Calculations for the salt of a weak acid and a weak base
One Example

The hydrolysis of salts in water

Intro to hydrolysis calculations

Hydrolysis of the salt of a weak acid

Hydrolysis of the salt of a weak base

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Is an aqueous solution of NH4CN acidic or basic?

Discussion:

An aqueous solution of NH4CN contains NH4+ ions and CN¯ ions (ignoring water and the other stuff). By itself in solution, NH4+ ions are acidic (conjugate acid of NH3, a weak base ). By itself in solution, CN¯ ions are basic (conjugate base of HCN, a weak acid). To actually calculate the pH of this solution is somewhat complicated. We can, however, ascertain if the solution is overall acidic or neutral by comparing the Ka of NH4+ and the Kb of CN¯ .

Solution:

NH4+ + H2O ⇌ H3O+ + NH3

The Kb for NH3 is 1.77 x 10¯5

  Kw   1.00 x 10¯14  
Ka = –––  =  ––––––––––  = 5.65 x 10¯10 (the Ka for the ammonium ion)
  Kb   1.77 x 10¯5  

CN¯ + H2O ⇌ HCN + OH¯

The Ka for HCN is 6.17 x 10¯10

  Kw   1.00 x 10¯14  
Kb = –––  =  ––––––––––  = 1.62 x 10¯5 (the Kb for the cyanide ion)
  Ka   6.17 x 10¯10  

This is the key comparison:

1.62 x 10¯5 > 5.65 x 10¯10

The Kb of CN¯ is greater than the Ka of NH4+

In other words, CN¯ is a stronger base than NH4+ is an acid.

The solution will be basic, with the pH being greater than 7.


The hydrolysis of salts in water

Intro to hydrolysis calculations

Hydrolysis of the salt of a weak acid

Hydrolysis of the salt of a weak base

Return to the Acid Base menu