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pH of a weak acid
-When you are with a weak acid in a reaction, it will not completly dissociate like strong acids. Therefore when you are looking for the pH of the acid you have to take this into consideration. You must use the equilibrium constant equation to find the pH. That way you will see how many H+ ions are produced in the reaction. With this information you can find the pH.
Procedure
Step 1. Find the balanced equation for the reaction.
Step 2. Find the equilibrium constant and write out the equilibrium equation.
Step 3. Plug in your known concentrations to the equlibrium equation.
Step 4. Use a chart to find your other concentrations(variables)
Step 5. Plug in your variables to the equilibrium equation and solve for x.
Step 6. With the H+ concentration find the pH.(pH = -log[H+])
Example
-If 0.01M Acetic acid dissolves in water, find the pH of the solution.
Step 1. HC2H3O2 + H20 <==> H30+ + C2H3O2-
Step 2. equilibrium constant of Acetic acid = 1.76E-5
Ka = [H30+[C2H3O2-]/[HC2H3O2]
(rewrite)
Step 3. Ka = [H30+[C2H3O2-]/0.01M
Step 4.
pH vs. Type of Solution |
|
Initial |
Eqilibrium |
| [HC2H3O2] |
0.01M |
0.01M-x (5% rule) |
| H30+ |
0 |
x |
| [C2H3O2-] |
0 |
x |
Step 5.
Ka = x2/0.01M
1.76E-5 = x2/0.01M
x2 = 1.76E-7
x = 4.1952E-4
Step 6. x = [H+] = 4.20E-4
pH = -log(4.20E-4)
pH = 3.37
Another example
- 0.10M Hydrofluoric acid is dissolved in water. Find the pH of the solution.
Answer: pH = 2.08