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A hexose (molar mass 180.16 g/mol) is treated with excess acetic anhydride…

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A hexose (molar mass $180.16\ \text{g/mol}$ ) is treated with excess acetic anhydride. A sample of $1.80\ \text{g}$ of the sugar gives $3.90\ \text{g}$ of the acetylated product, with every reactive $\text{-}\text{OH}$ group changed to $\text{-}\text{OCOCH}_{3}$ . Each change replaces an H atom by a $\text{COCH}_{3}$ group and adds $42.04\ \text{g/mol}$ . Find how many $\text{-}\text{OH}$ groups the sugar has.

Need a reminder? The key ideaShow

A carbohydrate is a polyhydroxy aldehyde or ketone. An aldose has $n - 2$ stereocentres and a ketose has $n - 3$ , where $n$ is the number of carbons in the chain. The number of stereoisomers is $2^{\text{centres}}$ , and half of them belong to the D series. An empirical formula comes from the moles of each element in 100 g.

The worked solution

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