Moles & Molar Mass practice questions

Question 1

A student measures a sample of pure titanium metal, $\text{Ti}$ (molar mass $48 \text{ g/mol}$), and finds its mass to be $24 \text{ g}$. Which of the following is closest to the total number of titanium atoms in the sample?

Accepted Answer

$3.0 \times 10^{23}$

Answer

$6.0 \times 10^{23}$

Answer

$1.2 \times 10^{24}$

Answer

$1.4 \times 10^{25}$

Question 2

A student heats a sample of pure titanium metal, $\text{Ti}$, in a crucible with excess oxygen until the metal is completely converted to titanium oxide. The student records the mass measurements in the table below.

| Measurement | Mass (g) |
|---|---|
| Mass of empty crucible and lid | 15.00 |
| Mass of crucible, lid, and $\text{Ti}$ sample | 17.40 |
| Mass of crucible, lid, and titanium oxide | 19.00 |

Based on the data in the table, what is the empirical formula of the titanium oxide?

Accepted Answer

$\text{TiO}_2$

Answer

$\text{TiO}$

Answer

$\text{Ti}_2\text{O}$

Answer

$\text{Ti}_2\text{O}_3$

Question 3

A student determines the molar mass of an unknown volatile liquid by vaporizing a sample in a container of fixed volume at a constant temperature and pressure. Under these conditions, the density of the unknown vapor is measured to be $3.60 \text{ g/L}$. Under the exact same temperature and pressure, the density of pure $\text{O}_2\text{(g)}$ (molar mass $32.0 \text{ g/mol}$) is $1.20 \text{ g/L}$. Based on these data, what is the molar mass of the unknown liquid?

Accepted Answer

$96.0 \text{ g/mol}$

Answer

$10.7 \text{ g/mol}$

Answer

$32.0 \text{ g/mol}$

Answer

$115 \text{ g/mol}$

Question 4

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 5

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 6

A student observes the particulate representations below, representing a pure substance $X$ undergoing a physical change in a sealed, rigid container of constant volume. Which of the following correctly compares the particle density and translational mobility of substance $X$ in Box $2$ relative to Box $1$?

Accepted Answer

In Box $2$, the particle density is lower and the translational mobility of the particles is greater than in Box $1$.

Answer

In Box $2$, the particle density is higher and the translational mobility of the particles is greater than in Box $1$.

Answer

In Box $2$, the particle density is lower and the translational mobility of the particles is smaller than in Box $1$.

Answer

In Box $2$, the particle density is higher and the translational mobility of the particles is smaller than in Box $1$.

Question 7

A student measures a sample of anhydrous aluminum sulfate, $\text{Al}_2(\text{SO}_4)_3$, with a mass of $34.2 \text{ g}$ for a water treatment experiment. Which of the following is the total number of oxygen atoms present in the sample?

Accepted Answer

$7.2 \times 10^{23}$

Answer

$6.0 \times 10^{22}$

Answer

$2.4 \times 10^{23}$

Answer

$2.5 \times 10^{24}$

Question 8

A 1.50 g sample of a pure organic compound containing only carbon, hydrogen, and oxygen is completely burned in excess oxygen gas. All of the carbon in the compound is converted to $\text{CO}_2\text{(g)}$ and all of the hydrogen is converted to $\text{H}_2\text{O(g)}$. The masses of the products collected are shown in the table below.

| Product | Mass (g) |
|---|---|
| $\text{CO}_2\text{(g)}$ | 2.20 |
| $\text{H}_2\text{O(g)}$ | 0.90 |

Based on these data, what is the empirical formula of the compound?

Accepted Answer

$\text{CH}_2\text{O}$

Answer

$\text{C}_2\text{H}_4\text{O}$

Answer

$\text{CH}_4\text{O}$

Answer

$\text{C}_2\text{H}_2\text{O}_3$

Question 9

A student prepares a gas mixture in a rigid container by combining 2.8 g of $\text{N}_2\text{(g)}$, 6.4 g of $\text{O}_2\text{(g)}$, and 8.0 g of $\text{Ar(g)}$. The total pressure inside the container is measured to be 1.50 atm. What is the partial pressure of $\text{N}_2\text{(g)}$ in the mixture?

Accepted Answer

$0.30 \text{ atm}$

Answer

$0.50 \text{ atm}$

Answer

$0.60 \text{ atm}$

Answer

$1.20 \text{ atm}$

Question 10

A student conducts a reaction to synthesize ammonia gas according to the balanced equation below.

$$\text{N}_2\text{(g)} + 3\text{H}_2\text{(g)} \rightarrow 2\text{NH}_3\text{(g)}$$

The student combines $56 \text{ g}$ of $\text{N}_2\text{(g)}$ and $9.0 \text{ g}$ of $\text{H}_2\text{(g)}$ in a closed reaction vessel and allows the reaction to proceed to completion. Assuming a 100% yield, what is the theoretical yield of $\text{NH}_3\text{(g)}$ produced?

Accepted Answer

$51 \text{ g}$

Answer

$26 \text{ g}$

Answer

$34 \text{ g}$

Answer

$68 \text{ g}$

Question 11

A student investigates the kinetics of the gas-phase reaction $2\text{ NO}_2(g) \rightarrow 2\text{ NO}(g) + \text{O}_2(g)$ at a constant temperature. Based on experimental data, the reaction is determined to be second order with respect to $\text{NO}_2(g)$. The student collects the data shown in the table below.

| Trial | Initial $[\text{NO}_2]$ (M) | Initial Rate of Consumption of $\text{NO}_2$ (M/s) |
|---|---|---|
| 1 | 0.20 | $1.2 \times 10^{-3}$ |
| 2 | 0.40 | ? |

Which of the following best predicts the initial rate of consumption of $\text{NO}_2(g)$ in Trial 2 and provides the correct justification?

Accepted Answer

The initial rate will be $4.8 \times 10^{-3}\text{ M/s}$ because doubling the concentration of a second-order reactant increases the reaction rate by a factor of $2^2$.

Answer

The initial rate will be $4.8 \times 10^{-3}\text{ M/s}$ because two moles of $\text{NO}_2(g)$ react for every one mole of $\text{O}_2(g)$ produced.

Answer

The initial rate will be $2.4 \times 10^{-3}\text{ M/s}$ because the rate of a reaction is directly proportional to the reactant concentration.

Answer

The initial rate will be $2.4 \times 10^{-3}\text{ M/s}$ because the stoichiometric coefficient of $\text{NO}_2(g)$ in the balanced chemical equation is 2.

Question 12

A student electrolyzes an aqueous solution of $\text{CuSO}_4\text{(aq)}$ to deposit copper metal onto a cathode. To deposit $0.635 \text{ g}$ of $\text{Cu(s)}$ in $965 \text{ s}$, what constant electrical current, in amperes, must be passed through the cell? (Faraday's constant = $96,500 \text{ C/mol } e^-$; molar mass of $\text{Cu} = 63.5 \text{ g/mol}$)

Accepted Answer

$2.00 \, \text{A}$

Answer

$0.500 \, \text{A}$

Answer

$1.00 \, \text{A}$

Answer

$4.00 \, \text{A}$

Question 13

A student performs two calorimetry trials in an insulated vessel containing water. In Trial 1, a process transfers heat $q$ to $50.0\text{ g}$ of water, resulting in a temperature change of $\Delta T_1$. In Trial 2, the same process transfers the same amount of heat $q$ to $100.0\text{ g}$ of water, resulting in a temperature change of $\Delta T_2$. Assuming no heat is lost to the surroundings, what is the ratio $\dfrac{\Delta T_1}{\Delta T_2}$?

Accepted Answer

$2$

Answer

$\dfrac{1}{4}$

Answer

$\dfrac{1}{2}$

Answer

$4$

Question 14

A student measures out a sample containing exactly $0.500 \text{ mol}$ of pure sucrose, $\text{C}_{12}\text{H}_{22}\text{O}_{11}\text{(s)}$. What is the mass of carbon present in this sample?

Accepted Answer

$72.0 \text{ g}$

Answer

$144 \text{ g}$

Answer

$171 \text{ g}$

Answer

$342 \text{ g}$

Question 15

An iron ore sample contains various iron oxides. A student analyzes three pure samples of iron oxides: $\text{FeO}$, $\text{Fe}_2\text{O}_3$, and $\text{Fe}_3\text{O}_4$. Which of the following lists the three oxides in order of greatest to least mass percent of iron ($\text{Fe}$)?

Accepted Answer

$\text{FeO} > \text{Fe}_3\text{O}_4 > \text{Fe}_2\text{O}_3$

Answer

$\text{Fe}_2\text{O}_3 > \text{Fe}_3\text{O}_4 > \text{FeO}$

Answer

$\text{FeO} > \text{Fe}_2\text{O}_3 > \text{Fe}_3\text{O}_4$

Answer

$\text{Fe}_3\text{O}_4 > \text{Fe}_2\text{O}_3 > \text{FeO}$

Question 16

A student uses atom models to assemble models of $\text{H}_2\text{O}$ molecules. The available numbers of atom models are shown in the table. Avogadro's constant is $N_A=6.02\times10^{23}\ \text{mol}^{-1}$.

| Atom model | Number available |
|------------|------------------|
| $\text{H}$ | $6.02\times10^{23}$ |
| $\text{O}$ | $1.806\times10^{23}$ |

What is the maximum amount of $\text{H}_2\text{O}$ models that can be assembled, and which atom model is the limiting constituent?

Accepted Answer

$0.300\text{ mol}$ of $\text{H}_2\text{O}$; $\text{O}$ is limiting

Answer

$0.150\text{ mol}$ of $\text{H}_2\text{O}$; $\text{O}$ is limiting

Answer

$0.500\text{ mol}$ of $\text{H}_2\text{O}$; $\text{H}$ is limiting

Answer

$0.600\text{ mol}$ of $\text{H}_2\text{O}$; $\text{O}$ is limiting

Question 17

A student recovers ethanol used in a fragrance extraction by heating the liquid in a simple distillation apparatus. The boiling process is represented by the following equation.

$$ \text{CH}_3\text{CH}_2\text{OH(l)} \rightarrow \text{CH}_3\text{CH}_2\text{OH(g)} $$

Which statement correctly classifies and explains the boiling of ethanol?

Accepted Answer

Boiling is a physical change because intermolecular attractions are overcome while the covalent bonds within each ethanol molecule remain intact.

Answer

Boiling is a physical change because the covalent $\text{C-C}$, $\text{C-O}$, and $\text{O-H}$ bonds temporarily break and then re-form in the vapor.

Answer

Boiling is a chemical change because intermolecular attractions between intact ethanol molecules are overcome as the molecules move farther apart.

Answer

Boiling is a chemical change because the change from $\text{(l)}$ to $\text{(g)}$ indicates that gaseous ethanol has a different molecular structure.

Question 18

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

$23.1\%$

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