The weak acid where both approximations can be made

2 The weak acid where both approximations can be made 

This argument will hold for acids with piifa lying in the range 4-10, provided the solution is not too dilute. 

3 The weak acid where there is extensive ionisation and approximation 1 is invalid 

This will happen with moderately strong weak acids of less than about 4, or for weaker acids in dilute solution. 

Where there is fairly extensive ionisation, the contribution to the actual concentration of HaO (aq) from the ionisation of HA(aq) 3 than that from the self ionisation of H20(l). The weak acid is then effectively the only source of both A (aq) and H30+(aq) and the self ionisation of water can be ignored  

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