Q.Write the open-chain (Fischer projection) structure of D-glucose and identify all the functional groups present in it.

Numbering the six carbons from the aldehyde end, D-glucose's open-chain (Fischer projection) structure is: C1 $-\text{CHO}$ (aldehyde), C2 $-\text{OH}$ on the right, C3 $-\text{OH}$ on the left, C4 $-\text{OH}$ on the right, C5 $-\text{OH}$ on the right, and C6 the terminal $-\text{CH}_2\text{OH}$. Written in condensed form: $\text{CHO}{-}(\text{CHOH})_4{-}\text{CH}_2\text{OH}$.

Counting functional groups: there is exactly one aldehyde group ($-\text{CHO}$, at C1) and five hydroxyl groups ($-\text{OH}$, at C2, C3, C4, C5 and the primary $-\text{OH}$ of C6), for a total molecular formula of $\text{C}_6\text{H}_{12}\text{O}_6$.

Number the chain from the carbonyl-bearing carbon as C1, then read off each carbon's substituent in turn to identify and count every functional group present.

Miscounting the hydroxyl groups by forgetting the primary $-\text{OH}$ on the terminal C6 carbon, or mistakenly placing the aldehyde group at C6 instead of C1.