Acid-Base Properties of Salts: Hydrolysis

Learning Goals

You will learn how to calculate the changes in hydronium and hydroxide ion concentrations when a salt is hydrolyzed.

Synopsis

A salt is an ionic compound that could have originated from the reaction of an acid with a base. [e.g., NaCl could have originated from the reaction of NaOH and HCl.] A hydrolysis reaction results in changes in the concentrations of the hydronium and hydroxide ions from dissolving a salt. When we are working with salts we need to remember (page 801) that the cations of groups IA and IIA do not affect the acidity of solutions. Let's consider the salt sodium hypochlorite, NaClO, which is used as a source of chlorine in swimming pools and other antibacterial applications. The addition of this salt to water will change the pH of the water due to the presence of the OCl- ion, which is the conjugate base of a weak acid, hypochlorous acid. So what would be the pH of a 0.015 M solution of sodium hypochlorite? First we will write the reaction of the conjugate base:

OCl-(aq) + H2O(l) --> HOCl(aq) + OH-(aq)

From Table 17.4 we see that Kb = 2.9 x 10-7so we have

2.9 x 10-7= [HOCl][OH-]/[OCl-]

So we need another table to set up the calculation of these equilibrium values.

OClHOClOH-
Initial,M 0.015 0 0
Change,M x +x+x
Equilibrium,M 0.015 - x x x

Because the stoichiometric coefficients of the products are the same, the concentrations will be the same. Therefore,

2.9 x 10-7= x2/(0.015 - x) = x2/0.015

Again we have made the approximation that x is much smaller than the initial concentration of the base. With this approximation we easily determine that x = [OH-] = 6.6 x 10-5 which is a pOH of 4.18 and thus the pH is 14.00 - 4.18 = 9.82.

Knowing that the cations of Groups IA and IIA do not affect the acidity of a water solution, and knowing that extremely weak acids and weak bases do not affect the acidity of a solution, we can often intuit if the addition of a particular salt will change the pH of a solution. So what are extremely weak acids and bases? The acids and bases that will not affect the pH of a solution are those that have a Ka or a Kb less than that of water.

If the salt has an anion which is amphiprotic, the acidic or basic nature of the resulting solution will be determined by the larger equilibrium constant. If its Ka is larger than its Kb then the solution will be acidic. If the salt is made up of a basic and an acidic component such as ammonium fluoride, again the acidic or basic nature of the solution will be determined by whether Ka is less than or greater than Kb.

Review Questions

1. For each of the following salts, predict whether the pH will be less than7, >7, or =7.

a.) NaCl b.) AlCl3 c.) NH4NO3 d.) NaHSO4

2. Calculate the pH of a 1.25 M solution of Na2SO3.


E-Mail

Web Author: Dr. Leon L. Combs
Copyright 2000 by Dr. Leon L. Combs - ALL RIGHTS RESERVED