Handout5-MonoproticAcidBase

Handout5-MonoproticAcidBase - CEM 834 ACIDBASE EQUILIBRIA...

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CEM 834 ACID–BASE EQUILIBRIA FALL 2005 I. Monoprotic Acid–Base Systems For the general monoprotic weak acid (HA) and its conjugate base ( 29 A - , the following equilibrium reaction occurs in water HA H O H O A 2 3 + + + - This reaction may be expressed as the independent contributions from dissociation of the weak acid HA H A K a a a a d H A HA + = + - + - (1) and dissociation of water H O H OH K a a 2 w H OH + = + - + - (2) For certain applications, it is necessary to determine the exact concentration of these species. In this case, rigorous solution of the stoichiometric equations describing the acid– base system, perhaps with subsequent activity corrections, must be performed. For other applications, however, approximate knowledge of the concentrations or pH is sufficient. In this case, a rapid solution by graphical methods is appropriate. Both the simultaneous equation and graphical approaches will be described below. A. Simultaneous Equation Approach 1. Write down all equilibria and dissociation constants for the acid–base system, as shown in equations (1) and (2). 2. Write a mass balance equation that relates the analytical concentrations of weak acid and conjugate base ( 29 C and C respectively HA A - , to the equilibrium concentrations [ ] [ ] ( 29 HA and A respectively - , [ ] [ ] C C HA A HA A + = + - - (3) 3. Write a charge balance equation that relates the equilibrium concentrations of all cations and anions [ ] [ ] [ ] [ ] H M OH A + + - - + = + (4)

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Note that M + , the cation associated with the conjugate base, must have a concentration equal to the analytical concentration of the conjugate base C A - . By substitution and rearrangement of the charge balance equation (4) [ ] [ ] [ ] ( 29 A C H OH A - + - = + - - (5) and by rearrangement of the mass balance equation (3) [ ] [ ] HA C C A HA A = + - - - (6) By substitution of equation (5) into equation (6) [ ] [ ] [ ] ( 29 HA C H OH HA = - - + - (7) From the equilibrium constant for the weak acid in equation (1) [ ] [ ] γ γ = = - - + - A HA d a A HA
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Handout5-MonoproticAcidBase - CEM 834 ACIDBASE EQUILIBRIA...

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