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 (Kb) for this reaction is
Kb = [HC2H3O2][OH-] / [C2H3O2-] (1)
The value of Ka for acetic acid is known (1.8 x 10-5),
Ka x Kb = [H+][C2H3O2-] / [HC2H3O2] x [HC2H3O2][OH-] / [C2H3O2-] = [H+][OH-] = Kw = 1.0 x 10-14 (2)
Kb = Kw / Ka = 1.0 x 10-14/ 1.8 x 10-5 = 5.6 x 10-10 (3)
Using Kb or its formulae (1) the pH scale of the aqueous solution of NaC2H3O2 can be calculated.
In the 0.30 M NaC2H3O2 solution,
Kb = 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
For the above example, in the 0.30 M NaC2H3O2 solution(Ka = 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.