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Another biochemical question

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Another biochemical question

Postby thewax » Wed Dec 24, 2008 5:41 am

This question is also excerpted from Deborah Goldberg's AP Biology:

"4. Which of the following is NOT a base?
(a) NaOH
(b) KOH
(c) Mg(OH)2 [2 is the subscript to OH, but I can't type subscripts here]
(d) C2H5OH
(e) BaOH"


Many lines, again, are skipped so that the reader can formulate his or her answer...

This again, is not a homework problem and is merely a question that I am using to study for a big test coming up soon.




















Solution: "(d) This is ethyl alcohol and is not a base. The other compounds contain hydroxide (OH-) [Note: in the OH-, the minus is a superscript] and are all bases."

The progress that I made: Literally none. Even after reading the solution many times, I still have no clue. I mean, if you somehow memorized that C2H5OH was ethyl alcohol then you would probably get this question right. But I don't think that the author of this book wanted the reader to memorize a whole bunch of molecular formulas (or are they structural formulas?). So I thought that maybe there was some sort of way of just looking at the molecular formula and somehow deducing that it was not a base. I looked through the chapter, but I could not find it. So I was wondering if someone could tell me how to, just by looking at the molecular formula, somehow deduce that it is, or not, a base. Or in other words, if one did not memorize that C2H5OH was not a base, then how would one answer the question (correctly)????????? My other question is this: The solution, at least to me, makes it sound so that if it's an alcohol, then it's not a base. Is that proposition correct? I did do some research, but the definitions of alcohol (at least the ones that I have looked at) don't seem to mention anything about it being (or not) a base.
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Postby blcr11 » Wed Dec 24, 2008 2:52 pm

All the other options were "metal" hydroxides. These compounds are salts which yield hydroxyl ions when they dissolve in water. Ethanol is a modestly polar organic molecule. It is miscible with water (remember the dictum, like dissolves like?), but unlike metal hydroxides, ethanol does not dissociate when it "dissolves" so it does not generate any base.
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Re: Another biochemical question

Postby thewax » Wed Dec 24, 2008 7:28 pm

Thanks blcr11 for the wonderful insight !!!!!!!! :)

I just have a couple of questions:
1) What's a metal hydroxide?
2) How do you tell, just by looking at the molecular formula, that something is a metal hydroxide?
3) Does the fact that a molecule is a metal hydroxide mean automatically that it is a base?
4) A minor detail - In your post, isn't hydroxyl ion a typo - shouldn't it be hydroxide ion (because hydroxyl group is a functional group)??????
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Postby blcr11 » Thu Dec 25, 2008 11:32 am

1. Group I/II elements are the alkali/alkaline earth metals. They most readily form salts. Sodium + chlorine forms sodium chloride; sodium + water forms sodium hydroxide and hydrogen gas. Sodium hydroxide is a metal hydroxide. Any of the Group I/II elements will form a similar hydroxide when placed in water.

2. If the element in combination with the hydroxide is a Group I or II element (first two columns on the left of the periodic table) it's a pretty good bet it's a metal hydroxide. Metallics tend to be found to the left on the periodic table, non-metallics to the right. Things in the middle aren't often easy to classify as either metals or non-metals--but they don't form soluble hydroxide salts anyway.

3. If the metal involved is a Group I or Group II element, yes. They don't call them alkali or alkaline earth metals for nothing.

4. Maybe. I don't know if it's proper to call an isolated hydroxide ion a hydroxyl ion or not. Certainly it still retains its functionality. If it bothers you, then just stick with hydroxide.
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Postby thewax » Thu Dec 25, 2008 11:30 pm

Thanks!!!!!!!! :)

4. What I was really trying to say was that in my book, they too used the word hydroxyl ion instead of hydroxide ion and that confused me. When I saw that someone on this forum used it, I thought perhaps I might inquire to learn what was the correct writing (as I did go through a textbook and it warned the readers not to get hydroxyl mixed up with hydroxide).
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Postby MrMistery » Fri Dec 26, 2008 9:41 pm

You should definitely not use the terms hydroxyl and hydroxide interchangeable. While hydroxide is an ion (it carries an electric charge), hydroxyl is the neutral form of the ion, that is neutral, has an unpaired electron and is thus a free radical. You should especially not confuse the two if you are a bio person, because the hydroxyl radical is a very dangerous compound to have in cells, whereas hydroxide is pretty much safe at low concentrations.

http://en.wikipedia.org/wiki/Hydroxyl
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Postby MrMistery » Fri Dec 26, 2008 9:42 pm

PS: name your post properly please..
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Postby thewax » Sat Dec 27, 2008 5:17 pm

Thanks everyone!!!!!!!!!!!! :)
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Postby mayoorraj » Mon Dec 29, 2008 6:36 am

well here i want to comment on fourth ques: what i belive that,as far as organic chem is concerned -OH is called hydroxyl group but inorganic chem where mostly it forms ionic bond u can say hydroxide ion
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Postby mayoorraj » Mon Dec 29, 2008 6:39 am

i agree MrMIstery
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Postby thewax » Mon Dec 29, 2008 6:44 am

so what do YOU think the post should be named?
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Postby MrMistery » Mon Dec 29, 2008 8:38 pm

you could name it "Which one is not a base?" or something like that
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