pH of salt composed of the conjugate of strong acid and base

salt like [HA]B dissloves in water (here we don't consider its solubility) :  

$$\ce{HA <=>[$K\_\mathrm a$] A- + H+}$$


and


$$\ce{B- + H2O <=>[$K\_\mathrm b$] HB + OH-}$$



> 
> usually we combine two and get 
>  $$\ce{HA + B- <=> A- + HB}$$
> 
> 
> (Ex: $\ce{NH4CN}$)
> 
> 
> and suggest this to be the dominant reaction so as 
>  to calculate pH
> 
> 
> **that is , the amount of HA and $\ce{B-}$ being consumed are assumed to be the same**
> 
> 
> let x be the amount being consumed and solve $\frac{x^2}{(C-x)^2}=\frac{K\_aK\_b}{K\_w}$ where C is the initial concentration of the salt
> 
> 
> 


My question is: **under what circumstance the suggestion their amount (being consumed) are the same would be invalid ?**


( Maybe both of $K\_\mathrm a$ and $K\_\mathrm b$ is really closed to $K\_\mathrm w$ and three of their dissociation must be carefully inspected to get the right answer ? )


**and if this is the case, how to solve it ?**




For strong acid and base, the reaction from H+ and A- to HA simply 'happens negligibly' because of the common ion phenomenon. So, the conjugate base of acid (or vice versa) is considered neutral.
(that's why we don't taste any sour or bitterness from NaCl)


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