Salt Producing Acidic Solution

Basic Concept

When a salt dissolves in water, it breaks up into its ions. Under certain conditions, these ions can behave as acids. 

This module calculates the pH of the salts that produce acidic solutions.

For example, in an aqueous solution of NH₄Cl, the major species are

        NH₄⁺, Cl^- and H₂O 

The NH₄⁺behaves as a weak acid:

        NH₄⁺(aq)  = NH₃ (aq) + H⁺ (aq) 

The Cl^- ion does not affect the pH of the solution.

        K_a = [NH₃][H⁺] / [NH₄⁺]       (1)

The K_b of NH₃ is 1.8 x 10^-5,

        K_a x K_b  = K_w = 1.0 x 10^-14    (2)

Thus

        K_a =  K_{w /} K_a = 1.0 x 10^-14 / 1.8 x 10^-5 = 5.6 x 10^-10    (3)

Using K_a or its formulae (1) the pH scale of the aqueous solution of NH₄⁺can be calculated.

In the 0.10 M NH₄⁺ solution,

        K_a = 5.6 x 10^-10 = [NH₃][H⁺] / [NH₄⁺] = (x)(x) / (0.10 - x) x² / 0.10

        x 7.5 x 10^-6

        pH = 5.13 

User Instructions

For the above example,  in the 0.10 M NH₄Cl solution(K_b = 1.8 x 10^-5 ).

This is one step process, enter the known data and press Calculate to output the unknowns.

1. Select Salt Producing Acidic Solution link from the front page or Salt Producing Acidic Solution tab from the Acid, Baseand Salt module. The Input and Output screen appears.

2. In the Input area, enter the known quantities with a proper significant figure.

3. Click Calculate to output the answer.

 

4. The Show Work area on the right shows you step-by-step how your problem has been solved.

To start a new problem, click Reset. All Input fields will be cleared. Follow Step 1-3 again.