Q: A chemist wants to determine the enthalpy change for the reaction: $$\ce{C(s) + O2(g) -\u003e CO2(g)}. $$Given the following bond enthalpies: $$\ce{C=O} in$$ $$\ce{CO2} is$$ $$799\ \mathrm{kJ/mol}$$, $$\ce{O=O} is$$ $$498\ \mathrm{kJ/mol}$$, and $$\ce{C-C} is$$ $$348\ \mathrm{kJ/mol}. $$Which calculation is correct?

$$\Delta H = [498] - [2 \times 799]$$

Question 1

Which of the following statements best explains why the boiling point of water is significantly higher than that of methane, even though both are small molecules?

Accepted Answer

Water molecules form strong hydrogen bonds, which require more energy to break compared to the weak London dispersion forces in methane.

Question 2

A chemist wants to determine the enthalpy change for the reaction: $$\ce{C(s) + O2(g) -\u003e CO2(g)}. $$Given the following bond enthalpies: $$\ce{C=O} in$$ $$\ce{CO2} is$$ $$799\ \mathrm{kJ/mol}$$, $$\ce{O=O} is$$ $$498\ \mathrm{kJ/mol}$$, and $$\ce{C-C} is$$ $$348\ \mathrm{kJ/mol}. $$Which calculation is correct?

Accepted Answer

$$\Delta H = [498] - [2 	imes 799]$$

Question 3

Which of the following molecules is polar according to VSEPR theory and molecular geometry?

Accepted Answer

$$\ce{NH3} ($$ammonia)

Question 4

A solution is prepared by dissolving $$10.0\ \mathrm{g} of $$NaCl in $$100.0\ \mathrm{g} of $$water. What is the mole fraction of NaCl in the solution? (Molar mass of NaCl = $$58.44\ \mathrm{g/mol}$$, water = $$18.02\ \mathrm{g/mol})$$

Accepted Answer

$$0.029$$

Question 5

Which of the following best describes the process of vaporization at the molecular level?

Accepted Answer

Molecules at the surface of a liquid gain enough kinetic energy to overcome intermolecular forces and enter the gas phase.

Question 6

A sample of water is heated at a constant rate. The temperature increases until it reaches $$100\ ^\circ\mathrm{C}$$, then remains constant as the water boils. Which of the following best explains why the temperature does not increase during boiling?

Accepted Answer

The energy is used to break intermolecular forces rather than increase kinetic energy.

Question 7

Which of the following would increase the rate of vaporization of a liquid?

Accepted Answer

Increasing the surface area of the liquid.

Question 8

A solution is made by dissolving $$2.0\ \mathrm{mol} of $$glucose ($$\ce{C6H12O6}$$, a non-electrolyte) in $$1.0\ \mathrm{kg} of $$water. What is the expected freezing point of the solution? ($$K_f$$ for water = $$1.86\ \mathrm{^\circ C\,kg/mol}$$, freezing point of pure water = $$0.00\ ^\circ\mathrm{C})$$

Accepted Answer

$$-3.72\ ^\circ\mathrm{C}$$

Question 9

Which of the following best explains why ionic compounds generally have higher melting points than molecular compounds?

Accepted Answer

Ionic compounds are held together by strong electrostatic forces between ions, which require more energy to break.

Question 10

A chemist needs to prepare $$250.0\ \mathrm{mL} of$$ $$0.200\ \mathrm{M}$$ NaCl solution. How many grams of NaCl are required? (Molar mass of NaCl = $$58.44\ \mathrm{g/mol})$$

Accepted Answer

$$2.92\ \mathrm{g}$$

Question 11

Which of the following correctly describes the effect of adding a nonvolatile solute to a solvent on the vapor pressure of the solution?

Accepted Answer

The vapor pressure of the solution decreases compared to the pure solvent.

Question 12

Which of the following is the correct form of the Clausius-Clapeyron equation for determining the enthalpy of vaporization from vapor pressure data at two temperatures?

Accepted Answer

$$\ln \left( \frac{P_2$}{P_1$} \right) = -\frac{\Delta H_{vap}}{R} \left( \frac{1}{T_2$} - \frac{1}{T_1$} \right)$$

Question 13

Which of the following best explains why salt is spread on icy roads in winter?

Accepted Answer

Salt lowers the freezing point of water, causing ice to melt at lower temperatures.

Question 14

Which of the following is NOT a property of an ideal solution according to Raoult's Law?

Accepted Answer

The interactions between solute and solvent are much stronger than those between solvent molecules.

Question 15

Which of the following best describes the molecular orbital theory's explanation for the paramagnetism of $$\ce{O2}$$?

Accepted Answer

$$\ce{O2}$$ has two unpaired electrons in its antibonding $$\pi^*$$ orbitals.

Question 16

A phase diagram for a substance shows a negative slope for the solid-liquid equilibrium line. What does this indicate about the densities of the solid and liquid phases?

Accepted Answer

The liquid phase is denser than the solid phase.

Question 17

Which of the following is the correct expression for the boiling point elevation of a solution?

Accepted Answer

$$\Delta T_b = i K_b m$$

Question 18

Which of the following factors will increase the solubility of a gas in a liquid solvent?

Accepted Answer

Increasing the pressure of the gas above the liquid.

Question 19

Which of the following best explains why ethanol ($$\ce{C2H5OH}) is $$miscible with water in all proportions?

Accepted Answer

Ethanol and water can form hydrogen bonds with each other.

Question 20

A $$0.10\ \mathrm{m}$$ aqueous solution of NaCl has a lower vapor pressure than pure water at the same temperature. Which law explains this phenomenon?

Accepted Answer

Raoult's Law

General Chemistry II Exam III Review: Chemical Bonding, Intermolecular Forces, and Solutions

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