Laboratory Activity: Teacher Notes

Continued

    1. 1M NaOH: Dissolve 4 g in distilled H2O and dilute to 100 mL.
    2. 0.5 M H2SO4 to 100 mL - carefully!
    3. 0.01 M NaCl: 0.06 g NaCl per 100 mL solution.
  1. Assume that 1.0 mL AgNO3 solution is approximately equal to 0.825 mg NaCl
    (0.0141 mol Ag+ is approximately equal to 0.0141 mol NaCl) from equation.

    (0.0141 mol NaCl) x (58.5 g NaCl)/(molNaCl) = (0.825 g NaCl)/(L) = (0.825 mg/mL)

    Pre-Laboratory Discussion

    1. Discuss the dangers of too much salt in our diet. [Hypertension, kidney disease, heart disease]
    2. Show Fritos® package, read list of ingredients and all nutritional information on label. Make overhead projection of label. Point out the amount of "sodium" on the label. Discuss with students: Is this metallic, elemental sodium or sodium in the form of a compound? [Na ions] How can we determine the actual amount of NaCl present? [By dissolving the salt in H2O and analyzing for one of the ions.]
    3. Explain that these reactions are the basis of Mohr chloride analysis. It depents on the solubility of the two silver precipitates, AgCl and Ag2CrO4. If Ag+ ion is added to a solution containing both Cl- and CrO42- ions, the AgCl will be precipitated first, because it is "less soluble in water" than the Ag2CrO4. The solubility concentrations are AgCl = 1.3 x 10-5 M, and Ag2CrO4 = 1.3 x 10-4 M. When all the chloride ion has been precipitated, the Ag2CrO4 starts to precipitate and the color changes from a creamy yellow to an orange almost like orange juice. This end-point is somewhat subjective, and students should keep all titration flasks for comparison until they finish at least three trials.
    4. Show the reactions below. AgCl(s) is white; Ag2CrO4(s) is red.

      Use a beaker or flask to illustrate the above reactions. Take 10 mL 0.01 M NaCl solution; add 20 drops of K2CrO4 solution; note color. Then add the AgNO3 solution, dropwise, with swirling. Point out the color changes to the orange color that signals the end-point of the titration.

    5. Discuss the reason for using a blank. A blank is used to correct for any chloride ion that may be present in the water being used. It normally has a value of about 0.2-0.3 mL, and must be subtracted from the amount of AgNO3 solution used in the analysis of the sample.
    6. Make sure that students understand the proper use of burets, how to fill them, and read the volume. Stress that they must rinse the burets 4-5 times with distilled water when the laboratory is over.
    7. Review the proper use of transfer pipets and demonstrate prior to student use.
    8. The following "Pictures in the Mind" can be used to develop an understanding of the phenomena occurring during titration.

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