# Find the number of water molecules “embedded” in sucrose

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?".

• 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." – Ivan Neretin Feb 24 '16 at 17:32
• 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. – Curt F. Feb 24 '16 at 17:36
• @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. – Murtuza Vadharia Feb 25 '16 at 3:42
• It really is a misleading 'trick' question. I just does not help to understand the structure of sucrose, in fact quite the opposite. – porphyrin May 5 at 7:23
• It's not really a trick question. It requires stoichiometric thinking (all they had in the old days) instead of structural thinking, though. – Curt F. May 5 at 15:35