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ChemWiki: The Dynamic Chemistry E-textbook > Physical Chemistry > Acids and Bases > Ionization Constants > Calculating a Ka Value from a
Known pH
Calculating a Ka Value from a Known pH
The quantity pH, or "power of hydrogen," is a numerical representation of the acidity or basicity of a solution. It can be used to calculate the
concentration of hydrogen ions [H+] or hydronium ions [H3O+] in an aqueous solution. Solutions with low pH are the most acidic, and solutions with
high pH are most basic.
Definitions
Although pH is formally defined in terms of activities, it is often estimated using free proton or hydronium concentration:
or
Ka, the acid ionization constant, is the equilibrium constant for chemical reactions involving weak acids in aqueous solution. The numerical value
of Ka is used to predict the extent of acid dissociation. A large Ka value indicates a stronger acid (more of the acid dissociates) and
small Ka value indicates a weaker acid (less of the acid dissociates). For a chemical equation of the form
, the equation for Ka is the following:
HA is the undissociated acid and A- is the conjugate base of the acid. (H2O, a pure liquid, has an activity equal to one, and is therefore ignored
in the equilibrium constant equation.)
Solving for K a
When given the pH value of a solution, solving for Ka requires the following steps:
1. Set up an ICE table for the chemical reaction.
2. Solve for the concentration of H3O+ (denoted x) using the equation for pH:
3. Use the concentration of H3O+ to solve for the concentrations of the other products and reactants.
4. Plug all concentrations into the equation for Ka and solve.
Example 1
Calculate the Ka value of a 0.2 M aqueous solution of propionic acid, CH3CH2CO2H, with a pH of 4.88.
SOLUTION
ICE TABLE
http://chemwiki.ucdavis.edu/Physical_Chemistry/Acids_and_Bases/Ionization_Constants/Calculating_A_Ka_Value_From_A_Measured_Ph
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3/18/2015
Calculating a Ka Value from a Known pH - Chemwiki
ICE
Initial Concentration
Change in Concentration Equilibrium Concentration According to the definition of pH, 0.2 M
-x M
(0.2 - x) M
0M
+x M
xM
0M
+x M
xM
Thus,
Related Articles
The Determination of pKa of Multiprotic, Weak Acids
The pKa of a Weak Acid as a Function of Temperature and Ionic Strength
Graphical Correlation Between pH Values, Molarities and Dissociation Constants of Weak Acids
References
1. Petrucci,et al. General Chemistry:Principles & Modern Applications; Ninth Edition, Pearson/Prentice Hall; Upper Saddle River, New Jersey 07.
Contributors
Paige Norberg, Gabriela Mastro (UCD)
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