Calculations on equilibria weak acids

This is an equation in three unknowns, provided the magnitude of is known. 

The general procedure to be followed in all calculations requires consideration of the following points. 

A decision will have to be made as to whether it is valid to approximate this expression to 

This reduces the expression to one in two unknowns. To solve this equation it is necessary  

Approximation 1 relating to the extent of ionisation of the weak acid 

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