A weak acid of dissociation constant `10^(-5)` is being titrated with aqueous NaOH solution . The pH at the point of one third of neutralization of the acid will be

Options

A. ` 5+ log 2 - log 3`
B. ` 5 - log2 `
C. `5 - log 3`
D. `5 - log 6`

Answer

B

Explanation

`K_(a)=10^(-5)rArr pK_(a)= - log K_(a) = - log 10^(-5) = 5`
`HA + NaOH to NaA +H_(2)O`
`{:("Initial",1 "mole 0",,0,0),("Final ",(1-1//3)"mole",,1//3"mole",1//3 "mole"),(,2//3 "mole",,1//3 "mole",):}`
(Assumed weak acid to be monoprotic, since only one dissociation constant value is provided) Final solution acts as an acidic buffer.
`rArr pH =pK_(a) +log. (["Salt"])/(["Acid"]) rArr pH = 5 +log . (1/3)/(2/3)= 5 + log. 1/2 `
`rArr pH = 5 - log 2 . `