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>> Salt of Strong Acid and Weak Base-Part1
Salt of a strong acid and a weak base:
The solution of such a salt is acidic in nature. The cation of the salt which has come from weak base is reactive. It reacts with water to form a weak base and H+ ions.
B+ + H2O ↔ BOH + H+
Consider, for example, NH4Cl. It ionises in water completely into NH4 and CF ions. ions react with water to form a weak base (NH4OH) and H+ ions.
NH+4 + H2O ↔ NH4OH + H+
C(1-x) Cx Cx
Thus, hydrogen ion concentration increases and the solution becomes acidic.
Applying law of mass action,
Kh = [Hx ][NH4 OH]/[NH4+ ]=(Cx.Cx)/C(1-x) = (x2 C)/((1-x)) ...... (i)
where C is the concentration of salt and x the degree of hydrolysis.
Other equilibria which exist in solution are
NH4OH ↔ NH+4 + OH-, Kb = [NH+4][OH-]/[NH4Oh] .... (ii)
H2O ↔ H+ + OH-, Kb = [H+][H-] ..... (iii)
From eqs. (II) and (iii)
Kw/Kb =[H+ ][NH4 OH]/[NH4+ ] =Kh .... (iv)
[H+] = [H+ ][NH4+]/[NH4OH] = Kw/Kb ×[NH4+ ]/[NH4 OH]
log [H+] = log Kw - log Kb + log[salt]/[base]
-pH = -pKw + pKb + log[salt]/[base]
pKw - pH = pKb + log[salt]/[base]
pOH = pKb + log[salt]/[base]