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JoVE Lab Manual
Lab: Chemistry

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Beer's Law

Beer's Law

Learning Objectives

At the end of this lab, students should know...

What is chemical equilibrium?

Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal and the concentrations of reactants and products do not change. At chemical equilibrium, the concentration of each component is related to each other through the equilibrium constant, Keq.

How do you express the equilibrium constant for a chemical reaction?

The equilibrium constant of a chemical reaction is the ratio of product concentrations to reactant concentrations, each raised to the power of their stoichiometric coefficients.

How can you determine equilibrium concentration?

One method to determine equilibrium concentration is to use a spectrophotometer to measure absorbance at a specific wavelength of light. The absorbance corresponds to the concentration of the absorbing compound in the sample. By using a standard curve of absorbances of samples with known concentrations, a linear trend is observed that corresponds to the Beer-Lambert Law. If the absorbance of the unknown sample is known, the equilibrium concentration can be calculated.

What is the Beer-Lambert Law?

The Beer-Lambert Law states that the concentration of a sample is linearly proportional to its absorbance of light. Absorbance equals the molar attenuation coefficient multiplied by the pathlength and concentration of the sample.

What is the molar attenuation coefficient

The molar attenuation coefficient is a measure of how strongly a chemical species absorbs light at a given wavelength. A higher molar attenuation coefficient means that the species absorbs more energy. This constant is specific to a molecule or compound.

List of Materials

  • 1-mL volumetric pipette
  • 5-mL volumetric pipette
  • 10-mL volumetric pipette
  • Pipette controller
  • 3.5-mL cuvette with cap
  • 50-mL volumetric flask
  • 50-mL glass burette
  • 50-mL glass beaker
  • 150-mL glass beaker
  • 100-mL glass beaker
  • 400-mL glass beaker
  • Glass stirring rod
  • Glass funnel
  • Watch glass
  • 10-mL glass graduated cylinder
  • 50-mL glass graduated cylinder
  • Ring stand
  • Burette clamp
  • Baking soda
    5 box
  • pH paper
    5 packages
  • Laboratory tape
    5 roll
  • Laboratory wipes
    5 box
  • Data acquisition device
  • Spectrophotometer attachment
  • Flash drive
  • 15.7 M HNO3
    40 mL
  • FeCl3·6H2O
    40 g
  • NaSCN
    15 mg
  • Powder funnel
  • 250-mL glass graduated cylinder
  • 250-mL brown glass bottle and acid-resistant cap
  • 125-mL brown glass bottle and acid-resistant cap
  • 1-L brown glass bottle and acid-resistant cap
  • Magnetic wand
  • Stir bar
  • Mortar and pestle
  • 250-mL glass Erlenmeyer flask
  • 1-L glass volumetric flask
  • Stirring hotplate
  • Weighing paper and boats
  • Small spatula
  • Large spatula
  • Analytical balance
  • Deionized water
    Dependent on lab size
  • Disposable plastic pipettes
    Dependent on lab size
  • Aluminum foil (minimum one box)
    Dependent on lab size
  • Plastic paraffin film (minimum one box)
    Dependent on lab size
  • Scissors (minimum one)
    Dependent on lab size

Lab Prep

Source: Smaa Koraym at Johns Hopkins University, MD, USA

  1. Preparation of Solutions

    Here, we show the laboratory preparation for 10 students working in pairs, with some excess. Please adjust quantities as needed.

    • To set up for this lab experiment, wear the appropriate personal protective equipment, including a lab coat, chemical splash goggles, and gloves. Note: You will be working with nitric acid, which is toxic and corrosive, so use caution when handling it. Let's start by preparing 1 L of 0.5 M nitric acid.
    • Measure out 32 mL of 15.7 M nitric acid, and carefully pour it into a 1-L volumetric flask via a funnel.
    • Add deionized water to the flask up to the mark. Place a stir bar in the flask and stir the solution. Label a 1-L brown glass bottle as '0.5 M nitric acid'.
    • Once the solution appears homogeneous, remove the stir bar and place a funnel in the mouth of the labeled bottle. Carefully pour the nitric acid solution into the bottle. Set the capped bottle of 0.5 M nitric acid at the back of the instructor's hood and store the concentrated nitric acid in a corrosives cabinet.
    • Prepare 125 mL of a 1 M iron(III) chloride solution in 0.5 M nitric acid. Measure out 125 mL of the just prepared 0.5 M nitric acid and pour it into a 250-mL Erlenmeyer flask using a clean funnel.
    • Bring a container of iron(III) chloride hexahydrate and a mortar and pestle to a top-loading balance. Measure at least 35 g and place it in the mortar.
    • Grind the clumps of iron(III) chloride hexahydrate to a fine consistency using the pestle. Then, measure 33.79 g of the powder and bring it to the fume hood. Transfer the powder to the flask of nitric acid with a powder funnel.
    • Place a magnetic stir bar in the flask and set it on a stirring hotplate. Stir the solution while heating at around 80 °C until it appears homogeneous. As the solution stirs, label a 125-mL brown glass bottle as '1.0 M iron(III) chloride in 0.5 M nitric acid'.
    • Once the iron(III) chloride solution appears homogeneous, remove the flask from the hotplate and retrieve the stir bar.
    • Allow the solution to cool to room temperature, then transfer it to the labeled bottle via the funnel. Cap the bottle and store the solution in a corrosives cabinet.
    • Lastly, prepare 250 mL of a 0.5 mM solution of sodium thiocyanate in 0.5 M nitric acid. Measure 250 mL of 0.5 M nitric acid and pour it into a 250-mL Erlenmeyer flask via a clean funnel.
    • Use an analytical balance to measure out 10.1 mg of sodium thiocyanate. Bring it back to the hood and pour it into the 250-mL flask of nitric acid. If necessary, use a small amount of nitric acid to wash any remaining sodium thiocyanate from the weighing boat.
    • Add a stir bar to the flask and stir the mixture at room temperature until it appears homogeneous. While the mixture stirs, label a 250-mL brown glass bottle as '5.00 x 10-4 M sodium thiocyanate in 0.5 M nitric acid'.
    • Once the solution is thoroughly mixed, remove the stir bar and transfer the solution to the labeled bottle. Cap the bottle, then store both the sodium thiocyanate solution and the nitric acid in a corrosives cabinet.
    • When you have finished making the solutions, neutralize any acid waste, and wash the glassware and equipment using your standard procedures.
  2. Preparation of the Laboratory
    • Place a waste container labeled for aqueous iron waste and a wash bottle filled with deionized water in a satellite waste hood.
    • Set aluminum foil, plastic paraffin film, and a pair of scissors in a central area. Ensure that each sink has a stock of paper towels.
    • Assemble the hand-held spectrophotometers and distribute them to the student workstations.
    • Set up a ring stand equipped with a burette clamp at each student hood.
    • Ensure that each workstation has the following glassware and lab equipment (we suggest that students work in pairs):
       1    Box of laboratory wipes
       1    Bag of baking soda
       1    Roll of pH paper
       1    Roll of laboratory paper
       1    Pen
       2    50-mL glass burettes
       1    1-mL volumetric pipette
       2    5-mL volumetric pipettes
       1    10-mL volumetric pipette
       1    Pipette bulb or controller
       1    50-mL volumetric flask
       3    150-mL beakers
       1    100-mL beaker
       5    50-mL beakers
       1    400-mL beaker
       1    50-mL graduated cylinder
       1    10-mL graduated cylinder
       3    Funnels
       1    Glass stirring rod
       1    3.5-mL cuvette with cap
    • Distribute boxes of disposable pipettes around the lab so that students have access to extras.
    • Just before the lab, place the 0.5 M nitric acid, the 0.5 mM sodium thiocyanate, and the 1 M iron(III) chloride solutions in a central hood.

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