Salt Producing Basic Solution

Basic Concept

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

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

For example, in an aqueous solution of sodium acetate (NaC₂H₃O₂), the major species are

        Na⁺, C₂H₃O₂^- and H₂O 

The C₂H₃O₂^- ion is the conjugate base of acetic acid and the reaction between acetate ion and water is

        C₂H₃O₂^-(aq)  + H₂O (l) = HC₂H₃O₂ (aq) + OH^- (aq) 

The equilibrium constant (K_b) for this reaction  is

        K_b = [HC₂H₃O₂][OH^-] / [C₂H₃O₂^-]                (1)

The value of K_a for acetic acid is known (1.8 x 10^-5),

        K_a x K_b = [H⁺][C₂H₃O₂^-]  / [HC₂H₃O₂] x [HC₂H₃O₂][OH^-] / [C₂H₃O₂^-] = [H⁺][OH^-] = K_w = 1.0 x 10^-14    (2)

Thus

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

Using K_b or its formulae (1) the pH scale of the aqueous solution of NaC₂H₃O₂ can be calculated.

In the 0.30 M NaC₂H₃O₂ solution,

        K_b = 5.6 x 10^-10 = [HC₂H₃O₂][OH^-] / [C₂H₃O₂^-] = (x)(x) / (0.30 - x) x² / 0.30

        x 1.3 x 10^-5

        [OH^-] = x = 1.3 x 10^-5 M

        pOH = 4.89

        pH = 9.11

User Instructions

For the above example,  in the 0.30 M NaC₂H₃O₂ solution(K_a  = 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 Basic Solution link from the front page or Salt Producing Basic 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.