Laboratory Activity: Teacher Notes

Activity 1: Analysis of Sodium Chloride in Snack Food

Major Chemical Concept

Students will use the Mohr Chloride Titration method (see Pre-Laboratory Discussion Items 3 and 4) to determine the amount of sodium chloride in a snack food such as Fritos®

Level

This activity is appropriate for all levels of chemistry classes.

Expected Student Background

Skills: Using a buret, pipet, and unit analysis calculations. Concepts: Solubility, averaging values of several trials in an analysis, using a "blank" in chemical analysis.

Time

40-50 min, if snack filtrate is prepared ahead of time.

Safety

Read the Safety Considerations in the Student Version.

Materials (For 24 students working in pairs)

Nonconsumables

• 48 Erlenmeyer flasks, 125-mL
• 12 Burets, 50-mL
• 12 Pipets, 10-mL
• 12 Ringstands and buret clamps

Consumables (see Advance Preparation)

• 0.0141 M Silver nitrate, AgNO₃, 600 mL
• 12 dropping bottles 5% Potassium chromate, K₂CrO₄
• Snack filtrate, 480 mL

Advance Preparation

1. Preparing the snack filtrate: Weigh out 20.0 g of crushed Fritos® soak 24 hr in 600 mL distilled water; filter through thick cheese cloth, rinse and bring total volume to 600 mL in a graduated cylinder. The solution will be cloudy. Record exact volume. Use pH paper to test pH of filtrate. If pH is below 7.0 or above 10.0, adjust with 1 M NaOH or 0.5 M H₂SO₄, respectively. The filtrate should be stored in a refrigerator. To prevent mold, add a few crystals of thymol or a few drops of Lysol disinfectant. WORD OF WARNING: Pretzels are suitable, but potato chips and cheese-flavored snacks are difficult to filter.
2. Solution preparation:
   1. AgNO₃ solution, 0.0141 M: Dissolve 2.4 g AgNO₃ (dried overnight at 140 degrees Celsius) in chloride-free water and dilute to 1.00 L. Store in a brown bottle.
   2. K₂CrO₄ indicator solution: Dissolve 10 g K₂CrO₄ in a little distilled water. Add AgNO₃ solution until a definite red precipitate is formed. Allow to stand 18-24 hr, filter, and dilute filtrate to 200 mL with distilled water. Put into dropper bottles.

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