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    So, there's a question that reads:
    Construct an equation using molecular formulae, for the elimination of water from pentan-2-ol.
    My answer is C_{5}H_{11}OH\rightarrow C_{5}H_{10}+H_{2}O
    The reason I write it like that is because the general frormula for an alcohol is C_{n}H_{2n+1}OH. In the mark scheme, however, it says reject C_{5}H_{11}OH and allow only C_{5}H_{12}O.
    Why?
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    You must know the difference between a molecular formula and a structural formula ( and empirical formula) :yy:

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    (Original post by StUdEnTIGCSE)
    You must know the difference between a molecular formula and a structural formula ( and empirical formula) :yy:

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    But C_{5}H_{11}OH isn't fully structural, neither fully molecular... I know the difference. It just seems like pedantry to me.
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    (Original post by halpme)
    So, there's a question that reads:
    Construct an equation using molecular formulae, for the elimination of water from pentan-2-ol.
    My answer is C_{5}H_{11}OH\rightarrow C_{5}H_{10}+H_{2}O
    The reason I write it like that is because the general frormula for an alcohol is C_{n}H_{2n+1}OH. In the mark scheme, however, it says reject C_{5}H_{11}OH and allow only C_{5}H_{12}O.
    Why?
    Because they don't test anything challenging at A-level they have to put marks up for crap like this
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    (Original post by halpme)
    But C_{5}H_{11}OH isn't fully structural, neither fully molecular... I know the difference. It just seems like pedantry to me.
    its not molecular at all, isn't it?


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    (Original post by JMaydom)
    Because they don't test anything challenging at A-level they have to put marks up for crap like this
    Woah.... bit harsh....
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    (Original post by halpme)
    But C_{5}H_{11}OH isn't fully structural, neither fully molecular... I know the difference. It just seems like pedantry to me.
    It is pedantry. That's a ridiculous thing to deduct a mark for, I would've thought structural formulae would actually have been preferred. I guess you should just look at past papers and see when your exam board prefers you to use a particular formula and remember for your exam.

    EDIT: The only issue I can see they took with it is the fact that because it's only partially structural the formula isn't correct. Because it's pentan-2-ol you can't just stick the OH group on the end. But that's still very harsh to not give you BOD for that imo.
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    (Original post by halpme)
    So, there's a question that reads:
    Construct an equation using molecular formulae, for the elimination of water from pentan-2-ol.
    My answer is C_{5}H_{11}OH\rightarrow C_{5}H_{10}+H_{2}O
    The reason I write it like that is because the general frormula for an alcohol is C_{n}H_{2n+1}OH. In the mark scheme, however, it says reject C_{5}H_{11}OH and allow only C_{5}H_{12}O.
    Why?
    I fell into a similar trap once or twice at A level.

    Molecular formula means you state the exact amounts of each element in order, no matter what functional groups are present.

    Yes, the most convenient way for stating an alcohol is altering the molecular formula denotation to denote the OH, but that is not the official molecular formula.

    C5H11OH isn't structural formula either; the structural formula is CH3CH(OH)CH2CH2CH3
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    (Original post by yepyepyep)
    It is pedantry. That's a ridiculous thing to deduct a mark for, I would've thought structural formulae would actually have been preferred. I guess you should just look at past papers and see when your exam board prefers you to use a particular formula and remember for your exam.
    Agreed :yy:. Chemistry is literally all about concentration, they have clearly asked for "molecular formula"

    (Original post by DurhamChemist)

    C5H11OH isn't structural formula either; the structural formula is CH3CH(OH)CH2CH2CH3
    It is structural but not fully structural, I reckon?

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    (Original post by StUdEnTIGCSE)
    Agreed :yy:. Chemistry is literally all about concentration, they have clearly asked for "molecular formula"



    It is structural but not fully structural, I reckon?

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    Ah I didn't think of that. I made the same mistake as OP and didn't even read the part that said molecular.:laugh: This is why I won't get an A* in chemistry. Yeah they do tend to specify what they want slyly in the question. The only thing that sprung to mind for me was that the OH group is on the second carbon so their formula was technically incorrect.
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    (Original post by StUdEnTIGCSE)
    Agreed :yy:. Chemistry is literally all about concentration, they have clearly asked for "molecular formula"



    It is structural but not fully structural, I reckon?

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    It simply indicates that an OH group is present, so yes in a sense it indicates a part of the molecule's structure.

    However, C5H11OH does not indicate the posistion of the OH group, neither does it indicate the arrangement of the C5H11.

    For most of A level chemistry, I could get away with denoting alcohols like OP did, but there was one specific question (I actually have it right next to me, F322 May 2012 (I paid for a photocopy to be sent to me)) that was about alcohols and required molecular formula. I made the mistake OP did.
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    (Original post by DurhamChemist)
    It simply indicates that an OH group is present, so yes in a sense it indicates a part of the molecule's structure.

    However, C5H11OH does not indicate the posistion of the OH group, neither does it indicate the arrangement of the C5H11.

    For most of A level chemistry, I could get away with denoting alcohols like OP did, but there was one specific question (I actually have it right next to me, F322 May 2012 (I paid for a photocopy to be sent to me)) that was about alcohols and required molecular formula. I made the mistake OP did.
    I'm doing F322 January 2013 and it's the same as May 2012! Question 3(a)(ii)... Pedantry
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    The way you wrote it implies that you have pentan-1-ol, which is incorrect.
 
 
 
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