Q.(a) Write the resonance structures of CH3COO^- and show the movement of electrons by curved arrows. [2]

(a) Acetate ion's negative charge is delocalised equally over both oxygens via two resonance structures connected by curved-arrow electron movement, stabilising the ion. (b) From the combustion data, the compound is about 21.95% C and 4.58% H by mass.

(a) Resonance structures of CH3COO^- (acetate ion):

Acetate ion can be drawn in two equivalent Lewis structures:

Structure I: CH3-C(=O)-O^- (double bond to one oxygen, negative charge and single bond to the other oxygen)

Structure II: CH3-C(-O^-)=O (single bond and negative charge on what was the first oxygen, double bond to the other)

These two structures are interconverted by curved arrows showing electron movement: a curved arrow starts from the lone pair on the negatively-charged O^- and moves to form a new pi bond (C=O) with the adjacent carbon, while simultaneously another curved arrow starts from the existing C=O pi bond and moves the electrons onto the other oxygen, converting it to O^- with a lone pair. Because these two structures are of equal energy (equivalent by symmetry) and differ only in the position of the double bond and the negative charge, the actual acetate ion is best described as a resonance hybrid of the two: both C-O bonds are identical, with an intermediate bond length/bond order (about 1.5), and the negative charge is shared equally (roughly half a unit) on each oxygen atom, rather than being fixed on just one. This delocalisation stabilises the acetate ion (and correspondingly makes acetic acid's O-H bond easier to ionise, contributing to it being a reasonably good weak acid among carboxylic acids).

(b) Percentage composition (%C, %H) from combustion analysis (Liebig's method): …