Chem 2002: Midterm Review

Midterm Exam Review Overview

• The session is a midterm exam review covering various lab experiments.

• Recorded on Zoom; interruptions expected due to a student finishing their exam.

Experiment 1: Volumetric Glassware and Statistical Tests

• Objective: Measurement of water volumes (10 mL, 20 mL) and their mass.

• Tools Used:

  • Beakers: Not calibrated; used for holding reactions, intermediate steps, or avoiding reactions over heat.

  • Volumetric Glassware: Calibrated for specific temperature (20°C); cannot be heated beyond calibration marks. Includes:

    • Burette: Ideal for titration experiments due to precise liquid dispensing control.

    • Volumetric Pipettes: Dispense specific volumes (1 mL, 2 mL, 5 mL, etc.) without variability.

    • More or Surgical Pipettes: Variable volumes, handy for different requirements.

    • Micropipettes: For small volumes, from 10 µL to 5,000 µL (less accurate over 1,000 µL).

    • Graduated Cylinders: Approximate volume measurements, less precise than volumetric glassware.

• Statistical Tests Applied: Case 2, Case 3, and F-tests; calibration plot referenced for further experiments.

Example Calculation

• Scenario: Comparing arsenic detection methods.

• Used pooled standard deviation and t-tests.

• Found the t-value of 2.785 through calculations and compared with t-table values for significance.

• Result indicated that the new method is not reliable for arsenic detection; original method preferred.

Experiment 2: Calcium Ion Determination in Antacid Tablets

• Method: Titration analysis for calcium ions.

• Key Steps:

  • Standardized sodium hydroxide due to concentration variations over time.

  • Used potassium hydrogen phthalate (KHP) as primary standard, which doesn’t degrade.

  • Calculated moles of leftover HCl to determine moles in antacid tablets.

  • Used indicators for endpoint determination during titration.

• Results: Percent mass of calcium in the tablets calculated based on titration results.

Experiment 3: Infrared Spectroscopy (FTIR)

• Parts:

  1. Identifying functional groups in pure molecules (pentanol, toluene, methyl ethyl ketone).

  2. Quantitative analysis using Beer's Law.

• Beer's Law: Absorbance = ε × b × c; maintaining constant path length for accurate concentration results.

• Example: Calculated absorbance of CH peak in hexane using tangent baseline method.

Experiment 4: Zinc Concentration in Vitamin Water

• Methods Used:

  • Calibration Plot Method: Standards with only zinc solutions.

  • Standard Addition Graphical Method: Samples contained vitamin water with varying zinc concentrations.

• Discussed matrix effects and importance of keeping conditions consistent for accurate results.

• Compared methods and highlighted that standard addition minimized matrix errors better than the calibration plot method.

Example for Zinc Concentration Calculation

• Analyzed absorbance data from both methods to determine zinc concentrations, accounting for dilutions.

• Dilution Equation: Used to track concentrations through calculated absorbance in solutions.

Experiment 5: Determination of Manganese in Steel Sample via Spectrophotometry

• Wavelength Selection: Identifying lambda max from absorbance spectra.

• Used standard addition two-point method for manganese concentration.

• Compared two solutions (unspiked and spiked with known concentrations).

• Result: Calculated manganese concentration in original sample.

• Weight Percent: Mass of manganese calculated from total sample mass.

Exam Preparation Tips

• Familiarize with lab techniques and calculations from each experiment.

• Bring necessary tools (calculator, pen, ruler) for all calculations during the exam.

• Review the recording of the session for details on difficult concepts.

• Be prepared to apply statistical tests discussed for evaluation of results.