Volumetric Analysis: Oxidation-Reduction
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Questions and Answers

What is the purpose of adding dilute sulfuric acid to the iron(II) solution in the conical flask?

  • To increase the volume of the solution
  • To neutralize the solution
  • To provide a reducing environment (correct)
  • To prevent the formation of iron oxides

    • During the titration, what indicates that the end-point has been reached?

  • The solution appears cloudy
  • The solution turns completely colorless
  • The first persisting pink color appears (correct)
  • A green color develops

    • What should be done to the burette before starting the titration?

  • Rinse it with potassium manganate(VII) solution (correct)
  • Leave it empty
  • Rinse it with deionized water
  • Fill it with distilled water

    • How is the concentration of the iron(II) solution calculated at the end of the titration process?

    <p>Using the molarity from titration and the stoichiometry of the reaction (D)</p> Signup and view all the answers

    What is the mole ratio between potassium manganate(VII) and iron(II) ions in the reaction?

    <p>1:5 (B)</p> Signup and view all the answers

    What is the total mass of five iron tablets mentioned in the procedure?

    <p>1.2 grams (C)</p> Signup and view all the answers

    Before beginning the titration, which step is crucial regarding the solutions in the pipette and burette?

    <p>The pipette should be rinsed with iron(II) solution and the burette with potassium manganate(VII) (D)</p> Signup and view all the answers

    What role does the white tile play during the titration?

    <p>It provides a contrast to observe the color change (B)</p> Signup and view all the answers

    What is the mass of iron in each tablet containing FeSO₄?

    <p>0.049 g (A)</p> Signup and view all the answers

    What is the percentage of FeSO₄ in each tablet?

    <p>55% (D)</p> Signup and view all the answers

    What is the first step in preparing the sodium thiosulfate solution for the titration?

    <p>Wash the pipette, burette, and conical flask with deionised water. (B)</p> Signup and view all the answers

    How many grams of FeSO₄ are in a 250 cm³ volumetric flask with a concentration of 2.63 g/L?

    <p>0.658 g (B)</p> Signup and view all the answers

    What is the molarity of the sodium thiosulfate solution given that 25 cm³ of a 0.045 M iodine solution is titrated to an average of 27.45 cm³?

    <p>0.065 M (B)</p> Signup and view all the answers

    What color change indicates the end-point of the titration with sodium thiosulfate?

    <p>Blue-black to colorless (D)</p> Signup and view all the answers

    What is the total mass of iron in 5 tablets of FeSO₄?

    <p>0.245 g (A)</p> Signup and view all the answers

    What procedure is indicated for dosing sodium thiosulfate during the titration?

    <p>Add sodium thiosulfate dropwise, swirling continuously before the end-point. (A)</p> Signup and view all the answers

    What is the molarity (M) of sodium thiosulfate after the titration calculation?

    <p>0.082 (C)</p> Signup and view all the answers

    How much sodium thiosulfate is required to reach the end-point of the titration?

    <p>27.45 cm^3 (C)</p> Signup and view all the answers

    What should be done after adding potassium iodide and dilute sulfuric acid to the conical flask?

    <p>Swirl the flask continuously. (D)</p> Signup and view all the answers

    What color change indicates the end-point of the titration?

    <p>Blue-black to colorless (C)</p> Signup and view all the answers

    What is the purpose of washing the pipette, burette, and conical flask with deionised water before use?

    <p>To remove any contaminants. (C)</p> Signup and view all the answers

    What was the reason for using excess potassium iodide during the procedure?

    <p>To keep the iodine in solution (D)</p> Signup and view all the answers

    The reaction equation shows that how many moles of thiosulfate are consumed for each mole of iodine?

    <p>2 moles (D)</p> Signup and view all the answers

    In the titration process, why is the conical flask placed on a white tile?

    <p>To make the colour change clearer (A)</p> Signup and view all the answers

    What is the first step in preparing the bleach solution for titration?

    <p>Dilute with deionised water. (D)</p> Signup and view all the answers

    What indicator is used in the titration to observe the color change?

    <p>Starch solution (A)</p> Signup and view all the answers

    What is the final volume of sodium thiosulfate used in the average titration?

    <p>16.1 cm^3 (D)</p> Signup and view all the answers

    What indicator is used in the titrations and what color change occurs at the endpoint?

    <p>Starch; blue to colorless (D)</p> Signup and view all the answers

    What is the concentration of NaClO in the original bleach solution expressed in moles per liter?

    <p>0.644 moles/L (A)</p> Signup and view all the answers

    What is the primary role of potassium permanganate in volumetric analysis?

    <p>It acts as an oxidising agent by gaining electrons. (D)</p> Signup and view all the answers

    Why must potassium permanganate be standardised before use?

    <p>It cannot be obtained in a state of high purity. (C)</p> Signup and view all the answers

    What ions are formed when Fe^2+ ions are oxidised by potassium permanganate?

    <p>Fe^3+ ions (B)</p> Signup and view all the answers

    How many Fe^2+ ions are needed to react with one MnO^4- ion?

    <p>Five Fe^2+ ions (A)</p> Signup and view all the answers

    What is the necessary action to prepare the iron(II) solution for titration?

    <p>Acidify the solution with sulfuric acid. (D)</p> Signup and view all the answers

    What property of ammonium iron (II) sulfate makes it suitable as a primary standard?

    <p>It can be obtained in a high degree of purity. (A)</p> Signup and view all the answers

    Which step is essential to take before filling the burette with potassium manganate(VII) solution?

    <p>Rinse the burette with the iron(II) solution. (C)</p> Signup and view all the answers

    From which part of the meniscus should readings be taken when measuring potassium manganate(VII) solution?

    <p>From the top of the meniscus (B)</p> Signup and view all the answers

    What is the concentration of sodium hypochlorite in the diluted bleach solution in moles per litre?

    <p>0.077 (A)</p> Signup and view all the answers

    How many grams of sodium hypochlorite are present in one litre of the diluted bleach solution?

    <p>5.737 (C)</p> Signup and view all the answers

    What is the percent weight/volume concentration (w/v) of sodium hypochlorite in the diluted bleach?

    <p>0.574% (A)</p> Signup and view all the answers

    If the original bleach solution was diluted 10 times, what is the concentration of sodium hypochlorite in the original bleaching solution in moles per litre?

    <p>0.77 (D)</p> Signup and view all the answers

    What is the molecular mass of sodium hypochlorite (NaClO)?

    <p>74.5 g/mol (A)</p> Signup and view all the answers

    During the titration, which compound was used as the titrant with a concentration of 0.12 M?

    <p>Sodium thiosulfate (D)</p> Signup and view all the answers

    What is the amount of grams of sodium hypochlorite present per 100 cm^3 in the original bleach?

    <p>5.74 (C)</p> Signup and view all the answers

    Which ion reacts with sodium hypochlorite in the titration process?

    <p>I^− (D)</p> Signup and view all the answers

    Flashcards

    Potassium permanganate

    A purple oxidizing agent, used for volumetric analysis.

    MnO4⁻

    The chemical formula for the permanganate ion.

    Primary Standard

    A substance used for precisely measuring concentration.

    Acidic solution

    Solution with excess H+ ions necessary for reaction.

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    Oxidizing Agent

    A substance that gains electrons in a reaction.

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    Reducing Agent

    A substance that loses electrons in a reaction.

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    Ammonium iron(II) sulfate

    A source of Fe²⁺ ion used for reaction and standardization.

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    Volumetric Analysis

    Technique to measure concentrations of solutions by titration.

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    Titration

    A technique used to determine the concentration of an unknown solution by reacting it with a solution of known concentration.

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    Endpoint

    The point in a titration where the reaction is complete, indicated by a change in color, often a persistent color change.

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    Molarity

    A concentration unit that expresses the number of moles of solute per liter of solution.

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    Titration Equation

    A balanced chemical equation representing the reaction occurring during a titration.

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    What is the purpose of the sulfuric acid in the titration?

    Sulfuric acid is added to the iron(II) solution to create an acidic environment, which is necessary for the reaction between potassium permanganate and iron(II) ions to proceed.

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    Why is the burette rinsed with potassium permanganate solution?

    To ensure that any water or previous solution residues are removed, preventing contamination and ensuring accurate measurements of potassium permanganate.

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    How is the endpoint of the titration determined?

    The endpoint is determined by the appearance of a persistent pink color in the solution, indicating that all of the iron(II) ions have reacted with the potassium permanganate.

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    What is the importance of repeating the titration?

    Repeating the titration ensures the accuracy of the results by reducing random errors and obtaining concordant values. This helps to verify the concentration of the unknown solution.

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    Standardization

    The process of determining the precise concentration of a solution using a primary standard.

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    Titrant

    The solution of known concentration used in a titration to determine the concentration of another solution.

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    Analyte

    The substance whose concentration is being determined in a titration.

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    Indicator

    A substance that changes color to signal the end-point of a titration.

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    Titration Procedure

    A stepwise process involving adding a titrant to an analyte until a color change indicates the reaction is complete.

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    Standard Solution

    A solution of precisely known concentration, used in titrations to determine the concentration of an unknown solution.

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    End Point Of Titration

    The point in a titration where the reaction is complete and the indicator changes color.

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    Iodine Titration

    A type of titration used to determine the concentration of a substance like sodium hypochlorite in bleach, by reacting it with a known concentration of thiosulfate.

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    Sodium Thiosulfate

    A chemical used in iodine titrations to react with iodine and determine the concentration of the unknown solution.

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    Liberate

    To release or set free, like by chemical reaction, in this case; to set free iodine from bleach.

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    Dilute

    To make a solution less concentrated by adding more solvent, typically water.

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    What is titration?

    A technique to determine the concentration of a solution by reacting it with a solution of known concentration (standard solution) until the reaction is complete.

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    What is the indicator used in the bleach titration?

    Sodium thiosulfate (Na2S2O3) is used as the titrant because it reacts with iodine (I2) in a known stoichiometric ratio, creating a colorless solution.

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    Why is the bleach diluted?

    The bleach solution is diluted to 10 times its original concentration. This ensures that the titration uses a manageable volume of the bleach and a reasonable amount of titrant (sodium thiosulfate).

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    What is the stoichiometric ratio of bleach (NaClO) to sodium thiosulfate (Na2S2O3)?

    The balanced equation shows that one mole of NaClO reacts with two moles of Na2S2O3. This is the stoichiometric ratio used in titration calculations.

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    How is the concentration of the bleach calculated?

    The concentration of the bleach is determined by using the volume of sodium thiosulfate used in the titration, the concentration of sodium thiosulfate, and the stoichiometric ratio between the two reactants.

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    What is the concentration of the bleach in moles per liter?

    The concentration of the original bleach is calculated to be 0.77 moles per liter of NaClO, meaning it is a relatively concentrated solution.

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    How is the concentration of bleach expressed in grams per liter?

    The concentration of the bleach can also be expressed in grams per liter by multiplying the concentration in moles per liter by the molar mass of NaCIO, which is 74.5 g/mol.

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    What is the concentration of bleach in % w/v?

    The concentration of bleach is 5.74% w/v. This means that 5.74 g of NaClO is present in 100 ml of the original bleach solution.

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    Excess Potassium Iodide

    Potassium iodide is used in excess for two main purposes:

    1. To keep iodine in solution: Iodine is not very soluble, but potassium iodide helps dissolve it and keep it stable for the reaction.
    2. To ensure complete bleach reaction: Excess potassium iodide reacts with any remaining bleach, ensuring all of it is used up for a reliable titration.
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    Conical Flask on White Tile

    The conical flask is placed on a white tile to make the color change during the titration more visible. This helps the observer to accurately determine the endpoint of the titration.

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    Titration Indicator

    Starch is the indicator used in this titration. It changes color from blue to colorless at the endpoint, indicating that all iodine has reacted.

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    NaClO Concentration in Diluted Bleach

    The concentration of NaClO (sodium hypochlorite) in the diluted bleach is calculated by using the titration data and the stoichiometry of the reaction. The final concentration is found to be 0.0322 moles per litre.

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    NaClO Concentration in Original Bleach

    The concentration of NaClO in the original bleach is found by multiplying the concentration in the diluted bleach by the dilution factor (20 in this case). The original bleach had a concentration of 0.644 moles per litre.

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    Study Notes

    Volumetric Analysis: Oxidation-Reduction

    • Volumetric analysis is used to determine the concentrations of oxidizing and reducing agents. Potassium permanganate (KMnO4) is a crucial oxidizing agent.
    • Potassium permanganate is a purple solid, not a primary standard (cannot be obtained in high purity).
    • It needs standardization by titration against a primary standard solution.
    • In acidic solution, KMnO4 gains five electrons and acts as an oxidizing agent: MnO4- + 8H+ + 5e- → Mn2+ + 4H2O (purple → colorless).
    • To react, dilute acid must be added to provide H+ ions.
    • KMnO4 is its own indicator.

    The Reaction of MnO4- and Fe2+

    • The concentration of KMnO4 solution can be determined by titrating it against a solution containing Fe2+ ions.
    • KMnO4 acts as an oxidizing agent, converting Fe2+ to Fe3+.
    • The reaction equation is: MnO4- + 8H+ + 5Fe2+ → Mn2+ + 5Fe3+ + 4H2O.
    • Five Fe2+ ions react with one MnO4- ion.
    • Ammonium iron(II) sulfate (source of Fe2+) is a primary standard, pure, and not affected by air.

    Mandatory Experiment: Standardization of KMnO4

    • Steps to prepare a standard solution of ammonium iron(II) sulfate and standardize a KMnO4 solution by titration:
      1. Wash glassware with deionized water. Rinse the burette and pipette with their respective solutions.
      2. Use a pipette to measure the iron(II) solution and transfer it to a conical flask. Add dilute sulfuric acid to acidify the solution.
      3. Fill the burette with KMnO4 solution, ensuring the part below the tap is filled to adjust to zero.
      4. Titrate the KMnO4 solution against the iron(II) solution, swirling the flask continuously and washing down the sides with deionized water.
      5. Watch for the first persistent pale pink color (endpoint), note the burette reading.
      6. Repeat the titration procedure until two titres agree within 0.1 cm3.

    Solving Volumetric Problems in Redox Reactions

    • Equations for calculating concentration:
      • Mox Vox nox = Mred Vred nred
      • Mo = (nr * Mre *Vre) / (no * Vo)

    Mandatory Experiment: Determining Iron Content in Tablets

    • Steps to determine the amount of iron in an iron tablet:
      1. Weigh five iron tablets.
      2. Crush the tablets and dissolve the ground material in dilute sulfuric acid.
      3. Transfer to a 250 cm3 volumetric flask, make up to the mark with deionized water and invert.
      4. Wash glassware with deionized water and rinse the burette and pipette with the appropriate solutions.
      5. Use a pipette to measure the iron(II) solution and transfer it to a conical flask. Acidify.
      6. Fill the burette with KMnO4 solution.
      7. Add the KMnO4 solution to the iron(II) solution in the conical (endpoint is pink colour).
      8. Record the final burette reading. Repeat twice.
      9. Calculate the concentration of the iron(II) solution.
      10. Calculate the mass of iron in each tablet.

    Mandatory Experiment: Sodium Thiosulfate Standardization

    • Steps to prepare a solution of sodium thiosulfate and standardize it by titration against a solution of iodine:
      1. Wash glassware. Rinse burette and pipette with the appropriate solutions.
      2. Fill a pipette with potassium iodate (KI) solution and transfer it to a conical flask. Add sulfuric acid and potassium iodide solution.
      3. Fill a burette with sodium thiosulfate (Na2S2O3) solution.
      4. Add Na2S2O3 solution to KI solution, swirling constantly and washing down the sides, until the colour changes (from blue/black to colourless).
      5. Record the burette reading. Repeat until two titres are within 0.1cm³.
      6. Calculate the concentration of Na2S2O3 solution.

    Mandatory Experiment: Determining Percentage of Sodium Hypochlorite in Household Bleach

    • Steps to determine the percentage (w/v) of sodium hypochlorite in household bleach:
      1. Dilute the bleach to a suitable concentration for titration.
      2. Wash glassware. Rinse burette and pipette with the appropriate solutions.
      3. Using a pipette, measure a portion of diluted bleach solution and place in a conical flask. Add potassium iodide and sulphuric acid.
      4. Fill a burette with Na2S2O3 solution
      5. Add Na2S2O3 solution to the bleach solution, swirling constantly, and wash down sides. Add starch indicator before end point.
      6. Record the burette reading. Repeat until two titres agree to within 0.1cm³.
      7. Calculate the concentration of sodium hypochlorite in household bleach.

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    Description

    This quiz focuses on volumetric analysis in oxidation-reduction reactions, specifically involving potassium permanganate (KMnO4) and its reactions with Fe2+. It explores the standardization of KMnO4 and its role as an oxidizing agent in acidic solutions. Test your understanding of these concepts and reactions!

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