Lab Experiment on Chloride Determination

Experiment 5: Determination of an Unknown Chloride

Overview of Experiment

  • Objective: To determine the concentration of chloride ions in an unknown salt solution using titration.

  • Key Components:

    • Titrant: Dilute solution of silver nitrate (AgNO3) with a known concentration.

    • Analyte: Salt solution containing an unknown amount of chloride ions (Cl-).

    • Precipitate: Silver chloride (AgCl), white insoluble solid produced during the reaction.

    • Indicator: Potassium chromate (K2CrO4), which is yellow and turns to a bright orange precipitate (silver chromate, Ag2CrO4) at the endpoint of titration.

Chemical Reactions

  • Immediate Precipitation Reaction:
    ext{Ag}^+(aq) + ext{Cl}^-(aq) ⇌ ext{AgCl}(s)

  • Indicator Reaction at Higher Concentration:
    2 ext{Ag}^+(aq) + ext{CrO}4^{2-}(aq) ⇌ ext{Ag}2 ext{CrO}_4(s)

Standardization of Solutions

  • Importance of Standardization: All standard solutions must be standardized due to potential evaporation.

  • Primary Standard:

    • Must be a stable solid that does not absorb water.

    • Primary Standard Used: Purified sodium chloride (NaCl).

Endpoint Determination

  • Indicator Color Change: The endpoint of the titration is indicated by the first permanent orange-red color of Ag2CrO4, signaling that all chloride has precipitated out.

  • Determining AgNO3 Concentration: This endpoint provides a means to calculate the concentration of AgNO3 used during the titration and subsequently allows for the calculation of percentage chloride in the salt sample.

Safety Note

  • Toxicity of Silver: Silver is a heavy metal toxin; all silver waste (silver nitrate and silver chloride) should not be flushed down the drain and must be disposed of in designated waste bottles.

Procedure

Part A: Determination of Percent Chloride in NaCl-KCl

  1. Sample Preparation:

    • Obtain an unknown chloride salt and record the ID number in your lab notebook.

    • Use an analytical balance to weigh 0.1 g samples of the chloride salt.

    • Place the sample into a 250 mL Erlenmeyer flask.

  2. Reagent Addition:

    • Add 50 mL of distilled water to the flask.

    • Add 3 drops of K2CrO4 indicator solution.

  3. Burette Preparation:

    • Fill the burette with a standardized 0.05 M silver nitrate solution.

  4. Titration Process:

    • Titrate the sample by adding silver nitrate until the first permanent orange-red color appears, indicating the endpoint.

Part B: Repeat Procedure for Second ID

  1. Repeat Steps 1-4 for the second unknown chloride salt ID.

Calculations

Percent Chloride Calculation

  • General Step: For both parts A and B, calculate the percentage of chloride in the sample using the molarity of AgNO3 determined in Part A.

  • Molar Mass Values:

    • Molar mass of NaCl = 58.4 g/mol

    • Molar mass of KCl = 74.5 g/mol

Post-Lab Questions

  1. Impact of Errors on % Chloride Calculation:

    • (a) If pure sodium chloride absorbed moisture, the calculated % chloride would decrease because excess water would increase the mass of the sample without adding more chloride.

    • (b) If the molarity of silver nitrate was actually 10% higher (0.055 M instead of 0.05 M), the calculated % chloride would increase since more chloride would be detected due to a higher amount of AgNO3 used.

    • (c) If 2 mL of silver nitrate was added beyond the chromate endpoint, this would also lead to an increase in the % chloride calculation as excess AgNO3 would falsely indicate more chloride precipitated than actually was the case.

  2. Calculation of Molarity of AgNO3:

    • Given: 35.46 mL of silver nitrate solution used to reach the endpoint with 0.12 g of pure NaCl.

    • Conversion to Molarity:

      • ext{Molarity} = rac{ ext{moles of solute}}{ ext{liters of solution}}

      • Moles of NaCl = rac{0.12 ext{ g}}{58.4 ext{ g/mol}} = 0.00205 moles

      • Volume in liters = rac{35.46 ext{ mL}}{1000} = 0.03546 ext{ L}

      • Molarity of AgNO3 = rac{0.00205 ext{ moles}}{0.03546 ext{ L}} = 0.0578 ext{ M}

  3. Calculation of Molar Mass of Xchloride:

    • Given: 0.1 g sample required 18.25 mL of 0.05 M AgNO3 to reach the chromate endpoint.

    • Moles of AgNO3 used = 0.05 * rac{18.25}{1000} = 0.0009125 ext{ moles}

    • Moles of Xchloride = Moles of AgNO3 (1:1 ratio) = 0.0009125 moles.

    • Molar Mass of Xchloride = rac{0.1 ext{ g}}{0.0009125 ext{ moles}} = 109.57 ext{ g/mol}

    • Percentage of Chloride in Sample = rac{35.46 ext{ g/mol (Cl)}}{109.57 ext{ g/mol}} * 100 = 32.36 ext{ \,\%}

  4. Reason for Using K2CrO4:

    • K2CrO4 serves as an effective visual indicator due to its yellow color and its ability to react with silver ions to produce an orange precipitate (Ag2CrO4) at the endpoint of the titration, allowing for precise determination of Cl- concentration.