AP Chem weak acid pH calculations practice questions

Define a weak acid.

A weak acid is an acid that partially dissociates in water, establishing an equilibrium between undissociated molecules and ions.

True or False: Weak acids completely ionize in solution.

False: Weak acids only partially ionize in solution, unlike strong acids.

List two examples of weak acids.

- Acetic acid (CH₃COOH) - Formic acid (HCOOH)

What is the equilibrium expression for a weak acid?

For a weak acid HA: $\displaystyle K_a = \\frac{[H^+][A^-]}{[HA]}$

Compare weak acids and strong acids.

Weak acids partially dissociate; strong acids fully dissociate in solution.

Fill in the blank: The strength of a weak acid is indicated by its _____ value.

Ka value

What does a larger Ka value indicate?

A larger Ka value indicates a stronger weak acid, meaning it dissociates more in solution.

Cause → Effect: What happens when a weak acid is added to water?

The weak acid partially dissociates into ions, establishing an equilibrium.

Identify one factor that affects weak acid dissociation.

Concentration of the acid; higher concentrations shift equilibrium, affecting pH.

True or False: All weak acids have the same pH at the same concentration.

False: Different weak acids have different Ka values, affecting their pH.

What is the dissociation reaction for acetic acid?

CH₃COOH ⇌ H⁺ + CH₃COO⁻

How does temperature affect weak acid dissociation?

Increasing temperature generally increases the dissociation of weak acids, affecting pH.

Provide a real-world example of a weak acid.

Vinegar, which contains acetic acid, is a common weak acid.

What is an acid-base conjugate pair?

An acid and its conjugate base (e.g., CH₃COOH and CH₃COO⁻).

What is the significance of pKa?

pKa is the negative logarithm of Ka; lower pKa indicates a stronger weak acid.

Identify the weak acid from its formula: H₂CO₃.

Carbonic acid, which partially dissociates in water.

What is the formula for pH?

pH = -log[H⁺]

Calculate the pH of 0.1 M acetic acid (Ka = 1.8 x 10⁻⁵).

Use ICE table: 1. Set up equilibrium expression: Ka = → [H⁺][A⁻]/[HA] 2. Solve for [H⁺] 3. pH = -log[H⁺]

True or False: Weak acids fully dissociate in solution.

False. Weak acids only partially dissociate.

What is the relationship between Ka and acid strength?

Higher Ka = stronger weak acid, more dissociation.

Fill in the blank: The higher the concentration of a weak acid, the _____ its pH.

lower

Identify the effect: Increasing temperature on weak acid pH.

Generally increases dissociation, decreasing pH.

How do you find [H⁺] from Ka and initial concentration?

Set up: Ka = → [H⁺][A⁻]/[HA] Solve for [H⁺] using I.C.E.

Calculate the pH of 0.05 M formic acid (Ka = 1.8 x 10⁻⁴).

1. Set up Ka expression: Ka = [H⁺]²/[HA - x] 2. Solve for x (H⁺) 3. pH = -log(x).

Comparing strong and weak acids: pH range?

Strong acids: pH ≤ 3 Weak acids: pH > 3

Find pH from [H⁺] concentration: [H⁺] = 0.001 M.

pH = -log(0.001) = 3.

What is the significance of pKa?

pKa = -log(Ka), indicates acid strength; lower pKa = stronger acid.

Calculate the pH of a 0.2 M weak acid with Ka = 1.0 x 10⁻⁵.

1. ICE table setup for dissociation. 2. Solve for [H⁺]. 3. pH = -log[H⁺].

True or False: pH can be calculated directly from Ka.

False. pH requires [H⁺] concentration.

Determine pH of a solution with 0.01 M acetic acid.

1. Use Ka to find [H⁺]. 2. Apply pH formula.

What does the term 'dissociation' refer to in weak acids?

The process by which a weak acid partially separates into ions.

How does dilution affect pH of a weak acid?

Increasing dilution increases pH (less concentration of H⁺).