(aq) CH COO (aq) + H O (l) CH COOH (aq) CH COO (aq) + H O (l) OH (aq) + (aq) H O (l) H O + (aq) H O + ( H O + ( O + These neutralization reactions are identical to those reactions that we have already discussed in common ion effect. XII U8-Ionic XII U8-Ionic - - - - Let us analyse the effect of the addition of . mol of solid sodium hydroxide to one litre of a buffer solution containing . M CH COOH and .
M CH COONa . Assume that the volume change due to the addition of NaOH is negligible. (Given: K a for CH COOH is . - CH -COOH(aq) CH COO (aq)+H (aq) H O .
- α α α ← H O . . . CH COONa(aq) CH COO (aq)+Na (aq) → The dissociation constant for CH COOH is given by K = [CH COO ][H ] [CH COOH] ; [H ]=K [CH COOH] [CH COO ] a a The above expression shows that the concentration of H + is directly proportional to [CH COOH] [CH COO ] Let the degree of dissociation of CH COOH be α then, [CH COOH]= .
- α and [CH COO ]= + . α ∴ [H ]=K ( . - ) ( . + ) << .
[H ]= K [H K a a ( . ) ( . ) ] ⇒ Given that K for CH COOH is . [H pH = - log ( .
a - ∴ ] ; = - log - . = - . pH = . Calculation of pH after adding .
mol NaOH to litre of buffer. Given that the volume change due to the addition of NaOH is negligible ∴ [OH .01M - ] The consumption of OH - are expressed by the following equations. CH COOH (aq) CH