Net Ionic Equations practice questions

Question 1

A student performs a gravimetric analysis experiment to determine the mass percent of $\text{Cl}$ in an unknown soluble salt mixture. The student dissolves a $0.500 \text{ g}$ sample of the mixture in distilled water and adds an excess of $\text{AgNO}_3\text{(aq)}$ to precipitate $\text{AgCl(s)}$. The precipitate is collected by filtration and weighed. Which of the following experimental errors would cause the calculated mass percent of $\text{Cl}$ in the mixture to be higher than the actual mass percent?

Accepted Answer

Failing to completely dry the $\text{AgCl(s)}$ precipitate before measuring its final mass

Answer

Adding an insufficient volume of $\text{AgNO}_3\text{(aq)}$, leaving some $\text{Cl}^-\text{(aq)}$ unreacted in solution

Answer

Spilling a small portion of the solution containing the dissolved salt mixture before adding $\text{AgNO}_3\text{(aq)}$

Answer

Rinsing the $\text{AgCl(s)}$ precipitate with distilled water while it is on the filter paper prior to drying

Question 2

A student titrates a $20.00 \text{ mL}$ sample of an $\text{HCl(aq)}$ solution of unknown concentration with a standardized $0.100 \text{ M NaOH(aq)}$ solution using phenolphthalein as an indicator. The student records the buret readings for three trials in the table below.

| Trial | Volume of $\text{HCl(aq)}$ ($\text{mL}$) | Initial buret reading ($\text{mL}$) | Final buret reading ($\text{mL}$) |
|-------|-------------------------------------------|--------------------------------------|------------------------------------|
| 1     | 20.00                                     | 0.50                                 | 10.50                              |
| 2     | 20.00                                     | 10.50                                | 20.50                              |
| 3     | 20.00                                     | 20.50                                | 30.50                              |

Based on the titration data, what is the molar concentration of the $\text{HCl(aq)}$ solution?

Accepted Answer

$0.0500 \text{ M}$

Answer

$0.100 \text{ M}$

Answer

$0.200 \text{ M}$

Answer

$0.400 \text{ M}$

Question 3

A student performs an experiment by adding solid sodium hydrogen carbonate, $\text{NaHCO}_3(s)$, to an aqueous solution of acetic acid, $\text{CH}_3\text{COOH}(aq)$. The student observes rapid formation of gas bubbles, and the temperature of the mixture decreases. Which of the following correctly classifies this process and identifies the primary particulate-level interactions involved?

Accepted Answer

| Classification | Primary Particulate-Level Interactions ||---|---|| Chemical change | Intramolecular chemical bonds broken and formed |

Answer

| Classification | Primary Particulate-Level Interactions ||---|---|| Physical change | Intermolecular forces overcome |

Answer

| Classification | Primary Particulate-Level Interactions ||---|---|| Physical change | Intramolecular chemical bonds broken and formed |

Answer

| Classification | Primary Particulate-Level Interactions ||---|---|| Chemical change | Intermolecular forces overcome |

Question 4

A student mixes 50.0 mL of 0.10 M $\text{AgNO}_3\text{(aq)}$ with 50.0 mL of 0.10 M $\text{NaCl(aq)}$ in a beaker, resulting in the immediate formation of a white precipitate of $\text{AgCl(s)}$. Which of the following identifies the spectator ions in this reaction?

Accepted Answer

$\text{Na}^+\text{(aq)}$ and $\text{NO}_3^-\text{(aq)}$

Answer

$\text{Ag}^+\text{(aq)}$ and $\text{Cl}^-\text{(aq)}$

Answer

$\text{Ag}^+\text{(aq)}$ and $\text{NO}_3^-\text{(aq)}$

Answer

$\text{AgCl(s)}$ and $\text{NaNO}_3\text{(aq)}$

Question 5

A student mixes aqueous sodium carbonate, $\text{Na}_2\text{CO}_3\text{(aq)}$, with excess aqueous hydrochloric acid, $\text{HCl(aq)}$, and observes vigorous bubbling due to the evolution of a gas. Which of the following is the correct balanced chemical equation for the overall reaction?

Accepted Answer

$\text{Na}_2\text{CO}_3\text{(aq)} + 2\text{HCl(aq)} \rightarrow 2\text{NaCl(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$

Answer

$\text{Na}_2\text{CO}_3\text{(aq)} + \text{HCl(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$

Answer

$\text{Na}_2\text{CO}_3\text{(aq)} + 2\text{HCl(aq)} \rightarrow 2\text{NaCl(aq)} + \text{H}_2\text{CO}_3\text{(aq)}$

Answer

$\text{Na}_2\text{CO}_3\text{(aq)} + 2\text{HCl(aq)} \rightarrow 2\text{NaCl(aq)} + \text{H}_2\text{(g)} + \text{CO}_3\text{(g)}$

Question 6

A student tests a water sample containing aqueous sodium sulfate by adding aqueous barium chloride. A white precipitate of barium sulfate forms.

Which of the following is the net ionic equation for the reaction?

Accepted Answer

$\text{Ba}^{2+}\text{(aq)} + \text{SO}_4^{2-}\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)}$

Answer

$\text{Ba}^{2+}\text{(aq)} + \text{SO}_4^{2-}\text{(aq)} \rightarrow \text{BaSO}_4\text{(aq)}$

Answer

$\text{BaCl}_2\text{(aq)} + \text{Na}_2\text{SO}_4\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)} + 2\text{NaCl}\text{(aq)}$

Answer

$\text{Na}^{+}\text{(aq)} + \text{Cl}^{-}\text{(aq)} \rightarrow \text{NaCl}\text{(s)}$

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 mixes $100.0 \text{ mL}$ of $0.200 \text{ M } \text{Pb(NO}_3)_2\text{(aq)}$ with $100.0 \text{ mL}$ of $0.300 \text{ M } \text{KI(aq)}$, resulting in the formation of a yellow precipitate according to the balanced net ionic equation below.

$$\text{Pb}^{2+}\text{(aq)} + 2\,\text{I}^-\text{(aq)} \rightarrow \text{PbI}_2\text{(s)}$$

Assuming the reaction goes to completion, how many moles of the excess reactant remain unreacted?

Accepted Answer

$0.0050 \text{ mol}$

Answer

$0.0100 \text{ mol}$

Answer

$0.0150 \text{ mol}$

Answer

$0.0200 \text{ mol}$

Question 9

A student combines $100.0 \, \text{mL}$ of $0.10 \, \text{M} \, \text{Pb(NO}_3\text{)}_2(\text{aq})$ with $100.0 \, \text{mL}$ of $0.10 \, \text{M} \, \text{KI}(\text{aq})$ at $25^\circ\text{C}$, causing a yellow precipitate of $\text{PbI}_2(\text{s})$ to form. Which of the following predictions correctly ranks the molar concentrations of the ions remaining in the solution from highest to lowest after the reaction goes to completion?

Accepted Answer

$[\text{NO}_3^-] > [\text{K}^+] > [\text{Pb}^{2+}] > [\text{I}^-]$

Answer

$[\text{NO}_3^-] > [\text{K}^+] = [\text{Pb}^{2+}] > [\text{I}^-]$

Answer

$[\text{NO}_3^-] > [\text{Pb}^{2+}] > [\text{K}^+] > [\text{I}^-]$

Answer

$[\text{K}^+] > [\text{NO}_3^-] > [\text{Pb}^{2+}] > [\text{I}^-]$

Question 10

A student mixes $50.0 \, \text{mL}$ of $0.20 \, \text{M}$ $\text{FeCl}_3\text{(aq)}$ with $50.0 \, \text{mL}$ of $0.50 \, \text{M}$ $\text{NaOH}\text{(aq)}$ at $25^\circ\text{C}$, causing a precipitate of $\text{Fe(OH)}_3\text{(s)}$ to form according to the balanced equation below.

$$
\text{FeCl}_3\text{(aq)} + 3\text{NaOH}\text{(aq)} \rightarrow \text{Fe(OH)}_3\text{(s)} + 3\text{NaCl}\text{(aq)}
$$

Which of the following identifies the limiting reactant and provides the correct justification?

Accepted Answer

$\text{NaOH}$, because completely reacting $0.010 \, \text{mol}$ of $\text{FeCl}_3$ requires $0.030 \, \text{mol}$ of $\text{NaOH}$, but only $0.025 \, \text{mol}$ of $\text{NaOH}$ is present.

Answer

$\text{FeCl}_3$, because the initial number of moles of $\text{FeCl}_3$ ($0.010 \, \text{mol}$) is less than the initial number of moles of $\text{NaOH}$ ($0.025 \, \text{mol}$).

Answer

$\text{FeCl}_3$, because $\text{Cl}^-$ ions bind to $\text{Na}^+$ ions in solution, preventing $\text{NaOH}$ from reacting fully with $\text{Fe}^{3+}$ ions.

Answer

$\text{NaOH}$, because the ratio of initial moles of $\text{NaOH}$ to $\text{FeCl}_3$ is $1:3$, which matches the required stoichiometric ratio.

Question 11

A student titrates a 20.0 mL sample of a diprotic acid solution, $\text{H}_2\text{A(aq)}$, with a standardized $0.100 \text{ M NaOH(aq)}$ solution using phenolphthalein as an indicator. The net ionic equation for the reaction that occurs during the titration is shown below.

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

The initial buret reading of $0.100 \text{ M NaOH(aq)}$ is 5.00 mL. At the pale pink endpoint, the final buret reading is 45.00 mL. What is the molarity of the $\text{H}_2\text{A(aq)}$ sample?

Accepted Answer

$0.100 \text{ M}$

Answer

$0.0333 \text{ M}$

Answer

$0.0667 \text{ M}$

Answer

$0.200 \text{ M}$

Question 12

A student evaluates a system that condenses water produced by a small fuel burner. The student burns a $22.8\text{ g}$ sample of pure octane, $\text{C}_8\text{H}_{18}\text{(l)}$, in excess oxygen according to the equation

$$2\text{C}_8\text{H}_{18}\text{(l)}+25\text{O}_2\text{(g)}\rightarrow16\text{CO}_2\text{(g)}+18\text{H}_2\text{O(g)}$$

Assuming complete combustion and complete collection of the condensed water, what mass of water is collected?

Accepted Answer

$32.4\text{ g}$

Answer

$16.2\text{ g}$

Answer

$64.8\text{ g}$

Answer

$324\text{ g}$

Question 13

A student compares the molar solubility of excess solid $\text{Mg(OH)}_2$ in pure water with its molar solubility in a pH-controlled acidic solution at the same temperature. The acidic solution does not initially contain $\text{Mg}^{2+}$. The relevant equilibria are

$\text{Mg(OH)}_2\text{(s)} \rightleftharpoons \text{Mg}^{2+}\text{(aq)} + 2\text{OH}^{-}\text{(aq)}$

$\text{H}_3\text{O}^{+}\text{(aq)} + \text{OH}^{-}\text{(aq)} \rightleftharpoons 2\text{H}_2\text{O(l)}$, which is strongly product-favored.

Which of the following correctly predicts and explains how the molar solubility of $\text{Mg(OH)}_2$ in the acidic solution compares with its molar solubility in pure water?

Accepted Answer

It is higher because $\text{H}_3\text{O}^{+}$ consumes $\text{OH}^{-}$, causing the dissolution equilibrium to shift toward the dissolved ions.

Answer

It is lower because $\text{H}_3\text{O}^{+}$ is a common ion in the dissolution equilibrium and shifts the equilibrium toward the solid.

Answer

It is lower because neutralization removes $\text{OH}^{-}$, causing the dissolution equilibrium to shift toward the solid.

Answer

It is higher because decreasing the pH increases the value of $K_{sp}$ for $\text{Mg(OH)}_2$.

Question 14

A student prepares two acetate buffer mixtures at $25^\circ\text{C}$. Each mixture has a volume of $50.0\text{ mL}$. The student adds $2.00\text{ mL}$ of $0.100\text{ M}$ $\text{NaOH(aq)}$ to each mixture and records the data shown below.

| Mixture | Initial $[\text{CH}_3\text{COOH}]$ | Initial $[\text{CH}_3\text{COO}^-]$ | Initial pH | pH after addition |
|---------|----------------------------------------|-----------------------------------------|------------|-------------------|
| 1 | $0.100\text{ M}$ | $0.100\text{ M}$ | $4.74$ | $4.78$ |
| 2 | $0.0100\text{ M}$ | $0.0100\text{ M}$ | $4.74$ | $5.11$ |

Which statement correctly compares the buffer capacities and explains the results?

Accepted Answer

Mixture 1 has the greater buffer capacity because both mixtures initially have the same $[\text{CH}_3\text{COO}^-]/[\text{CH}_3\text{COOH}]$ ratio, but mixture 1 contains larger amounts of both components, so the added base causes a smaller fractional change in composition.

Answer

Mixture 1 has the greater buffer capacity because its tenfold higher $[\text{CH}_3\text{COO}^-]$ makes the ratio $[\text{CH}_3\text{COO}^-]/[\text{CH}_3\text{COOH}]$ ten times larger than that of mixture 2.

Answer

Mixture 2 has the greater buffer capacity because its larger increase in pH indicates that it neutralized a larger amount of the added base.

Answer

The mixtures have the same buffer capacity because their identical initial pH values indicate that they can neutralize equal amounts of added base.

Question 15

A student tests the response of a water-quality calibration buffer to added strong base. The buffer and added solution have the following properties at $25^\circ\text{C}$.

| Solution | Volume | Solute concentrations |
|---|---:|---|
| Buffer | $50.0\text{ mL}$ | $0.100\text{ M NH}_3$ and $0.500\text{ M NH}_4\text{Cl}$ |
| Added solution | $50.0\text{ mL}$ | $0.100\text{ M NaOH}$ |

For $\text{NH}_4^+\text{(aq)}$, $pK_a=9.25$. Assume that the volumes are additive and that $\log 2=0.30$. What is the pH of the solution after the solutions are mixed?

Accepted Answer

$8.95$

Answer

$8.55$

Answer

$9.55$

Answer

$12.70$

Question 16

A student uses a volumetric pipet to transfer a $25.00\text{ mL}$ sample of an unknown monoprotic acid, $\text{HA(aq)}$, into a flask. The student titrates the sample with standardized $0.1000\text{ M}$ $\text{NaOH(aq)}$, using phenolphthalein as the indicator. The neutralization reaction has a $1:1$ mole ratio.

Near the endpoint, the student adds too much titrant, producing a persistent deep-pink solution rather than the first persistent pale-pink color. The student records $28.64\text{ mL}$ as the volume of titrant delivered and uses this volume to calculate the initial concentration of the acid. All other measurements are made correctly.

Which statement correctly describes how the error affects the calculated concentration of $\text{HA(aq)}$?

Accepted Answer

The calculated concentration is higher than the actual concentration because the recorded $\text{NaOH(aq)}$ volume is too large, causing too many moles of $\text{HA(aq)}$ to be inferred from the $1:1$ mole ratio.

Answer

The calculated concentration is lower than the actual concentration because, at fixed $\text{NaOH(aq)}$ concentration, a larger delivered volume corresponds to fewer moles of $\text{OH}^-\text{(aq)}$.

Answer

The calculated concentration is equal to the actual concentration because any $\text{OH}^-\text{(aq)}$ added after equivalence remains unreacted and therefore cannot affect the titration calculation.

Answer

The calculated concentration is higher than the actual concentration because the deep-pink color indicates that $\text{HA(aq)}$ remains in excess at the endpoint.

Question 17

A student determines the concentration of a monoprotic acid, $\text{HA(aq)}$, by titrating a $25.00\text{ mL}$ sample with $0.1000\text{ M}$ $\text{NaOH(aq)}$. The endpoint is assumed to coincide with the equivalence point.

Before each trial, the student rinses the buret with deionized water, drains it, and then fills it directly with the $\text{NaOH(aq)}$ titrant. A small, reproducible volume of water remains in the buret and mixes with the titrant. The student calculates the acid concentration using the labeled $0.1000\text{ M}$ titrant concentration and the recorded buret volume.

How does rinsing the buret only with deionized water affect the calculated concentration of $\text{HA}$?

Accepted Answer

The calculated concentration is too high because the residual water dilutes the $\text{NaOH(aq)}$, so a larger buret volume is required to deliver the equivalence-point moles of $\text{OH}^{-}\text{(aq)}$, and that larger volume is multiplied by $0.1000\text{ M}$.

Answer

The calculated concentration is too low because the residual water lowers the concentration of the $\text{NaOH(aq)}$ delivered from the buret.

Answer

The calculated concentration is too high because the residual water dilutes the acid in the flask, so more titrant must be added to neutralize the acid.

Answer

The calculated concentration is unchanged because adding deionized water does not change the number of moles of $\text{HA}$ originally present in the flask.

Question 18

The particulate diagram represents all the solute particles in separate aqueous solutions of [1m[0m$\text{MgCl}_2$ and $\text{NaOH}$ before the solutions are mixed. Each solute is completely dissociated. When the solutions are mixed, insoluble $\text{Mg(OH)}_2\text{(s)}$ forms, while $\text{Na}^{+}\text{(aq)}$ and $\text{Cl}^{-}\text{(aq)}$ are spectator ions.

Which particulate representation could show the contents of the container after precipitation is complete?

Accepted Answer

Diagram B

Answer

Diagram A

Answer

Diagram C

Answer

Diagram D

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