Question

The formulas in the "Solution/PH" chart are different from the initial formulas which are confusing me. Can you help solve this?

Answer 1

Acid is a substance which donates protons.

In water equilibrium of hypochlorous acid is

HClO + H₂O ⇌ H₃O⁺ + ClO^-

Dissociation constant =Ka = [ H₃O⁺][ClO^-] / [HClO ] = 3.0 x 10^-8

For HCN

HCN + H₂O ⇌ H₃O⁺ + CN^-

Ka = [ H₃O⁺][CN^-] / [HCN ] = 4.9 x 10^-10

High Ka value means High  [ H₃O⁺] and hence low pH.

So pH of HClO < HCN

Similarly dissociation of a base can be represented as,base is a proton acceptor.

NH₃ + H₂O ⇌NH₄⁺ + OH^-

Dissociation constant =Kb = [ NH₄⁺][ OH^-] / [NH₃ ] = 1.8 x 10^-5

CH₃NH₂ + H₂O ⇌ CH₃NH₃⁺ + OH^-

Kb = [ CH₃NH₃⁺][OH^-] / [CH₃NH₂ ] = 4.4 x 10^-4

High Kb value means High  [OH^-] and hence low pOH (or high pH).

{We have pH = 14 - pOH

Hence low pOH means high pH.}

So pH of CH₃NH₂ > NH₃

So increasing order of pH of

HClO < HCN < NH₃ < CH₃NH₂

Low pH High pH