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Complete Guide To Weak Acid pH and Ka Calculations

Weak acids react with water to produce hydronium ion,H3O+,( hydrogen ions (H+) is the concise version of hydronium ion) and conjugate base.

General Equation : HA (aq) <--> H+(aq) + A-(aq)

or HA(aq)+H2O(l) <---> H3O+(aq) + A-(aq)

Weak acids do not dissociate completely. Instead, the acid ionizes only partially in water. Acid ionization constant (equilibrium constant for acids) is Ka.

Ka =[H+][A-]/[HA]

Acid ionization constant describes the extent to which acid ionizes. The higher the Ka, the more acid ionizes and the stronger it is.

How to find pH of a weak acid:

1. Identify that you have a weak acid (If a question asks for Ka or gives Ka, most likely you are dealing with a weak acid).

2. Write out the equation for acid dissociation: HA(aq) <---> H+(aq) + A-(aq)

3. Create an ICE chart and plug in the initial concentrations.

4. Write out the Ka expression.

5. Plug in equilibrium values into the Ka expression and solve for X.

6. X =[H+]

7. pH = -log[H+]

Example: What is the pH of 0.10 M nicotinic acid, HC6H4NO2, at 25°C? Ka of nicotinic acid is 1.4*10^-5

Let's go over the steps:

Identify that you have a weak acid. We are given a small Ka value of nicotinic acid which implies that it is a weak acid. It is also not one of the strong acid that we memorized.

Write out the equation for acid dissociation: HA (aq) < --> H+(aq) + A-(aq).

Create an ICE chart and plug in the initial concentrations.

Write out the Ka expression.

Plug in equilibrium values into the Ka expression and solve for X. X =[H+].

To make calculations easier and not use the quadratic formula, we assume x is much smaller than 0.10 and cross it out. After we calculate the x value, we need to check the assumption by doing (x/0.10)*100% and if it is less than 5% our assumption works. Please note: different books and professors handle this assumption different. Please refer to your professor's notes.

pH = -log[H+]

How to find Ka of a weak acid when pH is given

1. Write out the equation for acid dissociation: HA(aq) < --> H+(aq) + A-(aq)

2. Create an ICE chart and plug in the initial concentrations

3. Determine [H+] from pH: [H+]= 10^-pH

4. [H+] = x . Plug in the concentration into the equilibrium values instead of x

5. Write out the Ka expression

6. Plug in equilibrium values into the Ka expression and solve for Ka

Example: Nicotinic acid (niacin) is a monoprotic acid with the formula HC6H4NO2. A solution that is 0.012 M in nicotinic acid has a pH of 3.39 at 25°C. What is the acid-ionization constant, Ka, for this acid at 25°C?

How to Calculate % ionization of a weak acid

1. Identify that you have a weak acid (If a question asks for Ka or gives Ka, most likely you are dealing with a weak acid

2. Write out the equation for acid dissociation:

3. HA(aq) <--> H+(aq) + A-(aq)

4. Create an ICE chart and plug in the initial concentrations

5. Write out the Ka expression

6. Plug in equilibrium values into the Ka expression and solve for X

7. % ionization = (X/[HA]initial)*100%

Example: Calculate the percent ionization of a 0.20M benzoic acid (HC7H5O2) solution at equilibrium at 25°C if Ka of benzoic acid is 6.3*10^-5.

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