This document discusses the equilibrium of weak acids. It defines weak acids as acids that partially dissociate in water, giving them a higher pH than strong acids. When a weak acid is added to water, an equilibrium is established between the undissociated acid molecules, hydronium ions, and conjugate base anions. The acid dissociation constant (Ka) characterizes the extent of dissociation and is used to calculate the pH of weak acid solutions. Common examples of weak acids and their Ka values are provided in a table. Methods for calculating pH from Ka using equilibrium expressions and assumptions are also described.

1. Equilibrium of weak
acids
Zumra Ahmad
Urwa Arshad

2. Contents
 Weak acid
 Equilibrium
 Equilibrium of weak acid
 Acid dissociation constant
 Table of weak acids(Ka values)
 Dissociation of weak acid
 Methods to calculation PH by using Ka values
 Examples
 Three ways to measure PH
 Calculating Ka from PH
 Percent dissociation

3. Weak acid or equilibrium
• Weak Acid: Weak acid is an acid that partially dissociates into its ions in an aqueous
solution or water.
 Weak acids having higher pH value.
 Weak acid is a weak electrolyte.
e.g: methanoic acid, benzoic acid, hydrofluoric acid, nitrous acid, phosphoric acid.
 When HF dissolve in water, only a fraction of the molecules ionize.
 HF(aq) + H2O(l) ⇌ H3O+(aq) + F-(aq)
• Equilibrium: When the rate of forward reaction takes place at the rate of reverse
reaction, the composition of the reaction mixture remains constant.

4. Equilibrium of weak acids
• When an uncharged weak acid is added to water, a homogenous
equilibrium forms in which aqueous acid molecules, HA(aq), react with
liquid water to form aqueous hydronium ions and aqueous anions, A-(aq)

5. Acid dissociation constant(1)
The dissociation of weak acid in water is characterized by an equilibrium
equation.
The equilibrium constant for the dissociation reaction, denoted by Ka is
called the acid-dissociation constant.
 HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)
Water is a base that react with acid HA, A is the conjugate base of the acid
HA and the hydronium ion is the conjugate acid of water.

6. Acid dissociation constant(2)
The acid dissociation constant; Ka qualifies the extent of dissociation of a
weak acid.
The smaller the value of Ka, the weaker the acid and vice versa.
The strength of weak acid depends on how much it dissociates; the more
it dissociates, the stronger the acid.
 Ka =
𝑯𝟑𝑶+ [𝑨−]
[𝑯𝑨]

7. Ka values of weak acids
Ka of Weak Acids
hydrocyanic HCN 6.2 x 10-10
hydrofluoric HF 6.3 x 10-4
hydrogen peroxide H2O2 2.4 x 10-12
hydrogen sulfate ion HSO4
- 1.2 x 10-2

8. Dissociation of weak acid
Weak acids ionize only partially, and the ionization reaction is reversible.
The ionization constant increase as the strengths of the acids increase.
Thus weak acids solutions contain multiple charge and uncharged species in
dynamic equilibrium.
In this article, we will discuss acid dissociation reaction and the related
equilibrium constant; Ka the acid dissociation constant.

9. Method to calculate PH using Ka
1) Write the chemical equation for the ionization equilibrium.
2) Write the equilibrium constant expression.
3) Set up a table for initial/change in/Equilibrium Concentration to
determine equilibrium concentrations as a function of charge (x)
4) Substitute equilibrium concentrations into the equilibrium constant
expression and solve for x. (make assumptions if possible)

10. Calculating PH from Ka
Calculate the PH of a 0.30Msolution of acetic acid, HC2H3O2, at 25∘C.
HC2H3O2(aq) +H2O(l) = H3O+(aq) + C2H3O2-(aq)
Ka for acetic acid at 25°C is 1.8× 10 ∗ −5
Ka =
𝑯𝟑𝑶+ [𝑪𝟐𝑯𝟑𝑶𝟐−]
[𝑯𝑪𝟐𝑯𝟑𝑶𝟐]

11. Three ways to measure PH
A PH meter
An indicator
Litmus paper
 Red to blue (basic)
o PH > 8
 Blue to red (acidic)
o PH < 5

12. Litmus Paper

13. Calculating Ka from PH
The PH of 0.100M solution of formic acid, HCOOH, at 25∘C is 2.38.
Calculate Ka for formic acid at this temperature.
HCOOH(aq) + H2O(l) = H3O+(aq) HCOO-(aq)
We know that; Ka =
𝑯𝟑𝑶+ [𝑯𝑪𝑶𝑶−]
[𝑯𝑪𝑶𝑶𝑯]
To calculate Ka, we need the equilibrium concentrations of all three
things.
We can find [H3O+], which is same as [HCOO-], from the PH.

14. Calculating percent dissociation
%Dissociation = 𝑪𝒐𝒏𝒄𝒆𝒏𝒕𝒓𝒂𝒕𝒊𝒏 𝒊𝒐𝒏𝒊𝒛𝒆𝒅
𝑶𝒓𝒊𝒈𝒏𝒂𝒍 𝒄𝒐𝒄𝒆𝒏𝒕𝒓𝒂𝒕𝒊𝒐𝒏 ×100
 %Dissociation =
𝑪𝒐𝒏𝒄𝒆𝒏𝒕𝒓𝒂𝒕𝒊𝒐𝒏 𝒐𝒇 𝒉𝒚𝒅𝒓𝒐𝒏𝒊𝒖𝒎
𝑪𝒐𝒏𝒄𝒆𝒏𝒕𝒓𝒂𝒕𝒊𝒐𝒏 𝒐𝒇 𝒕𝒉𝒆 𝒘𝒆𝒂𝒌 𝒂𝒄𝒊𝒅
×100
Lets the example of formic acid;
• [H3O+]eq = 4.2 × 10 ∗ −3M
• [HCOOH]initial = 0.10M
%Dissociation =
4.2×10∗−3
0.10
×100 =4.2%

15. THANK YOU!