Dissociation constants of two acids `HA` and `HB` are respectively `4xx10^(-10)` and `1.8xx10^(-5)`, whose `pH` value will be higher for a given molarity:

Options

A. `HA`
B. `HB`
C. Both same
D. Can't say

Answer

A

Explanation

`HAhArrH^(+)+A^(-)`
`HBhArrH^(+)+B^(-)`
For `HA`
`K_(a_(1))=alpha^(2)C`
`a_(1)^(2)=sqrt(k_(a_(1))/(C))=sqrt(4xx10^(-10))/(C)`
`alpha_(1)=2xx10^(-5)` for `C= 1M`
Similarly `implies[H^(+)]=C alpha=2xx10^(-5)impliespH=4.7`
For `HB,alpha_(2)^(2)=sqrt(k_(a_(2))/(C))=sqrt(1.8xx10^(-5))/(C)` for `C= 1 M`
`a_(2)=4.2xx10^(-3)implies [H^(+)]=Ca`
`4.2xx10^(-3)`
`pH=-log(4.2xx10^(-3))=2.37`