Fractional Distillation and Gas Chromatography
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Questions and Answers

What conditions allow for a mixture to be separated into individual components using fractional distillation?

  • The components must be miscible in water.
  • The components must have the same melting point.
  • The components must have different boiling points. (correct)
  • The components must have equal vapor pressures.

    • What is the significance of the total vapor pressure (Ptot) in the process of distillation?

  • It must remain constant during the distillation process.
  • It needs to be greater than the external pressure for boiling to occur. (correct)
  • It should be equal to the sum of the vapor pressures of all components.
  • It marks the threshold for the vaporization of all components.

    • In calculating the percentage composition of a vapor, what does the formula % A = (χA PA / Ptot) x 100% represent?

  • The ratio of the liquid component A to total liquid volume.
  • The mass of component A in the mixture.
  • The entire pressure of the vapor mixture.
  • The fraction of component A in the vapor phase. (correct)

    • What happens to the composition of the liquid after a simple distillation, given that A is more volatile than B?

    <p>It becomes enriched in the lower boiling component.</p> Signup and view all the answers

    If the vapor pressure of compound A at boiling point is 1000 torr and compound B is 520 torr, what is the total vapor pressure when their mole fractions are both 0.5?

    <p>760 torr</p> Signup and view all the answers

    After performing a second simple distillation, how does the percentage composition of A change if it's assumed that PA is 900 torr?

    <p>It remains at 77.9%.</p> Signup and view all the answers

    What is the likely outcome if sequential simple distillations are performed continuously on a mixture of A and B?

    <p>You will eventually achieve a pure A.</p> Signup and view all the answers

    What characteristic of compounds A and B ensures effective separation during fractional distillation?

    <p>They must have significantly different boiling points.</p> Signup and view all the answers

    What is the role of the packing material in fractional distillation?

    <p>It provides a surface area for vapor to condense.</p> Signup and view all the answers

    How is the height equivalent of theoretical plates (HETP) calculated?

    <p>HETP = height of packing material (cm) / # of theoretical plates</p> Signup and view all the answers

    Which equation is used to calculate the total number of plates in the fractional distillation process?

    <p>Total # of plates = log(nC / nT) / log(2.33)</p> Signup and view all the answers

    What is the relationship between nC and nT as determined by gas chromatography (GC)?

    <p>nC / nT = 1.2(PC / PT)</p> Signup and view all the answers

    How is the number of plates in the packing material determined?

    <h1>plates in packing material = total # of plates - 1</h1> Signup and view all the answers

    What is indicated by the presence of multiple theoretical plates in a fractional distillation column?

    <p>Higher purity of the collected distillate.</p> Signup and view all the answers

    Why should open flames be avoided around volatile organics during the procedure?

    <p>They pose a risk of fire or explosion.</p> Signup and view all the answers

    What is the purpose of collecting the first few drops during distillation?

    <p>To obtain the fraction richest in the lower boiling component.</p> Signup and view all the answers

    What is the main purpose of running all samples on gas chromatography (GC)?

    <p>To determine the peak areas related to molar ratios.</p> Signup and view all the answers

    What should be done if the amount of liquid in the Hickman Head fills during distillation?

    <p>Empty the Hickman Head and continue collecting distillate.</p> Signup and view all the answers

    Study Notes

    Fractional Distillation

    • Separates liquids based on their boiling points
    • Uses packing material in a column to increase surface area for condensation and vaporization
    • More “simple distillations” occur, resulting in a more purified vapor
    • Efficiency measured by HETP (Height Equivalent of Theoretical Plates)
    • HETP calculated by dividing height of packing material by the number of theoretical plates

    Gas Chromatography (GC)

    • Separates gases based on their volatility
    • Peak area on GC is proportional to the amount of material reaching the detector
    • Can be used to determine the molar ratio of components in a mixture
    • In this experiment, GC is used to determine the molar ratio of cyclohexane and toluene

    Experimental Procedure

    • Measure 4 mL of a 50/50 mixture of cyclohexane and toluene
    • Add a stir bar to the mixture
    • Assemble the distillation apparatus (round bottom flask, Vigreux column, Hickman head, water condenser, and thermometer)
    • Heat the mixture in a sand bath to achieve a slow and steady boil
    • Collect the first few drops of distillate (HETP fraction) and analyze by GC
    • Continue distilling and collect 2 mL of distillate (fraction 1) and analyze by GC
    • Analyze the remaining liquid in the round bottom flask (fraction 2) by GC
    • Record the temperature range over which the distillate is collected

    Calculating # of Theoretical Plates

    • Use the molar ratio of cyclohexane (nC) to toluene (nT) determined by GC
    • Calculate the total number of plates using the formula: Total # of plates = [log(nC /nT)/log2.33]
    • Subtract 1 from the total number of plates to account for the glassware outside the column: # plates in packing material = total # of plates – 1
    • Calculate the HETP using the formula: HETP = height of packing material (cm) / # of theoretical plates in packing material

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    Description

    This quiz covers the principles of fractional distillation and gas chromatography, including their methods and efficiencies. You'll understand how to separate liquids and gases based on physical properties and analyze mixtures. Test your knowledge on the experimental procedures involved in these techniques.

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