Determine the K, Equilibrium Constant

Question

Determine the K(Equilibrium constant) for the salt NH₄CN. 

Answer 1

Before writing an equilibrium constant, you first need to determine whether the salt would make an acidic or basic solution. To do that assume that the salt dissolves making NH4+ and CN- ions. Then study each ion carefully: NH4+ is the conjugate acid of NH3 while CN- is the conjugate base of HCN and write down the equilibrium reactions for each ion.

 

NH4+ <=> H+ + NH3

 

CN-   <=> HCN + OH-

 

Now you need to determine which of the above equilibrium reactions dominates. You should know that NH3 is a weak base and HCN is a weak acid so NH4+ is a strong acid and CN- is a strong base. Since you cannot tell qualitatively whether the the salt is acidic or basic you need to compare the Ka and Kb values of each ion respectively. To do that you need to use the following relation.

 

Ka*Kb = Kw    where Kw (the dissociation constant of water) = 1.0*10^-14.

 

Based on the Ka and Kb values of each ion, there are three cases that will help you determine the acidity or basicity of NH4CN

 

if Ka > Kb   then the salt is acidic

if Kb > Ka   then the salt is basic

if Ka = Kb   then the salt is neutral

 

From this you can determine which of the above equilibrium reactions will dominate.