the weak acid to the concentration of its conjugate base present in the solution i.e., H O =K [acid] [base] a eq eq ....( . ) XII U8-Ionic XII U8-Ionic - - - - The weak acid is dissociated only to a small extent. Moreover, due to common ion effect, the dissociation is further suppressed and hence the equilibrium concentration of the acid is nearly equal to the initial concentration of the unionised acid. Similarly, the concentration of the conjugate base is nearly equal to the initial concentration of the added salt.
H O =K [acid] [salt] a ....( . ) Here [acid] and [salt] represent the initial concentration of the acid and salt, respectively used to prepare the buffer solution Taking logarithm on both sides of the equation log [H O ]=logK +log [acid] [salt] a ....( . ) reverse the sign on both sides -log [H O ]=-logK -log [acid] [salt] a ....( . ) We know that pH = log [H O ] and pK = logK pH = pK log [acid] [salt a a a ⇒ ] ....( .
) ⇒ pH = pK log [salt] [acid] a ....( . ) Similarly for a basic buffer, pOH = pK +log [salt] [base] b ....( . ) Example . .
Find the pH of a buffer solution containing . mole per litre sodium acetate and . mole per litre acetic acid. K a for acetic acid is .
- pH = pK +log [salt] [acid] a Given that K a = . - ∴ = − pK a log( . log . = - .
= . - .. = . XII U8-Ionic XII U8-Ionic - - - - Example .
What is the pH of an aqueous solution obtained by mixing gram of acetic acid and . gram of sodium acetate making the