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Unformatted text preview: Robert G. Michel 1 Acid-base Titrations Fractional Composition diagrams Fractional Composition Equations Let C be the (formal) concentration of a weak acid, then the mass balance: C = [HA] + [A- ]. or [A- ] = C - [HA] Substitute into K a or .. eqn (1) K a = [ H + ]( C " [ HA ]) [ HA ] [ HA ] = [ H + ] C [ H + ] + K a Fraction in the form HA eq n (2) Dividing eq n 1 by C gives the fraction in the form HA: eqn (3) similarly fraction in form A- is: eqn (4) " HA = [ HA ] [ HA ] + [ A # ] = [ HA ] C " HA = [ HA ] C = [ H + ] [ H + ] + [ K a ] " A # = [ A # ] C = [ K a ] [ H + ] + [ K a ] Robert G. Michel 2 Monoprotic systems pK a = 5.00 below pH 5 HA is dominant above pH 5 A- dominates Note: if conjugate Pair BH + and B, instead of HA and A- Equation 3 gives the fraction in the form BH+ Equation 4 gives the fraction in the form B K a is then the acid dissociation constant for BH + (which is K w /K b ) Diprotic Systems...
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- Fall '08
- Michel, R