ethanoate 既 formula...

In writing the formula of an ionic compound with a metal cation and an organic anion, write the organic anion first, and then the cation.

For example :
CH3COONa (sodium ethanoate)

C6H5OK (potassium phenoxide)

(HCOO)2Mg (magnesium methanoate)

CH3CH2OK (potassium ethoxide)

CH3CH2CH2CH2CH2CH2CH2COONa (sodium octanoate)


Reason :
An organic anion (e.g. CH3COO-) usually has a long formula, at least longer than that of a common inorganic anion (e.g. O2-).

Consider the sodium octanoate, the last example above.
When the formula is written as NaCH3CH2CH2CH2CH2CH2CH2COO, it is equivalent to Na+CH3CH2CH2CH2CH2CH2CH2COO-. It is not good to separate the +ve and -ve charges a long distance.

Therefore, it is written as CH3CH2CH2CH2CH2CH2CH2COONa,
which is equivalent to CH3CH2CH2CH2CH2CH2CH2COO-Na+.