Q: A solution is prepared by dissolving 20.0 g of NaCl (molar mass = 58.44 g/mol) in 500.0 g of water. Calculate the freezing point depression ($\Delta T_f$) of the solution. ($K_f$ for water = $1.86 \, ^\circ \mathrm{C} \, \mathrm{kg}^{-1} \mathrm{mol}^{-1}$, $i$ for NaCl = 2).

$\Delta T_f = 2 \times 1.86 \times \frac{20.0/58.44}{0.500} = 1.27 \, ^\circ \mathrm{C}$

Question 1

Which of the following best describes the effect of adding a nonvolatile solute to a solvent on its vapor pressure, according to Raoult's Law?

Accepted Answer

The vapor pressure of the solution decreases because the solute particles lower the number of solvent molecules at the surface.

Question 2

A solution is prepared by dissolving 20.0 g of NaCl (molar mass = 58.44 g/mol) in 500.0 g of water. Calculate the freezing point depression ($\Delta T_f$) of the solution. ($K_f$ for water = $1.86 \, ^\circ \mathrm{C} \, \mathrm{kg}^{-1} \mathrm{mol}^{-1}$, $i$ for NaCl = 2).

Accepted Answer

$\Delta T_f = 2 	imes 1.86 	imes \frac{20.0/58.44}{0.500} = 1.27 \, ^\circ \mathrm{C}$

Question 3

Which of the following statements about the van’t Hoff factor ($i$) is correct when calculating the colligative properties of electrolyte solutions?

Accepted Answer

The van’t Hoff factor represents the number of ions produced per formula unit of solute dissolved.

Question 4

A reaction is found to be first order in A and second order in B. Which of the following is the correct rate law for the reaction?

Accepted Answer

$	ext{rate} = k[A][B]^2$

Question 5

Given the following data for a reaction: doubling the concentration of reactant A doubles the rate, while doubling the concentration of reactant B quadruples the rate. What is the overall order of the reaction?

Accepted Answer

Third order

Question 6

Which of the following integrated rate laws corresponds to a first-order reaction?

Accepted Answer

$\ln [A]_t = -kt + \ln [A]_0$

Question 7

A certain reaction has a rate constant $k$ that increases from $1.0 	imes 10^{-3} \, \mathrm{s}^{-1}$ to $4.0 	imes 10^{-3} \, \mathrm{s}^{-1}$ when the temperature is raised from $300 \, \mathrm{K}$ to $320 \, \mathrm{K}$. Using the Arrhenius equation, which of the following best explains this observation?

Accepted Answer

The activation energy is positive, and increasing temperature increases the fraction of molecules with enough energy to react.

Question 8

Which of the following best describes the role of a catalyst in a chemical reaction?

Accepted Answer

A catalyst provides an alternative pathway with a lower activation energy.

Question 9

For a reaction $A \rightarrow B$, the half-life for a first-order process is given by which of the following equations?

Accepted Answer

$t_{1/2} = \frac{0.693}{k}$

Question 10

Which of the following is the correct equilibrium constant expression ($K_c$) 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 11

A chemical system at equilibrium is disturbed by increasing the concentration of a reactant. According to Le Châtelier’s principle, what will happen?

Accepted Answer

The system will shift to produce more products.

Question 12

For the reaction $\mathrm{N}_2(g) + 3\,\mathrm{H}_2(g) \rightleftharpoons 2\,\mathrm{NH}_3(g)$, which of the following changes will increase the yield of ammonia ($\mathrm{NH}_3$) at equilibrium?

Accepted Answer

Increasing the pressure by decreasing the volume.

Question 13

Which of the following statements correctly distinguishes between $K_c$ and $K_p$ for a gaseous equilibrium?

Accepted Answer

$K_c$ is based on concentrations (mol/L), while $K_p$ is based on partial pressures (atm).

Question 14

A saturated solution of $\mathrm{CaF}_2$ is prepared. The solubility product constant ($K_{sp}$) expression for $\mathrm{CaF}_2$ is:

Accepted Answer

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

Question 15

A reaction mixture initially contains 0.50 M $\mathrm{A}$ and 0.50 M $\mathrm{B}$ for the reaction $\mathrm{A} + \mathrm{B} \rightleftharpoons \mathrm{C}$. At equilibrium, $[\mathrm{C}] = 0.30$ M. What is the value of the equilibrium constant $K_c$?

Accepted Answer

$K_c = \frac{[C]}{[A][B]} = \frac{0.30}{(0.20)(0.20)} = 7.5$

Question 16

Which of the following best explains why the boiling point of seawater is higher than that of pure water?

Accepted Answer

The dissolved salts in seawater lower the vapor pressure, requiring a higher temperature to boil.

Question 17

A student measures the rate of a reaction at several temperatures and plots $\ln k$ versus $1/T$. The slope of the resulting line is $-5000 \ \mathrm{K}$. What is the activation energy ($E_a$) for the reaction? ($R = 8.314 \ \mathrm{J} \, \mathrm{mol}^{-1} \mathrm{K}^{-1}$)

Accepted Answer

$E_a = -	ext{slope} 	imes R = 5000 	imes 8.314 = 41570 \ \mathrm{J} \, \mathrm{mol}^{-1}$

Question 18

Which of the following is true about the effect of a catalyst on a reaction energy diagram?

Accepted Answer

The catalyst lowers the activation energy, making the peak of the energy diagram lower.

General Chemistry Study Notes: Colligative Properties, Chemical Kinetics, and Chemical Equilibrium

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