Dissociation of acids and bases
Tutorial 1
Dissociation of acids and bases ; acidity and basicity constants
Dissociation of a strong acid $HB $
$HB + H_2O $ $\rightarrow$ $H_3O^+ + B^-$
Dissociation of a strong base $B $
$B + H_2O $ $\rightarrow $ $OH^- + HB $
(Charges may differ!)
Dissociation of a weak acid $HB$
$HB + H_2O$ $\leftrightarrows$ $H_3O^+ + B^-$
(Charges may differ!)
Constant of acidity
($\neq $ constant of the former equilibrium):
$K_a = \frac{[H_3O^+] [B^-]} {[HB]} $
Dissociation of a weak base $B $
$B + H_2O $ $\leftrightarrows $ $OH^- + HB^+ $
(Charges may differ!)
Basicity Constant
($\neq $constant of the former equilibrium):
$K_b=\frac{[OH^-][HB]}{[B]}$
Relations
$pK_a=-logK_a$
$K_a=10^{-pK_a}$
$pK_b=-logK_b$
$K_b=10^{-pK_b}$
$K_a\cdot K_b$ $=$ $10^{-14}$
$pK_a+pK_b$ $=$ $14$
In an aqueous solution of pyruvic acid (CH3-CO-COOH) maintained at a constant pH of 7, what is the ratio of the concentrations of the acid and base species?
Data: $ pKa $(pyruvic acid) $ = 2.5 $
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$K_a$ $=$ $10^{-2.5}$ $=$ $3.16\cdot 10^{-3}$
Isolate in $ K_a $ the ratio of the concentrations of acid and base species!
$\frac{[CH_3CO-COO^-][H_3O^+]}{[CH_3CO-COOH]}=K_a$
$\frac{[CH_3CO-COO^-]}{[CH_3CO-COOH]}=\frac{K_a}{[H_3O^+]}$
Calculate this ratio now !
$\frac{[CH_3CO-COO^-]}{[CH_3CO-COOH]}$ $=$ $\frac{3.16\cdot 10^{-3}}{10^{-7}}$ $=$ $3.16\cdot 10^4$
