Distillation

Dalton’s Law

P_total = P_A + P_B

Raoult’s Law

P_A = P_A° N_A

• P_total  =  total vapor pressure of the mixture

• P_A = vapor pressures  of pure compounds A

• N_A   = mole fractions of components A

Steps to finding mole fraction from a 50:50 mixture

ex: What is the mole fraction of a 20 ml 50:50 mixture of methyl acetate (MeOAc) and butyl acetate (BuOAc) ?

Solving for MeOAc…

1. Find the volume of component → methyl acetate in a 20 ml methyl-butyl mixture = 50% of 20 ml

   = 10 ml

2. Convert volume to mass of component

   →  volume x density = (10 ml)(0.934 g/cm3)

   = 9.34 g

3. Convert mass to moles of component

   → mass / molar mass = (9.34 g)/(74.08 g/mol) = 0.126 mol

4. Add moles of both components together to get Ntotal

5. Divide individual moles by Ntotal

   N_MeOAc = (0.126)/(0.202) = 0.624

Mole Fraction formula

mole fraction = Nmoles of one component / Ntotal moles of the mixture

Missing pressure value problem

ex: A 50:50 mixture of compound X and Z boils at 90 °C, if PX° at 90 °C is 576 torr what is PZ° at 90 °C?

Boiling point of a mixture problem

ex: What temperature (approximately) does a 75% A and 25% B solution boil?

Formula: P_total = P_A + P_B

Vapor composition at the boiling point of a mixture problem

ex: What is the composition of the vapor above the boiling solution in question above?

Formula → X_A = P_A°N_A / P_total

What liquid composition will provide a solution that will boil at 75 °C?

30% compound A, 70% compound B

Theoretical plates

•  the steps in a ladder in a distillation column; each step representing an evaporation-condensation cycle

  • (one theoretical plate = one vaporization-condensation cycle)

  • more plates = more separation efficiency

simple distillation

• only one vaporization-condensation cycle (less pure than fractional distillation)

• works best with big difference in boiling points between the liquids you want to separate.

fractional distillation

• multiple vaporization-condensation cycles within fractionating column

• more efficient for liquids with closer boiling points due to the fractionating column, which enhances separation.

  • generally more efficient separation of mixture components

What is HETP?

 Height of fractionating column Equivalent to one Theoretical Plate

what is reflux ratio?

ratio of the liquid returned to the distillation flask (reflux) to the liquid collected as distillate

what is gas chromatography and how does it work?

• technique used to separate and analyze volatile compounds in a mixture

  How does it work?

  • separates compounds based on their distribution between a mobile phase (gas) and a stationary phase (liquid on a solid support)

  • allows small scale separation of volatile compounds

  • ideal for determining composition

  • mobile phase is a gas

what is retention time?

• measures the time compounds take to elute from the column

retention times depend on what four factors?

1. boiling point of compound (can’t change)

2. column length (can’t change)

3. flow rate of carrier gas (easy to modify)

4. oven temperature (easy to modify)

How does a Thermal conductivity detector work?

• measuring changes in heat conductivity caused by the presence of different compounds eluting from the chromatography column.

  • When sample elutes, sample will conduct heat differently than pure He, so we can measure a change in the signal.

How do you calculate a wt/wt percentage using GC data?

wt/wt percentage → (area of individual peak) / (total area)

• ex: total area 1502.069, and peak 3 area is 63.179 → 63.179/1502.069 = 4.21 %

What are the steps to calculate a mole fraction, given a volume/volume ratio?

Formula → N = mole fraction / total moles

1. Start by finding mass by volume x density

2. Divide mass by molar mass of component

3. Find N total by adding both components mole fraction together

4. use formula!

Why should you not clean your glassware with water immediately before conducting your distillation?

• Because water can form azeotropic mixtures with many organic compounds.

What is an Azeotrope? (high boiling and low boiling)

• a mixture of liquids that boils at a constant temperature with constant composition

• azeotropes cannot be further separated by distillation, can’t reach max purity

  • ex: ethanol being expensive

High boiling azeotrope —> has a boiling point lower than that of any of its components

Low boiling azeotrope —> has a boiling point higher than that of any of its components

When performing a distillation, heating the solution too quickly will…

• cause uneven heating and bad component separation

When performing a distillation, opening a cold sample vial will…

• cause condensation and affect the purity of the sample

When performing a distillation, not capping a vial tightly will…

• cause the sample to evaporate and change composition

  
  

When performing a distillation, having the oven temperature too hot on the GC will…

• lead to decomposition of the components in the mixture leading to inaccurate results

When performing a distillation, having an air bubble in the syringe when injecting your GC sample will…

• bring an air pocket into the sample loop, which effects our accuracy for our injection volume, and our overall gc analysis

  
  

atmospheric pressure?

760 mmHg

What two factors determine the vapor composition above a liquid mixture at any given temperature?

1. Partial pressures (torr) of the individual components in the mixture

2. Their mole fractions