AP Chemistry Unit 8: Acids and Bases practice questions

Question 1

What does pH measure, and how is it calculated from hydronium ion concentration?

Accepted Answer

pH = −log[H₃O⁺], a measure of acidity on a logarithmic scale. Lower pH means more hydronium and higher acidity.

Answer

pH = [H₃O⁺] / [OH⁻], a direct ratio of ion concentrations

Answer

pH = log[H₃O⁺], without the negative sign

Answer

pH = −ln[H₃O⁺], using the natural logarithm instead of log base 10

Question 2

What is the ion-product constant of water (Kw) at 25°C, and what does it equal?

Accepted Answer

Kw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25°C

Answer

Kw = [H₃O⁺] + [OH⁻] = 1.0 × 10⁻⁷ at 25°C

Answer

Kw = [H₃O⁺] / [OH⁻] = 1.0 × 10⁻¹⁴ at 25°C

Answer

Kw = [H₂O] / ([H₃O⁺][OH⁻]) = 1.0 × 10⁻¹⁴ at 25°C

Question 3

At 25°C, pure water has equal [H₃O⁺] and [OH⁻]. What is the pH, and why is this the definition of neutral?

Accepted Answer

pH = 7.0 because [H₃O⁺] = [OH⁻] in pure water. Both equal 1.0 × 10⁻⁷ M, and equal concentrations define neutrality.

Answer

pH = 7 because water contains no ions whatsoever

Answer

pH = 7 at all temperatures because water is always neutral

Answer

pH = 7 because [H₃O⁺] = 0 in pure water

Question 4

At 80°C, Kw > 1.0 × 10⁻¹⁴. Is pure water at 80°C acidic, basic, or neutral? Explain.

Accepted Answer

Still neutral—[H₃O⁺] still equals [OH⁻]. The neutral pH is simply lower than 7.0 at higher temperatures.

Answer

Acidic, because the pH is below 7.0 at 80°C

Answer

Basic, because higher temperature increases OH⁻ production more than H₃O⁺

Answer

Neutral only if pH remains exactly 7.0, so it cannot be neutral at 80°C

Question 5

What distinguishes a strong acid from a weak acid in terms of ionization in aqueous solution?

Accepted Answer

Strong acids ionize completely, so [H₃O⁺] equals the acid's concentration. No equilibrium — the dissociation goes to completion.

Answer

Strong acids have a higher molar mass than weak acids

Answer

Strong acids partially ionize but have a larger Ka than weak acids

Answer

Strong acids only ionize when heated; weak acids ionize at room temperature

Question 6

A solution is prepared with 0.050 M Ca(OH)₂. What is [OH⁻], and how does the group II hydroxide rule apply?

Accepted Answer

[OH⁻] = 0.10 M because Ca(OH)₂ releases 2 mol OH⁻ per mole. It's a strong base that fully dissociates.

Answer

[OH⁻] = 0.050 M because molarity of the salt equals molarity of hydroxide

Answer

[OH⁻] = 0.025 M because only half of Ca(OH)₂ dissociates as a strong base

Answer

[OH⁻] = 0.10 M only if the solution is heated above 25°C

Question 7

How do you calculate the pH of a 0.025 M HCl solution?

Accepted Answer

pH = −log(0.025) ≈ 1.60, since HCl is strong and [H₃O⁺] = 0.025 M. Strong acids fully ionize, so no ICE table is needed.

Answer

Set up an ICE table using Ka for HCl to find equilibrium [H₃O⁺]

Answer

pH = 0.025 because pH equals the molar concentration of the acid

Answer

pH = −log(0.025/2) because only half the HCl molecules ionize at this concentration

Question 8

What is Ka, and what does a small Ka value indicate about an acid's behavior in water?

Accepted Answer

Ka is the acid dissociation constant; a low Ka means the acid is weak. Only a small fraction of its molecules ionize in water.

Answer

Ka measures the concentration of the acid solution, not its ionization

Answer

A small Ka means the acid is very concentrated but neutral

Answer

A small Ka means the acid completely ionizes but produces few H₃O⁺ ions

AP Chemistry — Unit 8: Acids and Bases