Recently I got a question as follows:

How many molecules of water are joined with 12 atoms of carbon in a molecule of sucrose ?

(A) 12

(B) 11

(C) 6

(D) 10

I am unable to understand the question. What actually is the question ?

  • Sucrose is a carbohydrate. This means that stoichiometrically, it can be regarded as adding water ($\ce{H2O}$) to elemental carbon $\ce{C}$.

  • The formula of sucrose is $\ce{C12H22O11}$.

  • Another way to write this formula is $\ce{C12(H2O)11}$.

  • Thus, sucrose can be regarded (stoichiometrically, but not structurally) as eleven waters plus twelve elemental carbons. Therefore the correct answer is (b).

  • The question is not well-worded, and would be better stated as "How many water molecules are required to join with the 12 carbon atoms in sucrose to give the correct empirical formula for sucrose?".

  • In the comments Ivan Neretin offers an even better variant, which I will re-word as "How many water molecules could extremely powerful dehydrating agents such as $\ce{H2SO4}$ or $\ce{SO3}$ extract from sucrose, if we assume the only non-water product is pure carbon?".

  • 2
    $\begingroup$ That would be not quite good either. Water molecules don't just up and join carbon atoms on their own. I don't know how to salvage this question. Maybe, "How many water molecules can be snatched from sucrose by $\ce{H2SO4}$, if the product is assumed to be pure carbon." $\endgroup$ Feb 24 '16 at 17:32
  • $\begingroup$ Thanks & good point. I added your idea to the answer. Perhaps a better word than "join" is "embed", which may be suitably ambiguous for this somewhat vague question. $\endgroup$
    – Curt F.
    Feb 24 '16 at 17:36
  • $\begingroup$ @Curt F I was quite thinking about how many water molecules can be joined with sucrose molecule with the help of hydrogen bond. But because there was no mention of Hydrogen bond so I didn't knew what actually it means. $\endgroup$ Feb 25 '16 at 3:42
  • $\begingroup$ It really is a misleading 'trick' question. I just does not help to understand the structure of sucrose, in fact quite the opposite. $\endgroup$
    – porphyrin
    May 5 '21 at 7:23
  • $\begingroup$ It's not really a trick question. It requires stoichiometric thinking (all they had in the old days) instead of structural thinking, though. $\endgroup$
    – Curt F.
    May 5 '21 at 15:35

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