Acid-Base Equilibria (Pt. 5) Weak Acid Equilibria and K a - Calculating the pH of a Weak Acid Solution By Shawn P. Shields, Ph.D. This work is licensed by Dr. Shawn P. Shields-Maxwell under a Creative Commons Attribution- NonCommercial - ShareAlike 4.0 International License .
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Chem 2 - Acid-Base Equilibria V: Weak Acid Equilibria and Calculating the pH of a Weak Acid Solution
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Acid-Base Equilibria (Pt. 5)
Weak Acid Equilibria and Ka- Calculating the pH of a Weak
Acid SolutionBy Shawn P. Shields, Ph.D.
This work is licensed by Dr. Shawn P. Shields-Maxwell under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Recall: Weak Acid SolutionsSuppose a 1.0 M solution of HF (a weak acid) is prepared.What is the concentration of hydronium (H3O+) in solution? We canโt do this by inspection. An equilibrium between the weak acid and the products exists.
We need to find [H3O+] by calculating the equilibrium concentrations of HF, H3O+, and F-
H3O+
F
HF
Calculating the pH of a Weak Acid Solution
An equilibrium exists between the weak acid and its products.
We can use the relationship between the value of the equilibrium constant K and the initial concentration of weak acid in solution. HOW?
The Equilibrium Constant Ka for Weak Acids
An equilibrium exists between the weak acid (HA) and its products.
conjugate base
weak acid
The equilibrium constant K is โrenamedโ for acids to Ka
The Equilibrium Constant Ka for Weak Acids
An equilibrium exists between the weak acid (HA) and its products.
๐ ๐=ยฟยฟ Recall heterogeneous equilibriaโฆ the activity for pure liquids and solids is โ1โ
Example: The Equilibrium Constant Ka for HF
Ka is called the โacid dissociation constant.โThe value of Ka for HF is 3.5 10-4
๐ ๐=ยฟยฟ
ICE Tables, Ka, and Calculating pH for a Weak Acid Solution
Use Ka and an ICE table to determine the [H3O+] at equilibrium.
Calculate the pH using the equilibrium [H3O+]
๐ ๐=ยฟยฟ
A 0.25 M HNO2 solution is prepared.
The Ka for HNO2 is 4.6 10-4.
Calculate the pH of this solution.
Example Problem: Calculate the pH of a Weak Acid Solution
A 0.25 M HNO2 solution is prepared. The Ka for HNO2 is 4.6 10-4. Calculate the pH of this solution.
The first stepโฆ Write the chemical equation for the weak acid equilibrium.
Example Problem: Calculate the pH of a Weak Acid Solution