Q: A solution is prepared by dissolving 10.0 g of NaCl (molar mass = 58.44 g/mol) in 100.0 g of water. What is the molality of the solution?

$$\text{molality} = \frac{\text{moles of solute}}{\text{kg of solvent}} = \frac{10.0/58.44}{0.100} = 1.71\ \mathrm{mol/kg}$$

Question 1

Which of the following statements best explains why gases are more compressible than liquids or solids?

Accepted Answer

Gases have much lower densities because their particles are far apart, allowing them to be compressed significantly.

Question 2

A sample of oxygen gas occupies 10.0 L at 2.00 atm and 300 K. What will be its volume at 1.00 atm and 300 K, assuming ideal gas behavior?

Accepted Answer

$$V_2$$ = $$V_1$$ \frac{$$P_1$$}{$$P_2$$} = 10.0\ \mathrm{L} 	imes \frac{2.00\ \mathrm{atm}}{1.00\ \mathrm{atm}} = 20.0\ \mathrm{L}$$

Question 3

Which of the following best describes the difference between intramolecular and intermolecular forces?

Accepted Answer

Intramolecular forces hold atoms together within a molecule, while intermolecular forces act between separate molecules.

Question 4

A 2.00 g sample of helium gas is placed in a 5.00 L container at 27°C. What is the pressure in the container? (R = 0.0821 L·atm·mol⁻¹·K⁻¹, molar mass of He = 4.00 g/mol)

Accepted Answer

$$n = \frac{2.00\ \mathrm{g}}{4.00\ \mathrm{g/mol}} = 0.500\ \mathrm{mol}$$ 
$$T = 27^\circ\mathrm{C} + 273 = 300\ \mathrm{K}$$ 
$$p = \frac{nRT}{V} = \frac{0.500 	imes 0.0821 	imes 300}{5.00} = 2.46\ \mathrm{atm}$$

Question 5

Which of the following statements about the Maxwell-Boltzmann distribution is correct?

Accepted Answer

At higher temperatures, the distribution of molecular speeds broadens and the most probable speed increases.

Question 6

Which of the following best explains why the solubility of gases in water decreases as temperature increases?

Accepted Answer

Dissolving a gas in water is usually exothermic, so increasing temperature shifts the equilibrium to favor the gas phase.

Question 7

A solution is prepared by dissolving 10.0 g of NaCl (molar mass = 58.44 g/mol) in 100.0 g of water. What is the molality of the solution?

Accepted Answer

$$	ext{molality} = \frac{	ext{moles of solute}}{	ext{kg of solvent}} = \frac{10.0/58.44}{0.100} = 1.71\ \mathrm{mol/kg}$$

Question 8

Which of the following phase transitions is exothermic?

Accepted Answer

Condensation of water vapor to liquid water.

Question 9

Which of the following is the correct expression for the equilibrium constant $$K_c$$ for the reaction: $$2\mathrm{NO}_2(g) \rightleftharpoons 2\mathrm{NO}(g) + \mathrm{O}_2(g)$$?

Accepted Answer

$$K_c = \frac{[\mathrm{NO}]^2[\mathrm{O}_2]}{[\mathrm{NO}_2]^2}$$

Question 10

A reaction has $$\Delta H = +50\ \mathrm{kJ}$$ and $$\Delta S = +100\ \mathrm{J/K}. At $$what temperature (in K) does the reaction become spontaneous?

Accepted Answer

$$T = \frac{\Delta H}{\Delta S} = \frac{50,000\ \mathrm{J}}{100\ \mathrm{J/K}} = 500\ \mathrm{K}$$

Question 11

Which of the following best describes the effect of a catalyst on a chemical equilibrium?

Accepted Answer

A catalyst increases the rate at which equilibrium is reached but does not affect the equilibrium composition.

Question 12

Which of the following statements about entropy ($$S) is $$correct?

Accepted Answer

The entropy of a pure, perfectly crystalline substance at 0 K is zero.

Question 13

Which of the following is the correct expression for the solubility product constant ($$K_{sp}) $$for $$\mathrm{CaF}_2$$?

Accepted Answer

$$K_{sp} = [\mathrm{Ca}^{2+}][\mathrm{F}^-]^2$$

Question 14

A 0.10 M solution of a weak acid, HA, is found to be 1.0% ionized at equilibrium. What is the value of the acid dissociation constant, $$K_a$$?

Accepted Answer

$$K_a = \frac{[H^+][A^-]}{[HA]} = \frac{(0.0010)(0.0010)}{0.099} = 1.0 	imes 10$$^{-5}$$

Question 15

Which of the following best explains why the vapor pressure of a solution is lower than that of the pure solvent?

Accepted Answer

The presence of solute particles reduces the number of solvent molecules at the surface, lowering the rate of evaporation.

Question 16

Which of the following is a correct application of Hess's Law?

Accepted Answer

The enthalpy change for a reaction is the sum of the enthalpy changes for any sequence of reactions that add up to the overall reaction.

Question 17

A mixture contains 2.0 mol of N$$_2$$, 3.0 mol of O$$_2$$, and 5.0 mol of Ar. What is the mole fraction of O$$_2 in $$the mixture?

Accepted Answer

$$\chi_{O_2$} = \frac{3.0}{2.0 + 3.0 + 5.0} = \frac{3.0}{10.0} = 0.30$$

Question 18

Which of the following statements about the van der Waals equation is correct?

Accepted Answer

The parameter $$a$$ corrects for intermolecular attractions, and $$b$$ corrects for the finite volume of gas molecules.

Question 19

Which of the following best describes the effect of adding a common ion to a saturated solution of a sparingly soluble salt?

Accepted Answer

The solubility of the salt decreases due to the common ion effect.

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