Dissociation of acids and bases
Tutorial 4
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
$pK_a=-logK_a$
$K_a=10^{-pK_a}$
$pK_b=-logK_b$
$K_b=10^{-pK_b}$
$K_a\cdot K_b$ $=$ $10^{-14}$
$pK_a+pK_b$ $=$ $14$
A few drops of methylorange ($ pK_a = 3.7 $, red acid form, yellow basic form) are poured into hydrochloric acid (strong acid $10^{-4}\; M$)
What color is there?
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Complete please this question before moving on to the next one!
What can be said about the dissociation of strong acids?
Dissociation of strong acids is complete
Calculate the pH of the acid.
$[H_3O^+]=10^{-4}$
$pH=4$
What is the turning area of methylorange ?
What will be the color of methylorange?
