`CH_3COOH+NaOH rarr CH_3COONa+H_2O`

Let us say we have 1L of `CH_3COOH ` solution.

According to the stoichiometry;

`CH_3COOH:NaOH = 1:1`

`CH_3COOH:CH_3COONa = 1:1`

Since we are adding enough NaOH full reaction will be occurred. We can assume that there is no partial reactions.

CH_3COOH moles present before tritration `= 0.018`

NaOH moles required for titration `= 0.018`

Required NaOH volume from 0.036M `= 1/0.036*0.018 = 0.5`

`CH_3COOH:CH_3COONa = 1:1`

Final CH_3COONa mole in the solution `= 0.018`

Final volume of the solution` = 1+0.5 = 1.5L`

`CH_3COONa rarr CH_3COO^-+Na^+`

`[CH_3COO^-] = 0.018/1.5 = 0.012M`

So the concentration of `CH_3COO^-` ions is 0.012M.