AP Chemistry Unit 8: Acids and Bases Practice Exam

Assessment for Unit 8: Acids and Bases

Estimated Time: 60 minutes

Exam Details

Questions: 30 total (30 multiple choice)
Time: 60 minutes (recommended)
Topics Covered: Unit 8: Acids and Bases
Format: Multiple Choice questions matching real exam difficulty
Scoring: Instant results with detailed explanations

A. Multiple-Choice Questions (30 Questions)

Select the one best answer for each question.

1. A student is reviewing the autoionization of water at 25°C. $2H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)$ At 25°C, $K_w = 1.0\times 10^{-14}$. Which of the following sets of values is consistent with a neutral aqueous solution made from pure water at 25°C? [Skill: 4.A | Topic: 8.1]

(A)$[H_3O^+] = 1.0\times 10^{-7}\,M$, $[OH^-] = 1.0\times 10^{-7}\,M$, pH = 7.00, pOH = 7.00(B)$[H_3O^+] = 1.0\times 10^{-14}\,M$, $[OH^-] = 1.0\times 10^{-14}\,M$, pH = 14.00, pOH = 14.00(C)$[H_3O^+] = 7.0\,M$, $[OH^-] = 7.0\,M$, pH = 1.0$\times $10^{-7}, pOH = 1.0$\times $10^{-7}(D)$[H_3O^+] = 1.0\times 10^{-7}\,M$, $[OH^-] = 1.0\times 10^{-14}\,M$, pH = 7.00, pOH = 14.00

2. A student investigates how the ion-product constant of water changes with temperature. The student uses published values for $K_w$ and assumes a sample of pure water is neutral at each temperature. Data: At 50°C, $K_w = 5.5\times 10^{-14}$. What is the pH of pure, neutral water at 50°C? [Skill: 4.A | Topic: 8.1]

(A)6.63(B)7.00(C)7.37(D)13.26

Refer to the figure below.

3. A student records the pH of a sample of pure water at different temperatures and obtains the trend shown. Which of the following best explains why the pH of pure water can be less than 7.0 at higher temperatures even when the water is neutral? [Skill: 3.A | Topic: 8.1]

(A)At higher temperatures, $K_w$ increases, causing both $[H_3O^+]$ and $[OH^-]$ to increase equally; therefore $[H_3O^+] = [OH^-]$ but pH decreases.(B)At higher temperatures, water becomes acidic because $[H_3O^+]$ increases while $[OH^-]$ decreases, so neutrality is lost.(C)At higher temperatures, pH decreases because dissolved $CO_2$ from the air forms carbonic acid, making pure water neutral but with pH below 7.(D)At higher temperatures, $K_w$ decreases, causing $[H_3O^+]$ to decrease and pH to decrease.

4. [Skill: 2B | Topic: 8.10] Two buffer solutions are prepared at 25°C using the conjugate acid-base pair $HA/A^-$. The initial concentrations are shown. Buffer 1: $[HA]=0.20\ \mathrm{M}$ and $[A^-]=0.20\ \mathrm{M}$ Buffer 2: $[HA]=0.020\ \mathrm{M}$ and $[A^-]=0.020\ \mathrm{M}$ Equal volumes of Buffer 1 and Buffer 2 are each treated with the same small amount of strong acid (same moles of $H^+$ added). Which statement is correct about the buffers’ initial pH and their relative buffer capacities?

(A)Buffer 1 has a higher initial pH and a higher buffer capacity than Buffer 2.(B)Buffer 1 has the same initial pH as Buffer 2 but a higher buffer capacity than Buffer 2.(C)Buffer 1 has the same initial pH as Buffer 2 but a lower buffer capacity than Buffer 2.(D)Buffer 1 has a lower initial pH and a higher buffer capacity than Buffer 2.

5. [Skill: 6A | Topic: 8.10] A student prepares two buffers at 25°C using the conjugate acid-base pair $HA/A^-$. Each buffer has the same total volume. Buffer X: $[HA]=0.10\ \mathrm{M}$ and $[A^-]=0.30\ \mathrm{M}$ Buffer Y: $[HA]=0.30\ \mathrm{M}$ and $[A^-]=0.10\ \mathrm{M}$ Which statement correctly compares the buffers’ capacities to neutralize added strong acid and added strong base?

(A)Buffer X has greater capacity for added strong acid, and Buffer Y has greater capacity for added strong base.(B)Buffer X has greater capacity for added strong base, and Buffer Y has greater capacity for added strong acid.(C)Buffer X has greater capacity for both added strong acid and added strong base because it contains more total buffer component.(D)Buffer X and Buffer Y have equal capacities for added strong acid and for added strong base because they have the same total buffer component concentration.

Refer to the figure below.

6. [Skill: 4A | Topic: 8.10] Two buffers, Buffer P and Buffer Q, are made from the same conjugate acid-base pair $HA/A^-$. Each buffer has the same initial pH. A student adds increasing amounts of strong acid ($HCl$) to equal volumes of each buffer and records the pH after each addition. The results are shown in a graph of pH vs. moles of $HCl$ added. One curve decreases in pH much more slowly as $HCl$ is added (flatter curve), while the other curve decreases more rapidly (steeper curve). Which claim is best supported by the graph?

(A)The buffer with the flatter curve has higher concentrations of $HA$ and $A^-$ (greater buffer capacity) than the buffer with the steeper curve.(B)The buffer with the flatter curve has a larger $[A^-]/[HA]$ ratio than the buffer with the steeper curve.(C)The buffer with the flatter curve has a lower initial pH than the buffer with the steeper curve.(D)The buffer with the steeper curve contains more conjugate base, so it better neutralizes added $H^+$.

7. [Skill: 2.A | Topic: 8.11] A student measures the molar solubility of three ionic solids in water at 25°C and then repeats the measurements after adjusting the solutions to pH 2 and pH 12. The results are shown. Salt I: NaNO3 (solubility at pH 2 ≈ solubility at pH 7 ≈ solubility at pH 12) Salt II: CaCO3 (solubility at pH 2 > solubility at pH 7 > solubility at pH 12) Salt III: Mg(OH)2 (solubility at pH 2 > solubility at pH 7 > solubility at pH 12) Which salt’s solubility is expected to be least sensitive to changes in pH, and why?

(A)Salt I, because its ions are the conjugates of a strong acid and a strong base and therefore do not react appreciably with added H+ or OH−.(B)Salt I, because Na+ reacts with added OH− to form NaOH(aq), pulling the dissolution equilibrium to the right.(C)Salt II, because CO3^2− is amphoteric and buffers the solution against changes in pH, keeping solubility constant.(D)Salt III, because OH− is produced only by strong bases and therefore Mg(OH)2(s) does not respond to changes in pH.

8. [Skill: 6.B | Topic: 8.11] Consider a saturated solution of calcium fluoride at 25°C. Relevant equilibria: $CaF_2(s) \rightleftharpoons Ca^{2+}(aq) + 2F^-(aq)$ $HF(aq) \rightleftharpoons H^+(aq) + F^-(aq)$ A small amount of strong acid is added to the saturated solution while keeping the temperature constant. Which of the following best predicts and justifies the effect on the molar solubility of $CaF_2(s)$?

(A)The solubility increases because added H+ reacts with F− to form HF, reducing [F−] and shifting $CaF_2(s)$ dissolution to the right.(B)The solubility decreases because added H+ increases [H+], shifting the HF equilibrium to the right and producing more F−, which shifts $CaF_2(s)$ dissolution to the left.(C)The solubility is unchanged because Ksp depends only on temperature and therefore adding acid cannot change how much dissolves.(D)The solubility decreases because adding acid increases ionic strength, which always decreases the solubility of ionic solids.

Refer to the figure below.

9. [Skill: 4.A | Topic: 8.11] A student investigates how pH affects the solubility of an ionic compound $MX(s)$ in water at 25°C. The student measures the molar solubility of $MX(s)$ at different pH values and obtains the trend shown in the graph. Which identity of the ion X− is most consistent with the data?

(A)$NO_3^-$(B)$ClO_4^-$(C)$CH_3COO^-$(D)$Br^-$

10. **[Skill: 2.A | Topic: 8.2]** A student prepares an aqueous solution by dissolving enough HCl(s) to make a final solution volume of 250.0 mL with an HCl concentration of 0.0200 M. Assume HCl is a strong acid that completely ionizes in water. What is the pH of the solution?

(A)1.70(B)2.00(C)12.30(D)0.0200

11. **[Skill: 2.A | Topic: 8.2]** A student transfers 25.0 mL of 0.150 M $Sr(OH)_2(aq)$ into a 100.0 mL volumetric flask and dilutes to the mark with water. Assume $Sr(OH)_2$ is a strong base that completely dissociates. What is the pH of the diluted solution?

(A)12.88(B)12.57(C)13.48(D)1.12

12. **[Skill: 2.A | Topic: 8.2]** Four 1.0 L beakers contain the following aqueous solutions. Assume all listed acids and bases are strong and completely ionize/dissociate. - Beaker 1: 0.010 M HBr - Beaker 2: 0.0050 M $Ca(OH)_2$ - Beaker 3: 0.020 M KOH - Beaker 4: 0.010 M $HNO_3$ Which beaker has the greatest pH?

(A)Beaker 1(B)Beaker 2(C)Beaker 3(D)Beaker 4

13. [Skill: 4.A | Topic: 8.3] A student prepares 0.10 M acetic acid, $HC_2H_3O_2(aq)$, at 25°C. The acid dissociation constant is $K_a=1.8\times 10^{-5}$. Assuming acetic acid is the only significant source of $H_3O^+$ and that the equilibrium approximation is valid, which of the following is the best estimate of the pH of the solution?

(A)2.87(B)3.74(C)4.74(D)11.13

14. [Skill: 4.A | Topic: 8.3] A monoprotic weak acid $HX$ has $\text{p}K_a=4.20$ at 25°C. Two solutions are prepared using the same acid: Solution 1: 0.10 M $HX$ Solution 2: 0.0010 M $HX$ Assuming the equilibrium approximation is valid for both solutions, which statement best compares the percent ionization of $HX$ in the two solutions?

(A)Solution 1 has the greater percent ionization because it has the greater initial acid concentration.(B)Solution 2 has the greater percent ionization because percent ionization increases as the initial concentration decreases.(C)Both solutions have the same percent ionization because $\text{p}K_a$ is the same.(D)Both solutions have approximately 100% ionization because $\text{p}K_a$ is less than 7.

15. [Skill: 4.A | Topic: 8.3] A student prepares 0.10 M sodium acetate, $NaC_2H_3O_2(aq)$, at 25°C. The acetate ion, $C_2H_3O_2^-$, is the conjugate base of acetic acid, which has $K_a=1.8\times10^{-5}$. Assuming $K_w=1.0\times10^{-14}$ and that the equilibrium approximation is valid, which of the following is the best estimate of the pH of the sodium acetate solution?

(A)2.87(B)7.00(C)8.87(D)11.13

16. [Skill: 2.B | Topic: 8.4] A student mixes 25.0 mL of 0.100 M HCl(aq) with 35.0 mL of 0.0800 M NaOH(aq) at 25°C. Assume volumes are additive and that both react completely. Which of the following is the best estimate of the pH of the resulting solution? Relevant reaction: $H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$

(A)2.30(B)7.00(C)11.70(D)12.60

17. [Skill: 6.B | Topic: 8.4] A student prepares a mixture by adding 30.0 mL of 0.100 M NaOH(aq) to 50.0 mL of 0.200 M $CH_3COOH$(aq). Assume volumes are additive. Relevant reaction: $CH_3COOH(aq) + OH^-(aq) \rightarrow CH_3COO^-(aq) + H_2O(l)$ Which statement best describes the major species present after the reaction goes to completion and the overall acid-base nature of the resulting solution?

(A)The solution is a buffer because both $CH_3COOH$ and $CH_3COO^-$ are present in significant amounts; $[CH_3COOH] > [CH_3COO^-]$, so $\text{pH} < \text{p}K_a$.(B)The solution is basic because the $OH^-$ added is in excess; significant $OH^-$ remains after reaction.(C)The solution is neutral because a weak acid and a strong base always neutralize to form water and a neutral salt.(D)The solution is acidic because the strong base converts all $CH_3COOH$ to $CH_3COO^-$, and $CH_3COO^-$ hydrolyzes to produce $H_3O^+$.

18. [Skill: 6.C | Topic: 8.4] Equal volumes of 0.10 M HA(aq) and 0.10 M B(aq) are mixed. The weak acid HA has $K_a = 1.0\times 10^{-5}$. The weak base B has $K_b = 1.0\times 10^{-4}$. Consider the reaction: $HA(aq) + B(aq) \rightleftharpoons A^-(aq) + HB^+(aq)$ Which statement best describes (i) whether products or reactants are favored at equilibrium and (ii) whether the resulting solution is acidic or basic?

(A)Reactants are favored because both species are weak; the resulting solution is neutral.(B)Products are favored because $K = \frac{K_aK_b}{K_w} \gg 1$; the resulting solution is slightly basic because $K_b$ for $A^-$ is greater than $K_a$ for $HB^+$.(C)Products are favored because $K = \frac{K_aK_b}{K_w} \gg 1$; the resulting solution is slightly acidic because $HB^+$ is present.(D)Reactants are favored because $K = \frac{K_w}{K_aK_b} \gg 1$; the resulting solution is basic because excess B remains.

19. **1. [Skill: 5.A | Topic: 8.5]** A student performs a titration to determine the concentration of a monoprotic strong acid, $HX(aq)$, of unknown concentration. The student titrates **25.00 mL** of the $HX(aq)$ with **0.250 M** $NaOH(aq)$. The equivalence point is reached after **18.60 mL** of $NaOH(aq)$ is added. Assume the reaction goes to completion and is represented by: $HX(aq) + OH^-(aq) \rightarrow X^-(aq) + H_2O(l)$ What is the concentration of the original $HX(aq)$ solution?

(A)0.0465 M(B)0.186 M(C)0.232 M(D)0.336 M

Refer to the figure below.

20. **2. [Skill: 3.A | Topic: 8.5]** A student titrates **20.0 mL** of a weak monoprotic acid, $HA(aq)$, with **0.100 M** $NaOH(aq)$. The student records pH as a function of volume of $NaOH(aq)$ added. > [Image Cue]: Titration curve, "Titration of weak acid HA with 0.100 M NaOH", x-axis = volume of NaOH added (mL), y-axis = pH. Key labeled points: initial pH = 2.90 at 0 mL; equivalence point at 30.0 mL; at 15.0 mL (half-equivalence) pH = 4.75; after equivalence pH rises sharply. Based on the titration curve, what is the value of $K_a$ for $HA$?

(A)1.8 × 10^-5(B)5.6 × 10^-10(C)3.2 × 10^-4(D)4.75

Refer to the figure below.

21. **3. [Skill: 4.A | Topic: 8.5]** A student titrates **50.0 mL** of a weak diprotic acid, $H_2A(aq)$, with **0.100 M** $NaOH(aq)$ and obtains the titration curve shown. > [Image Cue]: Titration curve, "Titration of diprotic acid H2A with 0.100 M NaOH", x-axis = volume of NaOH added (mL), y-axis = pH. The curve has two distinct steep rises (two equivalence points). Key labeled features: first equivalence point at 25.0 mL; second equivalence point at 50.0 mL; pH at second equivalence point is 9.2. Which of the following best explains why the pH at the **second equivalence point** is greater than 7.0?

(A)At the second equivalence point, excess $NaOH(aq)$ is present, which makes the solution basic.(B)At the second equivalence point, $Na^+(aq)$ hydrolyzes water to produce $OH^-(aq)$, increasing the pH.(C)At the second equivalence point, $A^{2-}(aq)$ acts as a base and reacts with water to form $HA^-(aq)$ and $OH^-(aq)$.(D)At the second equivalence point, the solution contains mostly $H_3O^+(aq)$ because the acid has two ionizable protons.

22. [Skill: 1A | Topic: 8.6] A student prepares four separate 0.10 M aqueous solutions at 25°C and measures the pH values shown. - Solution 1: HCl(aq), pH = 1.00 - Solution 2: HF(aq), pH = 2.10 - Solution 3: HBr(aq), pH = 1.00 - Solution 4: HOCl(aq), pH = 4.30 Which explanation best accounts for HF(aq) having a higher pH than HCl(aq), even though fluorine is more electronegative than chlorine?

(A)The conjugate base of HF is stabilized by resonance, whereas the conjugate base of HCl is not.(B)The H–F bond is weaker than the H–Cl bond, so HF dissociates less in water.(C)The conjugate base $F^-$ is less stable than $Cl^-$ because $F^-$ is smaller and has a more concentrated negative charge.(D)The HF solution must be less concentrated than the HCl solution because strong acids always have lower pH at any concentration.

23. [Skill: 3C | Topic: 8.6] Consider the two acids below. Acid I: acetic acid, $CH_3COOH$ Acid II: ethanol, $CH_3CH_2OH$ In water at 25°C, acetic acid is far more acidic than ethanol. Which statement best justifies this difference in acid strength based on molecular structure?

(A)The conjugate base of acetic acid ($CH_3COO^-$) is stabilized by resonance, while the conjugate base of ethanol ($CH_3CH_2O^-$) is not.(B)Acetic acid is stronger because it contains more oxygen atoms, which directly increases the concentration of $H^+$ in solution.(C)Ethanol is weaker because its conjugate base is stabilized by resonance, making ethanol less willing to donate a proton.(D)Acetic acid is stronger because its conjugate base contains a carbon–oxygen double bond, which makes it more reactive and therefore less stable.

24. [Skill: 2A | Topic: 8.6] A student compares the basicity of three species in water at 25°C: $CH_3COO^-$, $NH_3$, and $OH^-$ (from soluble group 1 hydroxides such as NaOH). Which species is the strongest base in water, and which reasoning best supports the choice?

(A)$CH_3COO^-$, because resonance makes it more reactive and therefore better at accepting $H^+$.(B)$NH_3$, because nitrogen is less electronegative than oxygen, so it always forms stronger bases than any oxygen-containing anion.(C)$OH^-$, because its conjugate acid ($H_2O$) is the weakest acid among the conjugate acids of the three species.(D)$OH^-$, because it is produced only by strong acids and therefore must be a strong base.

25. **1.** [Skill: 4.A | Topic: 8.7] A student is studying the acid-base indicator system $HIn(aq) \rightleftharpoons H^+(aq) + In^-(aq)$. The indicator has $pK_a = 4.2$. - The protonated form, $HIn$, is **yellow**. - The deprotonated form, $In^-$, is **blue**. The indicator is added to a solution with $pH = 3.5$. Which of the following best predicts the predominant form of the indicator and the observed color of the solution?

(A)Predominant form is $HIn$; the solution appears yellow(B)Predominant form is $HIn$; the solution appears blue(C)Predominant form is $In^-$; the solution appears yellow(D)Predominant form is $In^-$; the solution appears blue

Refer to the figure below.

26. **2.** [Skill: 6.A | Topic: 8.7] A student titrates 50.0 mL of a weak acid, $HA(aq)$, with 0.100 M $NaOH(aq)$. The student collects data and constructs the titration curve shown. Based on the titration curve, the pH at the equivalence point is approximately 8.8. Four indicators are available: | Indicator | $pK_a$ | Color change range (approx.) | |---|---:|---| | Methyl orange | 3.7 | 3.1–4.4 | | Bromothymol blue | 7.1 | 6.0–7.6 | | Phenolphthalein | 9.3 | 8.2–10.0 | | Alizarin yellow R | 11.0 | 10.1–12.0 | Which indicator is most appropriate for determining the equivalence point of this titration?

(A)Methyl orange(B)Bromothymol blue(C)Phenolphthalein(D)Alizarin yellow R

27. **1.** [Skill: 4.A | Topic: 8.8] A student prepares a solution by mixing 50.0 mL of 1.0 M $CH_3COOH(aq)$ and 50.0 mL of 1.0 M $CH_3COONa(aq)$, producing a buffered solution. The student then adds 1.0 mL of 1.0 M $HCl(aq)$ and observes that the pH decreases only slightly. Which of the following reactions best represents the primary process that minimizes the pH change when $HCl(aq)$ is added to the buffer?

(A)$CH_3COO^-(aq) + H_3O^+(aq) \rightarrow CH_3COOH(aq) + H_2O(l)$(B)$CH_3COOH(aq) + H_3O^+(aq) \rightarrow CH_3COO^-(aq) + H_2O(l)$(C)$CH_3COOH(aq) + OH^-(aq) \rightarrow CH_3COO^-(aq) + H_2O(l)$(D)$H_3O^+(aq) + OH^-(aq) \rightarrow 2H_2O(l)$

28. **2.** [Skill: 6.A | Topic: 8.8] Two buffers are prepared at 25°C using the $NH_3/NH_4^+$ conjugate acid-base pair. - Buffer 1: 1.00 L containing 0.10 mol $NH_3(aq)$ and 0.10 mol $NH_4Cl(aq)$ - Buffer 2: 1.00 L containing 0.010 mol $NH_3(aq)$ and 0.010 mol $NH_4Cl(aq)$ Each buffer is then treated with 0.0010 mol of $NaOH(aq)$. Which statement best predicts and justifies the difference in pH change between the two buffers after the addition of $NaOH(aq)$?

(A)Buffer 1 will have the smaller pH change because it contains a larger amount of $NH_4^+$, which reacts with added base to form $NH_3$ and $H_2O$.(B)Buffer 2 will have the smaller pH change because its lower concentrations shift the equilibrium to produce more $H_3O^+$, neutralizing the added base.(C)Both buffers will have the same pH change because they start with the same $\dfrac{[NH_3]}{[NH_4^+]}$ ratio, and buffer capacity depends only on this ratio.(D)Buffer 1 will have the larger pH change because its higher concentration increases the reaction rate with $OH^-$, producing a larger immediate increase in pH.

29. [Skill: 2.A | Topic: 8.9] A student prepares a buffer by mixing 50.0 mL of 0.200 M $CH_3COOH(aq)$ with 50.0 mL of 0.300 M $CH_3COONa(aq)$. The $pK_a$ of $CH_3COOH$ is 4.76. Assuming volumes are additive, what is the pH of the resulting buffer solution?

(A)4.58(B)4.76(C)4.94(D)5.24

30. [Skill: 2.A | Topic: 8.9] A buffer is prepared by dissolving 0.100 mol $NH_3(aq)$ and 0.100 mol $NH_4Cl(aq)$ in enough water to make 1.00 L of solution. The $pK_a$ of $NH_4^+$ is 9.25. The student then adds 0.0100 mol of $HCl(aq)$ to the buffer. Assume the volume change is negligible. What is the pH of the solution after the addition of $HCl(aq)$?

(A)9.16(B)9.25(C)9.34(D)8.34

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