The degree of dissociation of weak electrolyde is inversely proportional to the square root fo concentration. It is called Ostwald's dilution law.
`alpha = sqrt((K_(a))/(c))` As the tempertaure increases, degree of dissociation will increase.
`(alpha_(1))/(alpha_(2)) = sqrt((K_(a_(1)))/(K_(a_(2))))` if concentration is same.
`(alpha_(1))/(alpha_(2)) = sqrt((c_(2))/(c_(1)))` if acid is same.
`pH` of `0.005 M HCOOH [K_(a) = 2 xx 10^(-4)]` is equal to

Options

A. `3`
B. `2`
C. `4`
D. `5`

Answer

A

Explanation

`pH = 0.005M HCOOH : K_(a) = 2 xx 10^(-4)`
Since `C` is low and `K` is high. Solve the quadratic equaiton:
`K_(a) = (Calpha^(2))/(1-alpha)`
`rArr 2 xx 10^(-4) = ((0.005)alpha^(2))/(1-alpha) rArr alpha = 0.18`
`rArr pH =- log C alpha =- log (0.005xx0.18) ~~3`