Acid Base Equilibrium

Weak Acids & Bases

Recall From Yesterday….

pH = -log [H3O+]

[H3O+] = 10-pH

pOH = -log [OH-][OH-] = 10-pH

pKw = pH + pOH = 14

Weak AcidsDonate only one H+

Do not dissociate at 100%

For a Weak Acid:

The higher the Ka, the stronger the acid

Ka ◦– also known as the acid ionization constant.

Acid Dissociation Constant

The smaller the value of Ka, the less the acid ionizes in aqueous solution.

pH and Ka of a Weak Acid

Ka constants

the fraction of acid molecules that dissociate compared with the initial concentration of the acid, expressed as a percent.

depends on the value of Ka for the acid, as well as the initial concentration of the weak acid.

For a Weak Acid:

Or

1. % Ionization

A 0.25M solution of HF(aq) is ionized at 21.3%, calculate the pH

Try This:

% Dissociation

% dissociation = [HA]dissociated x 100%

[HA] initial

1. Balanced equation for acid equilibrium

2. Equilibrium constant (Ka) expression

3. ICE Table

2. pH of a Weak Acid, Given the Ka

4. Substitute the [equilibrium] into the Ka equation.

5. Solve for x i) 100 ruleii) quadratic

equation

6. Calculate pH from [H+]

pH of a Weak Acid, Given the Ka

Chloracetic acid, is a weak acid (Ka = 1.36 x 10-3).

Determine the pH of a 12.0 M solution of chloracetic acid.

Try This:

Step 1: [H+] = 10-pH

Step 2: [A-] = [H+]

Step 3:

3. Ka, Given the pH and [HA]

You measure the pH of a 0.10 M hypochlorous acid solution, HOCl(aq) and find it to be 4.23.

What is the Ka for hypochlorous acid?

Try This:

4. Polyprotic acids

A polyprotic acid is capable of donating more than one proton (H2CO3(aq), H2SO4(aq))

There is an ionization constant for each proton donation (Ka1, Ka2, etc.) as the ionization occurs in steps. (Table Pg. 803)

The Ka values become smaller with each ionization step, as the removal of a proton from a negatively charged object becomes more difficult.

4. Polyprotic acids

Polyprotic acids

However, the most ionization occurs in the first step.◦Ka1>> Ka2 > Ka3 .. . .

Consequently, the [H+] is predominantly established in the first ionization with the Ka1 value. Subsequent ionizations (Ka2 & Ka3) only add minimal amounts of [H+].

Use Ka1 to determine the pH of the solution at equilibrium.

Polyprotic acids