---
title: "AP Chemistry Unit 8 Practice Test"
description: "A timed AP Chemistry Unit 8 (Acids & Bases) practice test in the AP format, drawn from 3,639 multiple-choice questions and 72 FRQs written for the unit. The standard version takes about 60 minutes."
canonical: "https://fiveable.me/ap-chem/unit-test/8"
type: "unit-test"
subject: "AP Chemistry"
unit: "Unit 8"
---

# AP Chemistry Unit 8 Practice Test

## Overview

A timed AP Chemistry Unit 8 (Acids & Bases) practice test in the AP format, drawn from 3,639 multiple-choice questions and 72 FRQs written for the unit. The standard version takes about 60 minutes.

## Test Format

- 20 Multiple Choice Questions (30 minutes)
- 1 FRQ – FRQ 1 – Long Answer (15 minutes)
- 1 FRQ – FRQ 4 – Short Answer (15 minutes)

## Unit FRQs by Type

- [FRQ 1 – Long Answer](/ap-chem/frqs/frq-1?unit=8): 4 questions
- [FRQ 2 – Long Answer](/ap-chem/frqs/frq-2?unit=8): 3 questions
- [FRQ 3 – Long Answer](/ap-chem/frqs/frq-3?unit=8): 5 questions
- [FRQ 4 – Short Answer](/ap-chem/frqs/frq-4?unit=8): 15 questions
- [FRQ 5 – Short Answer](/ap-chem/frqs/frq-5?unit=8): 19 questions
- [FRQ 6 – Short Answer](/ap-chem/frqs/frq-6?unit=8): 9 questions
- [FRQ 7 – Short Answer](/ap-chem/frqs/frq-7?unit=8): 17 questions

## Practice Questions

### Question 1

A student determines the acid dissociation constant, $$K_a$$, for a weak acid $$HA$$ by measuring the pH of a 0.10 M solution. If the pH meter is improperly calibrated and reads 0.3 units higher than the actual pH, how will the calculated $$K_a$$ be affected?

- (A) Calculated $$K_a$$ is too low because the hydrogen ion concentration is underestimated.
- (B) Calculated $$K_a$$ is too high because the hydrogen ion concentration is underestimated.
- (C) Calculated $$K_a$$ is too low because the hydrogen ion concentration is overestimated.
- (D) Calculated $$K_a$$ is too high because the hydrogen ion concentration is overestimated.

**Answer: (A)** A higher pH reading corresponds to a lower calculated $$[H^+]$$. Since $$K_a \approx [H^+]^2 / [HA]$$, underestimating $$[H^+]$$ significantly reduces the calculated value of $$K_a$$.

### Question 2

A student determines the $$K_a$$ of a weak acid $$HA$$ by measuring the pH of a solution prepared by dissolving a known mass of $$HA$$ in water. If the student unknowingly uses a sample of $$HA$$ that is contaminated with a non-acidic impurity, how will the calculated $$K_a$$ compare to the true $$K_a$$?

- (A) Calculated $$K_a$$ will be lower than true $$K_a$$ because the measured $$[H^+]$$ is lower than expected for the assumed concentration.
- (B) Calculated $$K_a$$ will be higher than true $$K_a$$ because the measured $$[H^+]$$ is higher than expected for the assumed concentration.
- (C) Calculated $$K_a$$ will be lower than true $$K_a$$ because the impurity reduces the degree of dissociation of $$HA$$ through common ion effect.
- (D) Calculated $$K_a$$ will be lower than true $$K_a$$ because the impurity increases the solution volume without increasing the moles of $$HA$$ present.

**Answer: (A)** The impurity means the actual moles of acid are lower than calculated from the mass. The resulting pH is higher (lower $$[H^+]$$) than expected, leading to a smaller numerator in the $$K_a$$ expression.

### Question 3

A student compares the acid dissociation constants ($$K_a$$) of $$HClO$$ and $$HIO$$ to demonstrate the effect of halogen electronegativity on acid strength. The student determines the $$K_a$$ values by measuring the pH of equimolar solutions. If the measured pH of the $$HIO$$ solution is lower than the true value, which experimental error explains this?

- (A) The solution absorbed atmospheric carbon dioxide, producing acidic species.
- (B) The solution absorbed atmospheric carbon dioxide, producing basic species.
- (C) The solution was contaminated with a strong base, increasing the pH of the solution.
- (D) The solution was exposed to air, allowing volatile HIO to escape from the solution.

**Answer: (A)** Recognizing chemical interferences is key to error analysis. Carbon dioxide dissolves in water to form carbonic acid ($$H_2CO_3$$), which dissociates to release $$H^+$$ ions. This additional acidity lowers the pH of the solution, making the weak acid $$HIO$$ appear stronger than it actually is.

### Question 4

A particulate representation of a $$0.5$$ M $$HF$$ solution displays ten $$H^+$$ ions and ten $$F^-$$ ions in a given volume. To what extent does this model accurately describe the particle-level composition determining the solution's acidity?

- (A) It inaccurately depicts the solution by showing equal numbers of H⁺ and F⁻ ions when H⁺ ions should be more abundant.
- (B) It inaccurately depicts the solution by failing to account for the hydration of ions in the aqueous environment.
- (C) It inaccurately depicts the solution by failing to show that most $$HF$$ remains as molecules.
- (D) It inaccurately depicts the solution by showing dissociation products when HF should remain completely undissociated in water.

**Answer: (C)** Evaluating the model against the properties of weak acids reveals the error. $$HF$$ is a weak electrolyte, so the vast majority of particles should be represented as undissociated $$HF$$ molecules, not separate ions.

### Question 5

Diagram A shows 10 molecules of acid HX in water with 1 dissociated molecule. Diagram B shows 10 molecules of acid HY in water with 10 dissociated molecules. Which statement correctly compares the macroscopic properties based on these models?

- (A) Solution B has a lower pH because the concentration of hydronium ions is greater.
- (B) Solution A has a lower pH because the concentration of hydronium ions is greater.
- (C) Solution B has a higher pH because the concentration of hydronium ions is greater.
- (D) Solution A has a higher pH because the concentration of hydronium ions is greater.

**Answer: (A)** Diagram B shows complete dissociation (strong acid), resulting in a higher concentration of H3O+ ions compared to the partial dissociation in Diagram A. Higher [H3O+] corresponds to a lower pH.

### Question 6

A student creates particulate models for $$0.10$$ M $$HCl(aq)$$ and $$0.10$$ M $$HC_2H_3O_2(aq)$$. Which prediction best describes the distribution of solute particles in the two models?

- (A) $$HCl$$ shows mostly ions; $$HC_2H_3O_2$$ shows mostly molecules.
- (B) $$HCl$$ shows mostly molecules; $$HC_2H_3O_2$$ shows mostly ions.
- (C) Both models show mostly ions due to similar concentrations.
- (D) Both models show mostly molecules due to strong bonding.

**Answer: (A)** Predicting the dissociation behavior requires distinguishing between strong and weak electrolytes. $$HCl$$ is a strong acid that fully dissociates into ions, while $$HC_2H_3O_2$$ is a weak acid that remains primarily as intact molecules.

### Question 7

A student intends to draw a particulate representation of a 0.10 M aqueous solution of nitrous acid ($$HNO_2$$). To calculate the correct ratio of $$HNO_2$$ molecules to $$NO_2^-$$ ions to depict in the particulate drawing, which value is required?

- (A) The acid dissociation constant ($$K_a$$) of $$HNO_2$$
- (B) The molar mass of the $$HNO_2$$ molecules
- (C) The solubility product ($$K_{sp}$$) of $$HNO_2$$
- (D) The boiling point elevation constant ($$K_b$$)

**Answer: (A)** Determining the extent of dissociation for a weak acid requires the equilibrium constant ($$K_a$$). This value allows for the calculation of equilibrium concentrations of the molecular acid and its conjugate base, establishing the ratio needed for the drawing.

### Question 8

A particulate representation of acid HA shows 100% ionization, while a representation of acid HB shows only 5% ionization. Which conclusion about the relative strengths of these acids is supported by the models?

- (A) HA is a strong acid, while HB is a weak acid.
- (B) HA is a weak acid, while HB is a strong acid.
- (C) HA is a strong acid, while HB is a non-electrolyte.
- (D) HA is a weak acid, while HB is a non-electrolyte.

**Answer: (A)** Linking the degree of ionization to acid strength allows for classification. Complete (100%) ionization is the defining characteristic of a strong acid, while partial (5%) ionization characterizes a weak acid.

### Question 9

A student represents a $$1.0\ M$$ solution of hydrofluoric acid ($$HF$$) by drawing 10 dissociated $$H^+$$ ions and 10 dissociated $$F^-$$ ions with no intact $$HF$$ molecules. Why is this model inconsistent with the chemical behavior of hydrofluoric acid?

- (A) HF is a weak acid that exists primarily as undissociated molecules in solution.
- (B) HF is a strong acid that completely dissociates into H+ and F- ions in solution.
- (C) HF is a weak acid but exists primarily as dissociated ions due to the high electronegativity of fluorine.
- (D) HF is a weak acid that forms hydrogen bonds with water molecules instead of dissociating into ions.

**Answer: (A)** Classifying electrolytes is necessary to evaluate the extent of dissociation. $$HF$$ is a weak acid, meaning it only partially dissociates; a correct model would show mostly intact $$HF$$ molecules and very few ions.

### Question 10

A 0.10 M solution of acid HX has a pH of 1.0, while a 0.10 M solution of acid HY has a pH of 4.0. Which claim about the particulate composition of these solutions is best supported by the data?

- (A) HX exists primarily as separated ions, whereas HY exists primarily as intact molecules.
- (B) HX exists primarily as intact molecules, whereas HY exists primarily as separated ions.
- (C) HX exists primarily as separated ions, whereas HY exists primarily as separated ions.
- (D) HX exists primarily as intact molecules, whereas HY exists primarily as intact molecules.

**Answer: (A)** Interpreting the pH data shows that HX (pH 1.0) fully dissociates into H+ ions (strong acid behavior), while HY (pH 4.0) produces few H+ ions, indicating it remains mostly as undissociated molecules.

## Review the Unit

- [8.1 Introduction to Acids and Bases](/ap-chem/unit-8/intro-acids-bases/study-guide/rG2ZBgD9evdBohz4a7Mj)
- [8.2 pH and pOH of Strong Acids and Bases](/ap-chem/unit-8/ph-poh-strong-acids-bases/study-guide/AhVlrEQS1kkfZGGWdFNT)
- [8.3 Weak Acid and Base Equilibria](/ap-chem/unit-8/weak-acid-base-equilibria/study-guide/J3ggVgoQIyMYhq5nEKuw)
- [8.4 Acid-Base Reactions and Buffers ](/ap-chem/unit-8/acid-base-reactions-buffers/study-guide/aXiB6ONME0VEX1JR9Kwh)
- [8.5 Acid-Base Titrations](/ap-chem/unit-8/acid-base-titrations/study-guide/5ugPp0ykDKthSY3MiYan)
- [8.6 Molecular Structure of Acids and Bases](/ap-chem/unit-8/molecular-structures-acids-bases/study-guide/NVHPDVrJsTLaLzaOEL1i)
- [8.7 pH and pKa](/ap-chem/unit-8/ph-pka/study-guide/yKYJCv37E6gkI01yeRwG)
- [8.8 Properties of Buffers](/ap-chem/unit-8/properties-buffers/study-guide/PlRbvlggdbKMOXSUWfmD)
- [8.9 Henderson-Hasselbalch Equation](/ap-chem/unit-8/henderson-hasselbalch-equation/study-guide/9jNGg5JlJAYF5QilwfJW)
- [8.10 Buffer Capacity](/ap-chem/unit-8/buffer-capacity/study-guide/TRzbdif6DAyXpITT77wu)
- [8.11 pH and Solubility](/ap-chem/unit-8/ph-solubility/study-guide/QD1VMGuBFQJ1Rcw5ifOV)

## Other Unit Tests

- [Unit 1: Atomic Structure & Properties](/ap-chem/unit-test/1)
- [Unit 2: Compound Structure and Properties](/ap-chem/unit-test/2)
- [Unit 3: Properties of Substances and Mixtures](/ap-chem/unit-test/3)
- [Unit 4: Chemical Reactions](/ap-chem/unit-test/4)
- [Unit 5: Kinetics](/ap-chem/unit-test/5)
- [Unit 6: Thermochemistry](/ap-chem/unit-test/6)
- [Unit 7: Equilibrium](/ap-chem/unit-test/7)
- [Unit 9: Thermodynamics and Electrochemistry](/ap-chem/unit-test/9)

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