mole ratio for the hydroxide ion (from NaOH) and the hydrogen ion (from acetic acid)?5. How many moles of acetic acid were used to neutralize the NaOH? Show for both trials. Trial 1:Trial 2:6. What was the total volume of acetic acid used? Show for both trials.Trial 1:Trial 2:7. Using the information from questions 5 and 6, determine the concentration (molarity) of the acetic acid (vinegar). Show for both trials.Trial 1:Trial 2:8. Average the molarities found in #7. Your teacher will give you the vinegar manufacturer’s stated concentration. The actual molarity of the acetic acid is 0.25 M. Use this in your calculations.a. Calculate your % error? b. If your % error exceeds 10%, list one source of error (be specific). ................
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