Molarity & Solutions practice questions

Question 1

A student dissolves a sample of solid magnesium chloride, $\text{MgCl}_2\text{(s)}$, in distilled water. Which of the following best predicts the orientation of the surrounding $\text{H}_2\text{O}$ molecules in the hydration shells around the dissolved $\text{Mg}^{2+}\text{(aq)}$ and $\text{Cl}^-\text{(aq)}$ ions?

Accepted Answer

The oxygen atoms of $\text{H}_2\text{O}$ point toward $\text{Mg}^{2+}$, and the hydrogen atoms point toward $\text{Cl}^-$.

Answer

The hydrogen atoms of $\text{H}_2\text{O}$ point toward $\text{Mg}^{2+}$, and the oxygen atoms point toward $\text{Cl}^-$.

Answer

The oxygen atoms of $\text{H}_2\text{O}$ point toward both $\text{Mg}^{2+}$ and $\text{Cl}^-$.

Answer

The hydrogen atoms of $\text{H}_2\text{O}$ point toward both $\text{Mg}^{2+}$ and $\text{Cl}^-$.

Question 2

A student dissolves a sample of solid calcium chloride, $\text{CaCl}_2\text{(s)}$, in water contained in an insulated calorimeter. The student observes that the temperature of the water increases from $22.0^\circ\text{C}$ to $31.0^\circ\text{C}$. Which of the following claims and justifications correctly identifies the thermochemical nature of the dissolution process?

Accepted Answer

The process is exothermic because thermal energy flows from the dissolving $\text{CaCl}_2$ (system) to the water (surroundings).

Answer

The process is exothermic because thermal energy flows from the water (surroundings) to the dissolving $\text{CaCl}_2$ (system).

Answer

The process is endothermic because thermal energy flows from the water (surroundings) to the dissolving $\text{CaCl}_2$ (system).

Answer

The process is endothermic because thermal energy flows from the dissolving $\text{CaCl}_2$ (system) to the water (surroundings).

Question 3

A student adds a sample of solid ammonium nitrate, $\text{NH}_4\text{NO}_3\text{(s)}$, to a beaker of water at $25^\circ\text{C}$. The dissolution process is represented by the equation below:

$$\text{NH}_4\text{NO}_3\text{(s)} \rightarrow \text{NH}_4^+\text{(aq)} + \text{NO}_3^-\text{(aq)} \quad \Delta H^\circ > 0$$

Which of the following best predicts the temperature change of the solution as the salt dissolves, and provides the correct justification?

Accepted Answer

The temperature of the solution decreases because the endothermic dissolution process absorbs thermal energy from the solution.

Answer

The temperature of the solution increases because the dissolution process is exothermic and releases thermal energy to the solution.

Answer

The temperature of the solution increases because thermal energy is absorbed by the salt from the surrounding air.

Answer

The temperature of the solution decreases because the dissolution process releases thermal energy to the surroundings.

Question 4

A student performs paper chromatography on a mixture of two dyes, Dye 1 and Dye 2, using cellulose paper as the stationary phase and ethanol, $\text{C}_2\text{H}_5\text{OH}$ (a polar solvent), as the mobile phase. The experimental results are shown in the table below.

| Dye | Solvent Front Distance (cm) | Dye Distance (cm) |
|---|---|---|
| Dye 1 | $10.0$ | $8.0$ |
| Dye 2 | $10.0$ | $3.0$ |

If the student repeats the experiment using hexane, $\text{C}_6\text{H}_{14}$ (a nonpolar solvent), as the mobile phase, which of the following predictions regarding Dye 1 is correct, and why?

Accepted Answer

Dye 1 will travel a shorter distance because it forms weaker intermolecular attractions with nonpolar hexane than with polar ethanol.

Answer

Dye 1 will travel a greater distance because nonpolar hexane moves faster up the paper, carrying all solutes farther.

Answer

Dye 1 will travel a greater distance because it forms stronger London dispersion forces with hexane than the dipole-dipole forces it formed with ethanol.

Answer

Dye 1 will travel the same distance because the retention factor ($R_f$) depends only on the stationary phase and is independent of the mobile phase.

Question 5

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 6

A student prepares a solution by dissolving $14.2 \text{ g}$ of solid sodium sulfate ($\text{Na}_2\text{SO}_4$, molar mass $142 \text{ g/mol}$) in enough distilled water to make exactly $500.0 \text{ mL}$ of solution in a volumetric flask. What is the molarity of the $\text{Na}_2\text{SO}_4\text{(aq)}$ solution?

Accepted Answer

$0.200 \text{ M}$

Answer

$0.400 \text{ M}$

Answer

$0.600 \text{ M}$

Answer

$2.00 \text{ M}$

Question 7

A student determines the concentration of an unknown solution of $\text{CuSO}_4\text{(aq)}$ using a spectrophotometer set to a wavelength of $635\text{ nm}$. First, the student measures the absorbance of a $2.0 \times 10^{-3}\text{ M}$ standard $\text{CuSO}_4\text{(aq)}$ solution in a cuvette with a path length of $0.50\text{ cm}$, obtaining an absorbance of $0.20$. Next, the student measures the absorbance of the unknown $\text{CuSO}_4\text{(aq)}$ solution in a standard cuvette with a path length of $1.0\text{ cm}$ and records an absorbance of $0.60$. Based on these data, what is the molar concentration of the unknown $\text{CuSO}_4\text{(aq)}$ solution?

Accepted Answer

$3.0 \times 10^{-3}\text{ M}$

Answer

$1.5 \times 10^{-3}\text{ M}$

Answer

$6.0 \times 10^{-3}\text{ M}$

Answer

$1.2 \times 10^{-2}\text{ M}$

Question 8

A student dissolves a sample of solid calcium chloride, $\text{CaCl}_2\text{(s)}$, in an insulated calorimeter containing water at room temperature. The student claims that the dissolution of $\text{CaCl}_2\text{(s)}$ is an exothermic process. Which of the following experimental observations provides the best evidence to support the student's claim?

Accepted Answer

The temperature of the aqueous solution increases as the solid dissolves, demonstrating that thermal energy is transferred from the chemical system to the surroundings.

Answer

The temperature of the aqueous solution decreases as the solid dissolves, demonstrating that thermal energy is absorbed by the chemical system from the surroundings.

Answer

The solubility of the solid in water increases when the temperature is raised, demonstrating that thermal energy acts as a reactant in the dissolution process.

Answer

The electrical conductivity of the solution increases as the solid dissolves, demonstrating that thermal energy is produced alongside mobile hydrated ions.

Question 9

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 10

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 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 titrates a sample of a monoprotic acid, $\text{HA(aq)}$, with standardized $\text{NaOH(aq)}$. The reaction is represented by

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

| Quantity | Value |
|---|---:|
| Volume of acid sample | $25.00\text{ mL}$ |
| Concentration of $\text{NaOH(aq)}$ | $0.1000\text{ M}$ |
| Initial buret reading | $0.30\text{ mL}$ |
| Final buret reading | $25.30\text{ mL}$ |

Before the initial reading is recorded, an air bubble is trapped in the buret tip. During the titration, the bubble is expelled and the tip fills completely with the titrant. The student uses the difference between the final and initial buret readings to calculate the molarity of the acid. Which of the following correctly describes how the air bubble affects the calculated molarity of $\text{HA(aq)}$?

Accepted Answer

The calculated molarity is too high because the recorded volume includes titrant that fills the buret tip but does not enter the flask.

Answer

The calculated molarity is too high because expelling the air bubble increases the concentration of the $\text{NaOH(aq)}$ that subsequently enters the flask.

Answer

The calculated molarity is too low because some of the recorded titrant volume fills the buret tip, causing the student to use too few moles of $\text{NaOH}$ in the calculation.

Answer

The calculated molarity is unchanged because subtracting the initial buret reading from the final buret reading eliminates the effect of the air bubble.

Question 13

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 14

A student tests a barium hydroxide solution used to neutralize acidic wastewater. At $25^\circ\text{C}$, the analytical concentration of $\text{Ba(OH)}_2$ is $5.0 \times 10^{-3}\text{ M}$. Assume that the solute dissociates completely according to the equation

$\text{Ba(OH)}_2\text{(aq)} \rightarrow \text{Ba}^{2+}\text{(aq)} + 2\text{OH}^{-}\text{(aq)}$

What are the $\text{pOH}$ and $\text{pH}$ of the solution?

Accepted Answer

$\text{pOH}=2.00$ and $\text{pH}=12.00$

Answer

$\text{pOH}=1.70$ and $\text{pH}=12.30$

Answer

$\text{pOH}=2.30$ and $\text{pH}=11.70$

Answer

$\text{pOH}=12.00$ and $\text{pH}=2.00$

Question 15

A student prepares a solution of a tracer dye used to monitor water flow. The student obtains the following data at the wavelength of maximum absorbance.

| Dye concentration | Cuvette path length | Absorbance |
|---|---|---|
| $2.00 \times 10^{-5}\text{ M}$ | $2.00\text{ cm}$ | $0.800$ |

The Beer-Lambert relationship is $A=\varepsilon bc$, where $\varepsilon$ is the molar absorptivity. What is the molar absorptivity of the dye?

Accepted Answer

$2.00 \times 10^4\text{ M}^{-1}\text{cm}^{-1}$

Answer

$2.00 \times 10^3\text{ M}^{-1}\text{cm}^{-1}$

Answer

$4.00 \times 10^4\text{ M}^{-1}\text{cm}^{-1}$

Answer

$2.00 \times 10^6\text{ M}^{-1}\text{cm}^{-1}$

Question 16

A student determines the composition of a mixture containing only $\text{NaCl(s)}$ and sand. The student adds excess distilled water so that all the $\text{NaCl(s)}$ dissolves, filters the mixture, and dries the filter paper and residue to constant mass. Because the sand is insoluble, the dry residue is sand.

| Measurement | Mass |
|-------------|------|
| Original mixture | $12.0\text{ g}$ |
| Dry filter paper | $1.00\text{ g}$ |
| Dry filter paper and residue | $4.00\text{ g}$ |

What is the mass percent of sand in the original mixture?

Accepted Answer

$25.0\%$

Answer

$8.33\%$

Answer

$16.7\%$

Answer

$23.1\%$

Question 17

A student dissolves solid sodium chloride, $\text{NaCl(s)}$, in distilled water to prepare an aqueous solution. Which of the following particulate representations best shows the predominant orientation of water molecules around the $\text{Na}^+$ and $\text{Cl}^-$ ions in the solution?

Accepted Answer

Diagram A

Answer

Diagram B

Answer

Diagram C

Answer

Diagram D

Question 18

A student models the carbonation step of a beverage-production process. Identical samples of water are placed in pressure-rated vessels and allowed to reach equilibrium with pure $\text{CO}_2\text{(g)}$ headspaces maintained at the conditions shown. The process being studied is $\text{CO}_2\text{(g)} \rightleftharpoons \text{CO}_2\text{(aq)}$.

| Condition | Temperature | $P_{\text{CO}_2}$ |
|-----------|-------------|----------------------|
| 1 | $25^\circ\text{C}$ | $1.0\text{ atm}$ |
| 2 | $5^\circ\text{C}$ | $2.0\text{ atm}$ |

Which of the following best predicts the equilibrium concentration of $\text{CO}_2\text{(aq)}$ under condition 2 relative to condition 1?

Accepted Answer

It increases, because decreasing the temperature and increasing $P_{\text{CO}_2}$ both increase the solubility of $\text{CO}_2$ in water.

Answer

It increases, because increasing $P_{\text{CO}_2}$ increases gas solubility, but temperature does not affect gas solubility.

Answer

It remains unchanged, because identical volumes of water must dissolve identical amounts of $\text{CO}_2$.

Answer

It decreases, because lower temperature and higher $P_{\text{CO}_2}$ both favor transfer of $\text{CO}_2\text{(aq)}$ into the headspace.

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