Page 65 - Pharmaceutical_Analytical_Chemistry_1_Theoretical_Notes_Level_1

Mansoura National University
Pharm D-Clinical Pharmacy Program Level 1 Pharm. Anal. Chem. 1 (PC 101)

Acid-Base Equilibrium
Law of Mass Action

❖ The rate of a chemical reaction is proportional to the product of multiplication of the
active masses (molar concentrations) of the reacting substances.
aA + bB ⇌ cC + dD

K is a constant that is affected only by temperature & pressure.
eq

In Acid-Base Reactions

+
-
CH3COOH ⇌ CH3COO + H
1) In case of weak acids: K eq = Ka = Ionization constant of the acid or Acid dissociation
(e.g. CH3COOH) constant where (for acetic acid):

[CH3COO ][H ]
+
-
Ka = & pKa = - logKa
[CH3COOH]
-
+
NH4OH ⇌ NH4 + OH
K eq = Kb = Ionization constant of the base or base dissociation

constant where (for amm. hydroxide):
2) In case of weak bases:
-
+
(e.g. NH4OH) Kb = [NH4 ][OH ] & pKb = - logKb
[NH4OH]
N.B. Ka & Kb are calculated only for weak acids and weak
bases. Why??

+
3) In case of water H2O ⇌ H + OH -
(H2O): [H ][OH ]
-
+
Pure water is a very Dissociation constant of water (Kw) = =
weak electrolyte (very [H ][OH ] [H2O]
-
+
limited ionization).

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