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

Problem 15.63

Predict whether each of the following solutions is acidic, basic, or neutral. Write balanced equation(s) for any hydrolysis reaction(s) that occur.

(a) RbClO4(aq)

(b) CH3CH2NH3Br(aq)

(c) HCOONH4(aq)






Answer:

(a) Write the solubilization reaction:

RbClO4(aq) Rb+(aq) + ClO4(aq)

Rb+ is the cation of a strong base (RbOH) so no cation hydrolysis occurs.

ClO4 is the anion of a strong acid (HClO4) so no anion hydrolysis occurs.

Since no hydrolysis occurs, the solution will be neutral.


(b) Write the solubilization reaction:

CH3CH2NH3Br(aq) CH3CH2NH3+(aq) + Br(aq)

CH3CH2NH3+ is the cation of a weak base, CH3CH2NH2, hydrolysis occurs:

CH3CH2NH3+(aq) + H2O(l) H3O+(aq) + CH3CH2NH2(aq)

Br is the anion of a strong acid (HBr) so no anion hydrolysis occurs:

Since hydronium ion is formed by the cation hydrolysis, the solution will be acidic.


(c) Write the solubilization reaction:

HCOONH4(aq) NH4+(aq) + HCOO(aq)

NH4+ is the cation of a weak base, NH3, hydrolysis occurs:

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

HCOO is the anion of a weak acid, HCOOH, so anion hydrolysis ocurs:

HCOO(aq) + H2O(l) HCOOH(aq) + OH(aq)

The equilibrium constant for the cation hydrolysis is 1.0×10–14/1.8×10–5 = 5.6×10–10.

The equilibrium constant for the anion hydrolysis is 1.0×10–14/1.8×10–4 = 5.6×10–11.

The equilibrium reaction with the larger equilibrium constant will be favored, so the cation hydrolysis dominates, so that more hydronium ion will be formed than hydroxide ion and the solution will be acidic.