The pH of 0.01 M hydrocyanic acid (HCH) is 5.2 .What is the dissociation constant for the acid ?

Answer

`3.96 xx 10^(-10)`

Explanation

pH of the solution =5.2 , or `-log [H^(+)] =5.2`
` log [(1)/(H^(+))] =5.2 " or " [(1)/(H^(+))] = " Antilog " 5.2 = 158489.3`
`[H^(+)] =(1)/(158489.3) =6.3 xx 10^(-6)M`
According to Ostwald's dilution law,
`[H^(+)] =sqrt(K_(a) xx C) " or " (6.3 xx 10^(-6)) =(K_(a) xx C)^(1//2)`
`" or "" "K_(a) xx C =(6.3 xx 10^(-6)) ^(2) " or " K_(a) =((6.3 xx 10^(-6))/(0.1)) =3.96 xx 10^(-10)`