Acids & Bases: pH practice questions

Question 1

An aqueous solution contains dihydrogen phosphate ions and ammonia reacting according to the equilibrium below.

$\text{H}_2\text{PO}_4^-\text{(aq)} + \text{NH}_3\text{(aq)} \rightleftharpoons \text{HPO}_4^{2-}\text{(aq)} + \text{NH}_4^+\text{(aq)}$

Which of the following correctly identifies a Brønsted-Lowry acid and its conjugate base in this reaction?

Accepted Answer

Acid: $\text{H}_2\text{PO}_4^-\text{(aq)}$; Conjugate base: $\text{HPO}_4^{2-}\text{(aq)}$

Answer

Acid: $\text{NH}_3\text{(aq)}$; Conjugate base: $\text{NH}_4^+\text{(aq)}$

Answer

Acid: $\text{H}_2\text{PO}_4^-\text{(aq)}$; Conjugate base: $\text{NH}_4^+\text{(aq)}$

Answer

Acid: $\text{HPO}_4^{2-}\text{(aq)}$; Conjugate base: $\text{H}_2\text{PO}_4^-\text{(aq)}$

Question 2

A student prepares an aqueous solution of lactic acid, $\text{HC}_3\text{H}_5\text{O}_3\text{(aq)}$ ($p K_a = 3.86$), and adjusts its pH to $5.20$ by adding a small amount of $\text{NaOH(aq)}$. Which species, $\text{HC}_3\text{H}_5\text{O}_3\text{(aq)}$ or $\text{C}_3\text{H}_5\text{O}_3^-\text{(aq)}$, is present at a higher concentration in the solution, and why?

Accepted Answer

$\text{C}_3\text{H}_5\text{O}_3^-\text{(aq)}$, because the solution pH is greater than the $p K_a$, indicating that the deprotonated form predominated.

Answer

$\text{HC}_3\text{H}_5\text{O}_3\text{(aq)}$, because the solution pH is higher than the $p K_a$, indicating that protonation of the acid is favored.

Answer

$\text{HC}_3\text{H}_5\text{O}_3\text{(aq)}$, because lactic acid is a weak acid that only partially dissociates, so the protonated form remains predominant at any pH.

Answer

$\text{C}_3\text{H}_5\text{O}_3^-\text{(aq)}$, because the solution pH is greater than the $p K_a$, which means that $[\text{H}^+] > K_a$.

Question 3

A student prepares an aqueous solution of a weak monoprotic acid, $\text{HA(aq)}$. The particulate diagram below represents a sample of the solution after chemical equilibrium is established. Water molecules are omitted for clarity.

Based on the representation, what is the percent ionization of $\text{HA}$ in this solution?

Accepted Answer

$20\%$

Answer

$25\%$

Answer

$80\%$

Answer

$100\%$

Question 4

A student compares two monoprotic weak acids, $\text{HA(aq)}$ and $\text{HB(aq)}$, at $25^\circ\text{C}$. The acid ionization constant $K_a$ for $\text{HA}$ is $1.0 \times 10^{-4}$, and the $K_a$ for $\text{HB}$ is $1.0 \times 10^{-6}$. What is the value of the ratio $\dfrac{K_b(\text{A}^-)}{K_b(\text{B}^-)}$ for their conjugate bases at $25^\circ\text{C}$?

Accepted Answer

$1.0 \times 10^{-2}$

Answer

$1.0 \times 10^{-1}$

Answer

$1.0 \times 10^1$

Answer

$1.0 \times 10^2$

Question 5

A student prepares a buffer solution containing equal molar amounts of a weak acid, $\text{HA}$, and its conjugate base, $\text{A}^-$. Which of the following statements best explains why an equimolar mixture of $\text{HA}$ and $\text{A}^-$ achieves maximum buffer capacity for a fixed total solute concentration?

Accepted Answer

It provides maximum resistance to pH changes from both added acid and added base, because equal amounts of $\text{HA}$ (to react with added $\text{OH}^-$) and $\text{A}^-$ (to react with added $\text{H}_3\text{O}^+$) are present.

Answer

It provides maximum resistance to pH changes from both added acid and added base, because $\text{HA}$ neutralizes added $\text{H}_3\text{O}^+$ while $\text{A}^-$ neutralizes added $\text{OH}^-$.

Answer

It completely prevents any change in pH when acid or base is added, because $\text{HA}$ and $\text{A}^-$ continuously neutralize each other in solution.

Answer

It completely prevents any change in pH when acid or base is added, because an equimolar buffer solution always has a neutral pH equal to 7.0.

Question 6

A student prepares a solution by dissolving $\text{H}_2\text{SO}_4\text{(aq)}$ in distilled water to give a final concentration of $0.0050 \text{ M}$. Assuming that both protons dissociate completely in this dilute solution, what is the pH of the solution at $25^\circ\text{C}$?

Accepted Answer

$2.00$

Answer

$2.30$

Answer

$3.00$

Answer

$12.00$

Question 7

A student prepares a solution by mixing equal volumes of solution containing equal concentrations of standard aqueous solutions, specifically mixing equal volumes of stoichiometric equivalents of a weak monoprotic acid, 
$\text{HC}_2\text{H}_3\text{O}_2\text{(aq)}$, and a strong base, $\text{NaOH(aq)}$, at $25^\circ\text{C}$. Which of the following correctly predicts whether the resulting solution is acidic, neutral, or basic, and provides the correct justification?

Accepted Answer

The solution is basic ($\text{pH} > 7.0$) because the conjugate base reacts with water according to $\text{C}_2\text{H}_3\text{O}_2^-\text{(aq)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{HC}_2\text{H}_3\text{O}_2\text{(aq)} + \text{OH}^-\text{(aq)}$.

Answer

The solution is neutral ($\text{pH} = 7.0$) because equal moles of acid and base react completely to form a neutral salt and water.

Answer

The solution is acidic ($\text{pH} < 7.0$) because $\text{HC}_2\text{H}_3\text{O}_2\text{(aq)}$ is a weak acid that does not react completely with $\text{NaOH(aq)}$, leaving excess $\text{H}^+\text{(aq)}$ in solution.

Answer

The solution is basic ($\text{pH} > 7.0$) because $\text{NaOH(aq)}$ is a strong base that dissociates completely, leaving unreacted $\text{OH}^-\text{(aq)}$ in excess.

Question 8

A student titrates a sample of an unknown monoprotic weak acid with standard $0.10 \text{ M NaOH(aq)}$ using a pH meter and generates the titration curve shown. The student plans to repeat the experiment using a chemical indicator to signal the endpoint instead of using a pH meter. Based on the titration curve, which of the following correctly identifies the volume of $\text{NaOH(aq)}$ required to reach the equivalence point and selects the most appropriate indicator for the titration?

Accepted Answer

Volume: $25.0 \text{ mL}$; Indicator: Phenolphthalein (pH range 8.2–10.0), because its color-change range coincides with the steep pH jump surrounding the equivalence point.

Answer

Volume: $12.5 \text{ mL}$; Indicator: Methyl red (pH range 4.4–6.2), because the equivalence point occurs where the pH changes most gradually in the buffer region.

Answer

Volume: $25.0 \text{ mL}$; Indicator: Methyl red (pH range 4.4–6.2), because its color-change range matches the initial pH range of the weak acid solution.

Answer

Volume: $50.0 \text{ mL}$; Indicator: Phenolphthalein (pH range 8.2–10.0), because the equivalence point is reached when the solution pH levels off at its maximum value.

Question 9

A student has two separate saturated aqueous solutions at equilibrium: Beaker 1 contains solid calcium carbonate, $\text{CaCO}_3\text{(s)}$, in contact with its ions, and Beaker 2 contains solid silver chloride, $\text{AgCl(s)}$, in contact with its ions. A few drops of concentrated nitric acid, $\text{HNO}_3\text{(aq)}$, are added to each beaker, significantly decreasing the pH of both solutions without noticeably changing the total volume. Which of the following predictions best describes the effect of decreasing the pH on the solubility of each solid?

Accepted Answer

The solubility of $\text{CaCO}_3\text{(s)}$ increases because $\text{CO}_3^{2-}\text{(aq)}$ reacts with $\text{H}^+\text{(aq)}$, whereas the solubility of $\text{AgCl(s)}$ remains unchanged because $\text{Cl}^-\text{(aq)}$ is the conjugate base of a strong acid.

Answer

The solubility of both $\text{CaCO}_3\text{(s)}$ and $\text{AgCl(s)}$ increases because $\text{H}^+\text{(aq)}$ reacts with the anions in both solutions to form weak acids.

Answer

The solubility of $\text{CaCO}_3\text{(s)}$ remains unchanged, whereas the solubility of $\text{AgCl(s)}$ increases because $\text{Ag}^+\text{(aq)}$ forms a soluble complex with nitrate ions.

Answer

The solubility of both $\text{CaCO}_3\text{(s)}$ and $\text{AgCl(s)}$ remains unchanged because $\text{H}^+\text{(aq)}$ does not react with metal cations in aqueous solution.

Question 10

A student attempts to prepare a buffer solution by mixing equal volumes of two $0.20 \, \text{M}$ aqueous solutions. Which of the following pairs of solutions will produce an effective buffer solution?

Accepted Answer

$0.20 \, \text{M} \, \text{HF(aq)}$ and $0.20 \, \text{M} \, \text{NaF(aq)}$

Answer

$0.20 \, \text{M} \, \text{HCl(aq)}$ and $0.20 \, \text{M} \, \text{NaOH(aq)}$

Answer

$0.20 \, \text{M} \, \text{HF(aq)}$ and $0.20 \, \text{M} \, \text{NaOH(aq)}$

Answer

$0.20 \, \text{M} \, \text{HCl(aq)}$ and $0.20 \, \text{M} \, \text{NaF(aq)}$

Question 11

A student prepares a solution of acetic acid, $\text{CH}_3\text{COOH(aq)}$, by dissolving enough acid in water to create a $0.18 \text{ M}$ solution at $25^\circ\text{C}$. Given that the acid dissociation constant for acetic acid is $K_a = 1.8 \times 10^{-5}$ at $25^\circ\text{C}$, what is the percent ionization of acetic acid in this solution?

Accepted Answer

$1.0\%$

Answer

$0.0018\%$

Answer

$0.10\%$

Answer

$10\%$

Question 12

A student prepares a buffer solution by mixing $100. \, \text{mL}$ of a $1.0 \, \text{M}$ solution of a weak acid, $\text{HA}$ ($K_a = 1.0 \times 10^{-4}$), with $100. \, \text{mL}$ of a $0.10 \, \text{M}$ solution of $\text{NaA}$. What is the pH of the resulting buffer solution at $25^\circ\text{C}$?

Accepted Answer

$3.00$

Answer

$4.00$

Answer

$5.00$

Answer

$11.00$

Question 13

A student analyzing proton transfer in a water sample represents the reaction as follows.

$\text{H}_2\text{PO}_4^-\text{(aq)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{H}_3\text{O}^+\text{(aq)} + \text{HPO}_4^{2-}\text{(aq)}$

Which choice correctly identifies both the acid and its conjugate base and the base and its conjugate acid, with the species in each pair listed in that order for the forward reaction?

Accepted Answer

Acid/conjugate base: $\text{H}_2\text{PO}_4^-\text{(aq)}$, $\text{HPO}_4^{2-}\text{(aq)}$; base/conjugate acid: $\text{H}_2\text{O(l)}$, $\text{H}_3\text{O}^+\text{(aq)}$

Answer

Acid/conjugate base: $\text{H}_3\text{O}^+\text{(aq)}$, $\text{H}_2\text{O(l)}$; base/conjugate acid: $\text{HPO}_4^{2-}\text{(aq)}$, $\text{H}_2\text{PO}_4^-\text{(aq)}$

Answer

Acid/conjugate base: $\text{H}_2\text{PO}_4^-\text{(aq)}$, $\text{HPO}_4^{2-}\text{(aq)}$; base/conjugate acid: $\text{HPO}_4^{2-}\text{(aq)}$, $\text{H}_2\text{PO}_4^-\text{(aq)}$

Answer

Acid/conjugate base: $\text{H}_3\text{O}^+\text{(aq)}$, $\text{H}_2\text{O(l)}$; base/conjugate acid: $\text{H}_2\text{O(l)}$, $\text{H}_3\text{O}^+\text{(aq)}$

Question 14

A student titrates a $50.0\text{ mL}$ sample of a weak monoprotic acid, $\text{HA(aq)}$, with $0.100\text{ M }\text{NaOH(aq)}$. The titration curve for the process is shown below. Based on the titration curve, which of the following correctly identifies the approximate $K_a$ of $\text{HA}$ and provides the correct justification?

Accepted Answer

$1.0 \times 10^{-5}$, because at $20.0\text{ mL}$ of added $\text{NaOH(aq)}$, $[\text{HA}] = [\text{A}^-]$ and $\text{pH} = p K_a = 5.0$.

Answer

$1.0 \times 10^{-5}$, because at $40.0\text{ mL}$ of added $\text{NaOH(aq)}$, $[\text{HA}] = [\text{A}^-]$ and $\text{pH} = p K_a = 5.0$.

Answer

$1.0 \times 10^{-9}$, because at $40.0\text{ mL}$ of added $\text{NaOH(aq)}$, the equivalence point is reached where $\text{pH} = p K_a = 9.0$.

Answer

$1.0 \times 10^{-3}$, because at $0.0\text{ mL}$ of added $\text{NaOH(aq)}$, the initial $\text{pH}$ of the acid solution equals its $p K_a$.

Question 15

A student analyzes a treated-water sample at $25^\circ\text{C}$. The hydroxide ion concentration of the sample is $2.0 \times 10^{-4}\text{ M}$. At this temperature, $K_w=[\text{H}_3\text{O}^+][\text{OH}^-]=1.0 \times 10^{-14}$.

What is the hydronium ion concentration of the sample?

Accepted Answer

$5.0 \times 10^{-11}\text{ M}$

Answer

$2.0 \times 10^{-18}\text{ M}$

Answer

$5.0 \times 10^{-15}\text{ M}$

Answer

$2.5 \times 10^{-11}\text{ M}$

Question 16

A student titrates a sample of a weak diprotic acid, $\text{H}_2\text{A}$, with $0.10 \text{ M NaOH}$. The titration curve generated from the experimental data is shown below. Based on the titration curve, which of the following best identifies the value of $pK_{a2}$ for $\text{H}_2\text{A}$ and the reasoning used to determine it?

Accepted Answer

$7.2$, because it is the pH at the second half-equivalence point at $30.0 \text{ mL}$ of added $\text{NaOH}$

Answer

$3.5$, because it is the pH at the first half-equivalence point at $10.0 \text{ mL}$ of added $\text{NaOH}$

Answer

$6.0$, because it is the pH at the first equivalence point at $20.0 \text{ mL}$ of added $\text{NaOH}$

Answer

$10.0$, because it is the pH at the second equivalence point at $40.0 \text{ mL}$ of added $\text{NaOH}$

Question 17

A student prepares a buffer containing comparable amounts of aqueous ammonia, $\text{NH}_3\text{(aq)}$, and ammonium chloride, $\text{NH}_4\text{Cl(aq)}$. The student then adds a small amount of hydrochloric acid, $\text{HCl(aq)}$, without exceeding the buffer capacity. Which of the following is the net ionic equation for the principal reaction that occurs?

Accepted Answer

$\text{H}_3\text{O}^+\text{(aq)}+\text{NH}_3\text{(aq)}\rightarrow\text{NH}_4^+\text{(aq)}+\text{H}_2\text{O(l)}$

Answer

$\text{H}_3\text{O}^+\text{(aq)}+\text{OH}^-\text{(aq)}\rightarrow 2\text{H}_2\text{O(l)}$

Answer

$\text{NH}_4^+\text{(aq)}+\text{H}_2\text{O(l)}\rightarrow\text{NH}_3\text{(aq)}+\text{H}_3\text{O}^+\text{(aq)}$

Answer

$\text{H}_3\text{O}^+\text{(aq)}+\text{Cl}^-\text{(aq)}+\text{NH}_3\text{(aq)}\rightarrow\text{NH}_4^+\text{(aq)}+\text{Cl}^-\text{(aq)}+\text{H}_2\text{O(l)}$

Question 18

A student prepares a buffer at $25^\circ\text{C}$ by mixing the solutions shown in the table. The $pK_a$ of $\text{HA(aq)}$ is $5.00$.

| Solution | Concentration | Volume |
|----------|---------------|--------|
| $\text{HA(aq)}$ | $0.300\text{ M}$ | $50.0\text{ mL}$ |
| $\text{NaOH(aq)}$ | $0.100\text{ M}$ | $50.0\text{ mL}$ |

The $\text{OH}^-\text{(aq)}$ reacts completely with $\text{HA(aq)}$ according to $\text{HA(aq)}+\text{OH}^-\text{(aq)}\rightarrow\text{A}^-\text{(aq)}+\text{H}_2\text{O(l)}$. Which of the following correctly predicts the pH of the resulting buffer relative to the $pK_a$ of $\text{HA(aq)}$?

Accepted Answer

The pH is less than $pK_a$, because neutralization leaves twice as many moles of $\text{HA(aq)}$ as the moles of $\text{A}^-\text{(aq)}$ produced.

Answer

The pH is greater than $pK_a$, because producing some $\text{A}^-\text{(aq)}$ makes the conjugate base the predominant buffer component.

Answer

The pH is equal to $pK_a$, because mixing equal volumes produces equal amounts of $\text{HA(aq)}$ and $\text{A}^-\text{(aq)}$.

Answer

The pH is less than $pK_a$, because doubling the total volume decreases the $K_a$ of $\text{HA(aq)}$.

Acids & Bases: pH Multiple Choice Questions & Quizzes

Where This Topic Shows Up

Find More Questions On Related Topics

Practice Questions

Quizzes