Unknown substances retention times with their identities a) Discuss the retentio

Question

Unknown substances retention times with their identities

a) Discuss the retention times of the five substances with regard to their molecular weights. Describe any pattern that emerges.

b)Discuss the retention times of the five substances with regard to their boiling points. Describe any pattern that emerges.

c) Discuss the retention times of the five substances with regard to their polarity. Describe any pattern that emerges.

d)Identify the substances that are present in your unknown mixture. Support your identification.

Compound Retention Time (min) Methanol 1.371 Ethanol 1.495 Propyl acetate 4.225 Butyl acetate 6.340 2-butanone 2.345

Explanation / Answer

Retention time is defined as a measure of the time taken for a solute to pass through a column from injection to detection. Retention time has several factors which mainly includes polarity, boiling point, vapour pressure and column packing.

Basing on the type of the interaction between the solute and the mobile phase, there are different types of chromatography techniques. So, basing on the molecular weight, boiling point and polarity, the chromatogram also varies.

a) Regarding molecular weight :

Gel Permiation column (GPC) performs a separation based on molecular size of the compound.

Methanol : 32.04

Ethanol : 46.07

Propyl acetate : 102.13

Butyl acetate : 116.16

2-butanone : 72.11

In GPC, the separation is purely a physical partitioning and there is no interaction or binding. The lager molecular elutes first and smaller molecules elutes last.

2) Regarding boiling point :

Gas chromatography separates the component basing on polarity and boiling point. If a component has a low boiling point, then it spends more time in the gas phase. Therefore, its retention time will be lower than a compound with a higher boiling point.

Methanol : 64.7, Ethanol : 78.4, Propyl acetate : 102, Butyl acetate : 126, 2-butanone: 79.64

3) Regarding Polarity :

  If the polarity of the stationary phase and compound are similar, then the retention time increases because the compound interacts stronger with stationary phase. As a result, polar compounds have long retention times on polar stationary phases and shorter retention times on non-polar stationary phases.

As for the five compounds, the polarity order is

Butyl acetate<Propyl acetate<2-butanone<Ethanol<Methanol. So, the retention times are higher for medium polar compounds.

D) As ethanol and propyl acetate are both polar and medium polar compounds respectively. And the boiling point of ethanol is low when compared to propyl acetate. So, the retention time of ethanol will be of less than that of propyl acetate.

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