(a) The Henderson-Hasselbalch equation relates the pH of a solution containing a weak acid (HA) and its conjugate base (A^-) to the dissociation constant (Ka) of the acid:
pH = pKa + log([A^-]/[HA])
where pH is the negative logarithm of the hydrogen ion concentration, pKa is the negative logarithm of the dissociation constant, [A^-] is the concentration of the conjugate base, and [HA] is the concentration of the weak acid.
The Henderson-Hasselbalch equation is useful in calculating the pH of buffer solutions, which resist changes in pH upon the addition of small amounts of acid or base. A buffer solution is typically a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid.
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(b) The solubility product constant (Ksp) is a measure of the solubility of sparingly soluble salt in water and is defined as the product of the ion concentrations raised to the powers of their stoichiometric coefficients in the balanced equation for the dissolution of the salt. The Ksp expression for Pb(IO3)2 in water is:
Pb(IO3)2(s) ⇌ Pb2+(aq) + 2IO3^-(aq)
Ksp = [Pb2+][IO3^-]^2
The Ksp expression for AlCl3 in water is:
AlCl3(s) ⇌ Al3+(aq) + 3Cl-(aq)
Ksp = [Al3+][Cl^-]^3
The Ksp values for these salts can be experimentally determined and used to calculate their solubility in water. If the product of the ion concentrations in a solution exceeds the Ksp value for the salt, precipitation of the salt will occur until the ion product is equal to the Ksp value.