Year 12 Chemistry question

[Anthonaut]

Well I had my final exam for Year 12 Chemistry today (SACE) and i reckon I did pretty well but there was one question that I thought there was a mistake in.

The question was on chromatography (yuck, i know) and it involved 4 or 5 Nucleic Acid bases and their individual retention times. The solvent was polar and the stationary phase was non-polar.

The question then asked to explain why one of the bases was the most polar. This base had the shortest retention time of all of the bases and I always thought that the most polar substance should have the longest retention time because of the polar solvent?

I know i should put it behind me but i cant help wondering!

Cheers.

[Hopper]

Well I had my final exam for Year 12 Chemistry today (SACE) and i reckon I did pretty well but there was one question that I thought there was a mistake in.

The question was on chromatography (yuck, i know) and it involved 4 or 5 Nucleic Acid bases and their individual retention times. The solvent was polar and the stationary phase was non-polar.

The question then asked to explain why one of the bases was the most polar. This base had the shortest retention time of all of the bases and I always thought that the most polar substance should have the longest retention time because of the polar solvent?

I know i should put it behind me but i cant help wondering!

Cheers.

You are used to questions where the stationary phase is polar. What happens is the more polar the molecule the less adsorbtion it has with the stationary phase. The more adsobrtion and unadsoption the molecule has the longer it will take, as the stationary phase is non-polar, the molecule with the most polarity will have the less adsoption and hence shorter retention time.

Think of it this way, like attracts like in chromatography, the non-polar will be more attracted to the non polar stationary phase so takes longer to pass through the system. The opposite of this is the least polar will be quickest through the system.

[e30 guy]

dont know if im of much help having only just started yr 12 chem but likes dissolve likes that is a polar dissolves other polar substances and non-polar dissolve non-polar. Thus if the stationary is non-polar the most polar will be retentioned the least assuming my blind stabb in the dark is correct :p . Thats all i got

[Hopper]

dont know if im of much help having only just started yr 12 chem but likes dissolve likes that is a polar dissolves other polar substances and non-polar dissolve non-polar. Thus if the stationary is non-polar the most polar will be retentioned the least assuming my blind stabb in the dark is correct :p . Thats all i got

You got it exactly right, good luck with yr 12 chem, it's not that hard.

[Anthonaut]

Hmm i kind of understand. But i thought that if the base is polar, then it will want to "stay" with the moving phase longer and hence have a longer retention time...

Ah well, clearly im wrong but i just cant get my head around it.

[Anthonaut]

Ah I just got it then. I was thinking of different types of chromatography. Maybe even a type of chromatography i invented in my head.

Lucky I put the opposite to what I thought as my answer :P

Cheers guys.

[e30 guy]

Sweet hope the rest of chem carries on from prelime as much cause i kicked arse in that

[Anthonaut]

Its not too bad, as long as you get a decent teacher.

Good luck mate.