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

Problem 15.65

Which of the following 0.100 M solutions has the lowest pH: (a) NaNO3, (b) CH3COOK, (c) NH4I, or (d) Na3PO4?




Answer

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

(a) NaNO3(aq) Na+(aq) + NO3(aq)

(Sodium salts and nitrates are freely soluble in water, so the reaction goes to completion.)

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

Na+(aq) + H2O(l) No Reaction

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

NO3 (aq) + H2O(l) No Reaction

Thus, the solution is neutral, pH ~ 7.

(b) CH3COOK(aq) K+(aq) + CH3COO(aq)

(Potassium salts and acetates are freely soluble in water, so the reaction goes to completion.)

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

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

CH3COO is the conjugate base of CH3COOH, a weak acid (acetic acid), so does hydrolyze, i.e.

CH3COO(aq) + H2O(l)CH3COOH(aq) + OH(aq)

The solution generates hydroxide, so pH > 7.

(c) NH4I(aq) NH4+(aq) + I(aq)

(Ammonium salts and most halides are freely soluble in water, so the reaction goes to completion.)

NH4+ is the conjugate acid of NH3, a weak base (ammonia), so does hydrolyze, i.e.

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

I is the conjugate base of a strong acid, HI, so does not hydrolyze, i.e.

I(aq) + H2O(l) No Reaction

The solution generates hydronium ion, so pH < 7.

(d) Na3PO4(aq) 3 Na+(aq) + PO43– (aq)

(Sodium salts are freely soluble in water, so the reaction goes to completion.)

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

Na+(aq) + H2O(l) No Reaction

PO43– is the conjugate base of HPO42–, a weak acid, so does hydrolyze, i.e.

PO43– (aq) + H2O(l)HPO42– (aq) + OH(aq)

The solution generates hydroxide, so pH > 7.

The only acidic solution is (c) so has the lowest pH.