A-level Chemistry/OCR (Salters)/Buffer solutions

< A-level Chemistry‎ | OCR (Salters)

Calculating the pH of a buffer solutionEdit

 

DerivationEdit

For any equilibrium

 

the equilibrium constant, K, is defined as

 


Therefore, for the dissociation equilibrium of any acid

 

the acid dissociation constant, Ka, is defined as

 

This equation can be rearranged to make [H+(aq)] the subject:

 


Two assumptions are required:

1 Every A ion comes from the salt

Although this is not quite true, it is a close enough that the pH value we get from the final equation is very close to that found experimentally. It allows us to assume that
 


2 Every HA molecule remains undissociated

Again, despite being slightly inaccurate, this assumption creates the following useful equation
 


The equations in assumptions 1 and 2 allow us to replace [A(aq)] with [salt] and [HA(aq)] with [acid] as follows.


The effect of assumption 1 is that

 

becomes

 


The effect of assumption 2 is that

 

becomes

 


By definition,

 

so