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J. W. Hill, R. H. Petrucci, T. W. McCreary, & S. S. Perry General Chemistry 4th edition

Problem 15.64

Predict whether each of the following solutions is acidic, basic, or neutral. Write balanced equation(s) for any hydrolysis reaction(s) that occur.
(a) CH3CH2COOK(aq) (b) Mg(NO3)2(aq) (c) NH4CN(aq)





Answer

Hydrolysis occurs by the reaction of water with the conjugate species of weak acids or weak bases.

(a) CH3CH2COOK(aq) K+(aq) + CH3CH2COO(aq)

(The reaction goes to completion because potassium salts are freely soluble in water.)

K+ is the conjugate acid of KOH, a strong base, so does not hydrolyze, i.e.,

K+(aq) + H2O(l) No reaction


CH3CH2COO is the conjugate base of propionic acid, CH3CH2COOH, a weak acid so hydrolysis will occur, i.e.

CH3CH2COO(aq) + H2O(l)CH3CH2COOH(aq) + OH(aq)

Since the solution generates hydroxide, it is basic.

(b) Mg(NO3)2 Mg2+(aq) + 2 NO3(aq)

(The reaction goes to completion because nitrate salts are freely soluble in water.)

Mg2+ is the conjugate acid of Mg(OH)2, a strong base, so does not hydrolyze, i.e.,

Mg2+(aq) + H2O(l) No reaction

NO3 is the conjugate base of a strong acid, HNO3, so also does not hydrolyze, i.e.

NO3(aq) + H2O(l) No reaction

Since no additional hydronium ion or hydroxide ion is generated, the solution is neutral.

(c) NH4CN(aq) NH4+(aq) + CN(aq)

(The reaction goes to completion because ammonium salts are freely soluble in water.)

NH4+ is the conjugate acid of NH3, a weak base, so can hydrolyze, i.e.,

NH4+(aq) + H2O(l)NH3(aq) + H3O+(aq)

CN is the conjugate base of a weak acid, HCN, so also can hydrolyze, i.e.,

CN(aq) + H2O(l)HCN(aq) + OH(aq)

To determine whether the solution is acidic or basic requires use of the equilibrium constants.

Ammonium ion has Ka = 5.6×10–10. Kb(CN) = Kw/Ka(HCN) = 1.0×10–14/6.2×10–10 = 1.6×10–5.

Since the equilibrium constant for the base hydrolysis is much larger than the equilibrium constant for the acid hydrolysis, the base hydrolysis predominates and the solution is basic.