The `K_a` value for HCN is `10^(-9)`. What is the pH of decimolar HCN solution ?

Explanation

Dissociation constant, `K_a = 10^(-9)`, Concentration, `C=0.1 = 1 xx 10^(-3) mol L^(-1)`
`[H^+]= sqrt(K_aC) = sqrt(10^(-9) xx 10^(-1) = 10^(-5)) mol L^(-1)`
pH of the solution `pH=- log 10^(-5) = 5`