Preview of “Lab 9 PostLab - Titrations”

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Unformatted text preview: 11/6/12 Lab 9 PostLab -‐‑ Titrations WebAssign Lab 9 PostLab -­ Titrations (Postlab) Current Score : 21 / 25 Steve Yarush CH102, section 021, Fall 2012 Instructor: Brandon Zoellner Due : Thursday, November 1 2012 11:00 PM EDT The due date for this assignment is past. Your work can be viewed below, but no changes can be made. Important! Before you view the answer key, decide whether or not you plan to request an extension. Your Instructor may not grant you an extension if you have viewed the answer key. Automatic extensions are not granted if you have viewed the answer key. Request Extension www.webassign.net/web/Student/Assignment-‐‑Responses/view_key?dep=4449848 1/6 11/6/12 Lab 9 PostLab -‐‑ Titrations 1. 5/6 points | Previous Answers NCSUGenChem102LabV1 9.POST.01. Consider a different titrant for this exercise. Suppose Ca(OH)2 were used as the titrant, instead of NaOH. This will make the titrant twice as concentrated in hydroxide ion. The analyte will still be HC2H3O2. (a) What is the stoichiometry of HC2H3O2 to Ca(OH)2? 8:3 5:2 2:1 1:1 1:2 2:5 3:8 (b) Complete the following table for this titration. Data Table P1: Titration of acetic acid with calcium hydroxide concentration of Ca(OH)2 0.340 M volume vinegar solution 12.20 mL mass vinegar solution 12.26 g volume of Ca(OH)2 solution 12.88 mL mmol of Ca(OH)2 4.38 4.38 mmol mmol of HC2H3O2 8.76 8.76 mmol mass of HC2H3O2 .527 0.526 g mass % of HC2H3O2 in the original sample 4.09 4.29 % molarity of HC2H3O2 in the original sample .718 0.718 M Titrations www.webassign.net/web/Student/Assignment-‐‑Responses/view_key?dep=4449848 2/6 11/6/12 Lab 9 PostLab -‐‑ Titrations 2. 5/6 points | Previous Answers NCSUGenChem102LabV1 9.POST.02. Consider a different analyte for this exercise. Citric acid is found in many fruits and fruit juices. Sodium hydroxide (NaOH) is the titrant and citric acid (H3C6H5O7) the analyte according to the following balanced chemical equation. H3C6H5O7 + 3 OH – → C6H5O73– + 3 H2O (a) What is the stoichiometry of H3C6H5O7 to OH –? 8:3 5:2 3:1 1:1 1:3 2:5 3:8 (b) Complete the following table for this titration. Data Table P2: Titration of citric acid in orange juice with sodium hydroxide. concentration of OH – 0.0745 M volume orange juice 8.53 mL mass orange juice 8.67 g volume of OH – solution 24.32 mL mmol of OH – 1.81 1.81 mmol mmol of H3C6H5O7 .604 0.604 mmol mass of H3C6H5O7 .116 0.116 g mass % of H3C6H5O7 in orange juice 1.34 1.34 % molarity of H3C6H5O7 in orange juice .071 0.0708 M Titrations www.webassign.net/web/Student/Assignment-‐‑Responses/view_key?dep=4449848 3/6 11/6/12 Lab 9 PostLab -‐‑ Titrations 3. 5/6 points | Previous Answers NCSUGenChem102LabV1 9.POST.03. Consider a different titration for this exercise. Potassium permanganate (KMnO4) is the titrant and hydrogen peroxide (H2O2) the analyte according to the following balanced chemical equation. 2 MnO4– + 5 H2O2 + 6 H + → 2 Mn2+ + 5 O2 + 8 H2O (a) What is the stoichiometry of MnO4– to H2O2? 8:3 5:2 2:1 1:1 1:2 2:5 3:8 (b) Complete the following table for this titration. Data Table P3: Titration of hydrogen peroxide with potassium permanganate. concentration of MnO – 0.551 M 4 volume H2O2 solution 18.22 mL mass H2O2 solution 18.30 g volume of MnO – solution 16.14 mL 4 mmol of MnO – 4 8.89 8.89 mmol mmol of H2O2 22.2 22.2 mmol mass of H2O2 .755 0.756 g mass % of H2O2 in the original sample 4.13 4.13 % molarity of H2O2 in the original sample 1.38 1.22 M www.webassign.net/web/Student/Assignment-‐‑Responses/view_key?dep=4449848 4/6 11/6/12 Lab 9 PostLab -‐‑ Titrations Titrations 4. 6/7 points | Previous Answers NCSUGenChem102LabV1 9.POST.04. Green checks and red X's are not displayed for this question. Indicators are weak acids or bases that have different colors in their acid and base forms. Chemists make use of these noticeable color changes by using them in acid-­base titration experiments. The equivalence point of a titration is where the acid and the base have reacted in stoichiometric proportions. The end point of a titration is where the indicator changes color. Selecting an indicator that changes color at a point as close as possible to the equivalence point is crucial for accurate quantitative analysis using titration. (a) Consider the titration curve below and the provided table of indicators. Select the appropriate indicator(s) for the titration. indicator Bromophenol color change good for this acidic pH titration? form basic form 3.0 -­ 4.6 Yes Yes yellow Methyl Orange 3.1 -­ 4.4 Yes Yes red Alizarin Red S 4.6 -­ 6.0 Yes No yellow red Thymol Blue 8.0 -­ 9.6 No No yellow blue Phenolphthalein 8.2 -­ 9.8 No No colorless pink 10.2 -­ 12.0 No No yellow red Blue Alizarin Yellow R www.webassign.net/web/Student/Assignment-‐‑Responses/view_key?dep=4449848 purple orange-­ yellow 5/6 11/6/12 Lab 9 PostLab -‐‑ Titrations (b) Thymol Blue might be selected as an indicator for the titration of a weak acid with a strong base. Can you estimate the pH at the equivalence point? Yes. The equivalence point should be somewhere between a pH of 8.0 and 9.6. No. We would need to know the volumes of both the analyte and the titrant before we could do that. Yes. The equivalence point should be somewhere between a pH of 3.1 and 4.4. Yes. The equivalence point should be somewhere between a pH of 7.0 and 8.0. No. We would need to know the concentration of the titrant before we could do that. Yes. The equivalence point should be somewhere between a pH of 9.6 and 10.6. Titrations www.webassign.net/web/Student/Assignment-‐‑Responses/view_key?dep=4449848 6/6 ...
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