# pH of Weak Acid | H3O+ concentration

Weak acid partially dissociate in the water to its ions. Therfore their acidic strength is not strong like strong acids. Weak acid dissociation is a reversible reaction. Le Chatelier's principle and equilibrium formula is applied to calculate concentrations of each components in the solution .

## Weak acid definition

When anion of acid is not stable in the water, that acid is called as a weak acid. That anion reacts with a H2O molecule and takes a proton.

## Dissociation of weak acid

As previously mentioned, weak acid dissociate partially to its H3O+ and related anion. ### H3O+ concentration and pH

H3O+ concentration is very low when it compares with weak acid concentration. As an example, if weak acid concentration is 0.1 mol dm-3, H3O+ concentration may be 0.0001 mol dm-3. That solution pH is 4.

If there are two solution, one is a weak acid and other one is a strong acid. If both of them concentration are same, pH of weak acid is higher than strong acid.

### Weak acids examples

• Formic acid - HCOOH
• Ethanoic acid - CH3COOH
• Sulfurous acid - H2CO3
• Carbonic acid - H2CO3

#### Examples of H3O+ concentration, pH and weak acid concentration

CH3COOH concentration and H3O+ are listed below. Then you can understand that H3O+ concentration is very less. Later in this tutorial, we do few examples to calculate H3O+ concentration and pH of weak acid.

## How to calculate H3O+ concentration and pH

### How to calculate H3O+ concentration

When weak acid gained the equilibrium, we can apply equilibrium expression.

#### Equilibrium expression • Ka = Equilibrium Constant
• [HA(aq)] = Equilibrium concentration of acid
• [A-(aq)] = Equilibrium concentration of anion
• [H3O+(aq)] = Equilibrium concentration of hydronium ion

Ka depends on the acid and the temperature. When concentrations are taken in mol dm-3 units, Ka also has the mol dm-3 units.

Ka value is different for CH3COOH acid and HCOOH acid. Also Ka is different for 200C and 300C for HCOOH acid.

When Ka and weak acid concentration are given, H3O+ concentration and pH can be found easily making some assumption. These assumptions are explained in the examples.

### How to calculate pH of a weak acid

Substitute calculated H3O+ concentration to the pH equation.

pH = -log10[H3O+(aq)]

Example

## Calculating H3O+ concentration and pH of CH3COOH acidic solution.

There is a CH3COOH acid solution in the laboratory. Initial CH3COOH(aq) concentration is 1 mol dm-3. After obtaining the equilibrium, calculate followings and mention all the assumptions you made in the calculations.

• Equilibrium [H3O+(aq)] concentration
• pH of the solution

Ka of CH3COOH(aq) = 1.8 * 10-5 mol dm-3

Solution

First look what are the data given for you. Only Ka of CH3COOH and Initial CH3COOH(aq) concentration are given to solve the question.

#### Assumptions made in the calculations

• Dissociation of water is negligible when it comparing with weak acid dissociation. Therefore we can assume, H3O+(aq) is given by weak acid dissociation. And also pH of the solution totally depends on the weak acid.
• After gaining the equilibrium, weak acid concentration is same as the initial weak acid concentration because dissociated amount (concentration) is very small comparing with initial amount (initial concentration).

### Chemical reaction between CH3COOH and water

Write the chemical reaction between CH3COOH and water. Then construct a table as below and write down the three rows as initial, dissociated / formed, and equilibrium concentrations with mol dm-3.

We take dissociated concentration as a mol dm-3. When a amount concentration is dissociated, a amount concentrations of CH3COO- and H3O+ are formed.

Then we know the equilibrium concentrations. We neglect water concentration because water concentrations is very larger than other components. ### Substitute equilibrium concentrations to equilibrium expression According to the assumptions we made earlier, we can assume equilibrium CH3COOH concentration equals to the initial CH3COOH concentration. Therefore equilibrium CH3COOH concentration = 1.0 mol dm -3. ### Calculate pH of CH3COOH solution

Substitute calculated H3O+ concentration to the pH equation. Related Tutorials to Weak Acid pH

Physical Chemistry Tutorials