Complex Ion Equilibria Formation Constant Kf

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The equilibrium constant for the formation of a complex ion is called a formation constant (or stability constant) and given the symbol Kf. A typical example is 

The large K value means that the forward reaction goes virtually to completion. Addition of ammonia to a solution of AgN03 will convert nearly all the Ag+ ions to Ag(NH3)2 (see Example 15.1). 

Substitute the values for [NH3] and Kf into the expression to get the desired ratio. 

Focus on the reactant, Ag(NH3)2 , in the desired equation. It contributes NH3 as a product. Find an equation that liberates NH3 from Ag(NH3)2 . That equation is the reverse of the equation that forms Ag(NH3)2.  

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