begin mathsize 36px style p H space equals space 7 space plus thin space 1 half p K subscript a space plus 1 half log C end style

Ka =  Acid Dissociation Constant ; C = Concentration of the Acidic Soluiton ;

pH = -log[H+] ; pKa = -logKa





The above equation is valid for weak Bronsted base. For aqueous weak base the pH is aproximated and it bears the above relation with Concentration (C) and pKa.  The pKa value to be used is of the conjugate acid of the weak base under consideration, i.e for an aqueous solution of Ammonia (NH3 + H2O) the pKa of NH4+, is to be taken, which is the conjugate base of NH3. It must be noted that this equation is valid at 25oC. 

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