Is it possible for a weak acid, say acetic acid `(K_(a)=1.8xx10^(-5))` to have a pH = 7 by adding any other chemical and if so how much amount of it should be added ?

Explanation

If acid is completely neturalised by a base (say NaOH), the salt fromed will have `pH lt 7` . The only possibility to have `pH=7` for `CH_(3)COOH` can be obtained by addition of `CH_(3)COO^(-)` (a conjugate base say `CH_(3)COONa`) in it.
`CH_(3)COOH hArr CH_(3)COO^(-)+H^(+)`
`K_(a)=([CH_(3)COO^(-)][H^(+)])/([CH_(3)COOH])`
`:. ([CH_(3)COO^(-)])/([CH_(3)COOH])= (1.8xx10^(-5))/(10-7)=1.8xx10^(2)`
Thus addition of 180 times concentratgikon of `CH_(3)COONa` to `CH_(3)COOH` will give a solution of `pH=7`